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ABSTRACT

Studies on the corrosion inhibition potentials of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on mild steel and aluminium in 0.1 M solutions of HCl, H2SO4 and H3PO4 were carried out using electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation and gravimetric techniques. The study was carried out at 303 and 333 K respectively. The results obtained revealed that AD, GU, HYP and XN had moderate inhibitive effects on corrosion of mild steel in the three acid media in the decreasing order: AD> HYP> GU> XN in HCl; HYP> AD> GU> XN in H2SO4 and GU> AD> HYP> XN in H3PO4 at 303 K. The results obtained also showed that AD, GU, HYP and XN inhibited the corrosion of aluminium in HCl in the order: GU> HYP> AD> XN. It was observed that only HYP inhibited the corrosion of aluminium in H2SO4 solution while none of the purines inhibited the corrosion of aluminium in H3PO4 solution. Potentiodynamic polarisation studies showed that the purines surpressed both the anodic and cathodic half reactions of the corrosion processes, thereby acting as mixed inhibitors, while impedance data indicated that inhibition was achieved via adsorption of the inhibitor species on the mild steel and aluminium surfaces. Synergistic effects slightly increased the efficiency of the inhibitors in the presence of iodide ions. Among several adsorption isotherms assessed, Langmuir adsorption isotherm was found to best describe the adsorption behaviour of the inhibitors on the metal surfaces. According to Langmuir isotherm, calculated values of free energy of adsorption, ΔGads°, for the corrosion of mild steel in the presence of AD, GU, HYP and XN are as follows: -22.30 to -25.78 kJ mol-1 and 5.38 to -28.33 kJ mol-1 in HCl at 303 and 333 K respectively; -3.19 to -27.52 kJ mol-1 and -11.12 to -21.19 kJ mol-1 in H2SO4 at 303 and 333 K respectively; -6.70 to -27.52 kJ mol-1 and -18.39 to
-30.25 kJ mol-1 in H3PO4 at 303 and 333 K respectively. Calculated ΔGads° values for the corrosion of aluminium in the presence of AD, GU, HYP and XN are as follows:
-10.12 to -24.04 kJ mol-1 and -24.19 to 30.25 kJ mol-1 in HCl at 303 and 333 K respectively; -10.12 kJ mol-1 and -11.12 kJ mol-1 for HYP in H2SO4 at 303 and 333 K
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respectively. Calculated activation energy (Ea) values for the corrosion of mild steel in the presence of AD, GU, HYP and XN and the blank are as follows: 49.02 to 60.81 kJ mol-1 and 49.02 kJ mol-1 (blank) in HCl; 18.99 to 33.90 kJ mol-1 and
34.86 kJ mol-1 (blank) in H2SO4; 27.79 to 30.94 kJ mol-1 and 29.87 kJ mol-1 (blank) in H3PO4. Calculated Ea values for the corrosion of aluminium in the presence of AD, GU, HYP and XN and the blank are as follows: 61.51 to 88.98 kJ mol-1 and 52.13 kJ mol-1 (blank) in HCl; 54.27 to 72.19 kJ mol-1 and 50.54 kJ mol-1 (blank) respectively in H2SO4; 55.05 to 56.99 kJ mol-1 and 52.08 kJ mol-1 (blank) in H3PO4. Calculated values of heats of adsorption, ΔQads, of AD, GU, HYP and XN on mild steel surfaces are as follows: -3.86 to -10.58 kJ mol-1 respectively in HCl; 2.31 to 10.47 kJ mol-1 in H2SO4; 0.66 kJ mol-1, 0.40 to 9.27 kJ mol-1 in H3PO4. Calculated ΔQads values for the adsorption of AD, GU, HYP and XN on aluminium surfaces are as follow: -8.86 to -24.37 kJ mol-1 in HCl. The negative ΔGads values obtained in the study indicate the spontaneity of the adsorption processes while the calculated values of activation energy (Ea) for the adsorption of the purines on the metal surfaces shows that the adsorption mechanism may not be purely physisorption. The inhibition mechanisms, estimated from the temperature dependence of inhibition efficiency as well from activation energy (Ea) and heat of adsorption (ΔQads) parameters show that the purines functioned via mixed inhibition mechanism which was confirmed by polarization curves. The SEM micrographs, FTIR and impedance spectra of the metals in the presence of the inhibitors as well as molecular dynamics modeling of the adsorption of the single molecules on the metal surface confirmed the presence of protective layers over the mild steel and aluminium surfaces thereby providing evidence for the inhibition of the corrosion of the metals.

 

 

TABLE OF CONTENTS

Title page – – – – – – – – – iii
Declaration – – – – – – – – – iv
Certification – – – – – – – – – v
Dedication – – – – – – – – – vi
Acknowledgements – – – – – – – – vii
Abstract – – – – – – – – – viii
Table of Contents – – – – – – – – x
List of Figures – – – – – – – – – xv
List of Tables – – – – – – – – xxiii
List of Appendices – – – – – – – – xxv
Abbreviations and Symbols – – – – – – – xxviii
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study – – – – – – 1
1.2 Forms of Corrosion – – – – – – – 3
1.2.1 Uniform corrosion – – – – – – – 3
1.2.2 Galvanic corrosion – – – – – – – 3
1.2.3 Pitting corrosion – – – – – – – 3
1.2.4 Crevice corrosion – – – – – – – 4
1.2.5 Intergranular corrosion – – – – – – 4
1.2.6 Erosion corrosion – – – – – – – 4
1.2.7 Cavitation corrosion – – – – – – – 5
1.2.8 Interfilm corrosion – – – – – – – 5
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1.2.9 Fretting corrosion – – – – – – – 5
1.3 Corrosion Monitoring Techniques – – – – – 6
1.3.1 Gravimetric technique – – – – – – – 6
1.3.2 Gasometric technique – – – – – – – 7
1.3.3 Thermometric technique – – – – – – 8
1.3.4 Potentiodynamic polarisation techniques – – – – 8
1.3.5 Linear polarisation resistance (LPR) – – – – – 9
1.3.6 Electochemical noise – – – – – – – 9
1.3.7 Electrochemical impedance spectroscopy (EIS) – – – 9
1.3.8 Galvanic/potential monitoring – – – – – 10
1.4 Common Methods of Corrosion Prevention – – – 11
1.4.1 Selection of materials and design against corrosion – – – 11
1.4.2 Cathodic protection – – – – – – – 12
1.4.3 Protective coatings – – – – – – – 12
1.4.4 Environment modification and addition of inhibitors – – – 12
1.5 Inhibitors – – – – – – – – 13
1.5.1 Classification of inhibitors – – – – – – 13
1.6 Adsorption Isotherms – – – – – – 18
1.6.1 Langmuir adsorption isotherm – – – – – 19
1.6.2 Freundlich isotherm – – – – – – – 19
1.6.3 Temkin adsorption isotherm – – – – – – 19
1.6.4 Flory-Huggins adsorption isotherm – – – – – 20
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm – 20
1.6.6 Frumkin adsorption isotherm – – – – – – 21
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1.7 Purines – – – – – – – – 22
1.7.1 Adenine – – – – – – – – 22
1.7.2 Guanine – – – – – – – – 23
1.7.3 Hypoxanthine – – – – – – – – 23
1.7.4 Xanthine – – – – – – – – 23
1.8 Statement of the Problem – – – – – – 23
1.9 Justification for the Choice of Purines as Corrosion Inhibitors – 24
1.10 Aims of the Research – – – – – – 26
1.11 Objectives of the Research – – – – – – 26
CHAPTER TWO
LITERATURE REVIEW
2.1 Corrosion Inhibitors – – – – – – – 28
2.1.1 Triazoles and benzotriazoles derivativesas corrosion inhibitors – 28
2.1.2 Dyes as corrosion inhibitors – – – – – – 30
2.1.3 Amino acids as corrosion inhibitors – – – – – 32
2.1.4 Schiff bases as corrosion inhibitors – – – – – 33
2.1.5 Imidazoles as corrosion inhibitors – – – – – 33
2.1.6 Purines as Corrosion Inhibitors – – – – – 34
CHAPTER THREE
MATERIALS AND METHODS
3.1 Material Preparation – – – – – – 37
3.2 Gravimetric Method – – – – – – – 37
3.3 Electrochemical Impedance Spectroscopy (EIS) – – – 38
3.4 Potentiodynamic Polarisation – – – – – 39
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3.5 Synergistic Effects – – – – – – – 39
3.6 Fourier Transform Infrared Spectrophotometery(FTIR) – 39
3.7 Scanning Electron Microscopy (SEM) – – – – 40
3.8 Computational and Theoretical Considerations – – – 40
3.8.1 Quantum chemical calculations – – – – – 40
3.8.2 Molecular dynamics simulation – – – – – 41
CHAPTER FOUR
RESULTS
4.1 Gravimetric Measurements – – – – – – 42
4.2 Half Lives (t1/2) of Mild Steel and Aluminium in the Test Solutions 63
4.3 Electrochemical Tests at 303 K – – – – – 68
4.4 Synergistic Considerations – – – – – – 77
4.5 Infrared Spectroscopy Analysis – – – – – 77
4.6 Scanning Electron Microscopy – – – – – 77
4.7 Quantum Chemical Calculations – – – – – 77
4.8 Molecular Dynamics – – – – – – – 78
CHAPTER FIVE
DISCUSSION
5.1 Gravimetric Measurements – – – – – – 102
5.2 Half Lives (t1/2) of Mild Steel and Aluminium in the Test Solutions 106
5.3 Effect of Temperature /Thermodynamics Study – – – 123
5.4 Adsorption Study – – – – – – – 130
5.5 Electrochemical Tests at 303 K – – – – – 170
5.5.1. Electrochemical impedance measurements – – – – 170
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5.5.2 Potentiodynamic polarization measurements – – – – 172
5.6 Synergistic Considerations – – – – – – 173
5.7 Infrared Spectroscopic Analysis – – – – – 175
5.8 Scanning Electron Microscopy – – – – – 178
5.9 Quantum Chemical Calculations – – – – – 179
5.10 Molecular Dynamics Simulations – – – – – 180
5.11 Mechanism of Inhibition – – – – – – 182
CHAPTER SIX
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS
6.1 Summary and Conclusions – – – – – – 186
6.2 Recommendations – – – – – – – 188
REFERENCES – – – – – – – – 189
APPENDICES – – – – – – – – – 202

 

Project Topics

 

CHAPTER ONE

NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
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to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
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1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
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ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
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In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
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1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
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c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
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regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
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1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
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An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
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In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
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b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
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decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
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zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
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sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
23
1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
25
(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
23
1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
25
(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
23
1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
25
(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
23
1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
25
(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
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1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
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(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
23
1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
25
(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
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1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
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c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
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1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
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In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
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b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
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1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
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1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
24
Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
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(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
26
1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
27
employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
28NTRODUCTION
1.1 Background to the Study
Corrosion of metals is an electrochemical process that occurs whenever a metal is in contact with an aggressive medium such as acids, bases and salts.The susceptibility of a metal to corrosion depends on the nature of the metal and the environment.
Despite the invention and over-usage of plastics in most industrial applications, metals still rule manufacturing industries. Metals like steel (iron), aluminium, copper, zinc and tin are commonly used in most industries. Mild steel is one of the best preferred materials for industries due to its easy availability and excellent structural properties. Aluminium on the other hand, is the most abundant metal in the earth’s crust (8.1%), although it is not found free in nature. The versatility of aluminium makes it the most widely used metal after steel. Most often, during industrial processes such as pickling and etching, these metals come in contact with aggressive media such as acids, bases and salts thereby exposing them to corrosion attack.
Corrosion can cause dangerous and costly damages to oil, gas and water pipelines, bridges, public buildings, vehicles, water and waste water systems and even home appliances. The effects of corrosion include large loss of products and resources, and ecological damages (Günter, 2009).
Corrosion of metals costs the United States excess of $276 billion per year (Denny, 2004).This loss to the economy is more than the Gross National Product of many countries around the world. It has been estimated that 40% of U.S. steel production goes
2
to the replacement of corroded parts and products (Jorge and Leandro, 2005 ). Analysis of oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria showed corrosion as one of the major causes of failure (Achebe et al., 2012). SPE (2008) stated in their report that Nigeria oil and gas industry suffered greatly between 2000 and 2004.The total pipeline breakage loss figure due to corrosion in 2004 alone was 396,000 metric tons (about four super tankers) while the financial losses were estimated to be #19.66 billions (US $154.4).
This menace of corrosion of metals in the oil, metallurgical and other industries has been widely acknowledged and several researches have been carried out on the protection of metals against corrosion. The results obtained revealed that one of the best methods involves the use of inhibitors. However, owing to stringent environmental regulations, organic compounds are preferred to inorganic compounds especially heavy metals derivatives, as corrosion inhibitors. Organic compounds containing hetero atoms such as N, S, P or O in conjugated or aromatic systems have been reported to be effective corrosion inhibitors (Abdallah, 2004; Ashassi-Sorkhabi et al., 2006; Umoren and Ebenso, 2008). The presence of polar functional groups (such as –NH2, -COOH and –OH) as well as π-electrons facilitates the adsorption of the inhibitor on the surface of the metal (Ebenso et al., 2008; Eddy, 2008; Obot et al, 2009a).
In the absence of adequate information on corrosion rate (metal weight loss/unit area/unit time) and various methods of protecting a metal, overdesign (e.g. thicker tube wall, leading to greater power requirements for moving parts), lower efficiency of equipment, contaminations, plants shut down, loss of production and loss of equipment will be inevitable.
3
1.2 Forms of Corrosion
Based on the appearance of the corroded metal, eight forms of corrosion have been identified and are discussed below.
1.2.1 Uniform corrosion
Uniform corrosion is the attack of a metal at essentially the same rate at all exposed areas of its surface. It is characterized by laterally constant speed of corrosion. For example, in the atmospheric corrosion of galvanized steel, the speed of corrosion depends on the thickness of the steel, as such, the thicker the steel coating, the longer the service life of the metal. Uniform attack is the most common type of corrosion and causes the greatest destruction of metals on a weight basis (Moore, 1996).
1.2.2 Galvanic corrosion
Galvanic corrosion is a type of corrosion by which metals are preferentially corroded. This form of corrosion has the potential to attack junctions of metals, or regions where one construction metal is changed to another. The extent of galvanic corrosion increases with the potential difference of the metal. The relative size of the anode or cathode significantly affects the relationship between the active and inert metals. Galvanic corrosion can be prevented by keeping dissimilar metals apart or by the provision of insulating materials between the metals in order to interrupt current flow (Oldfield, 1988; Baboian et al., 1990; Eddy, 2008).
1.2.3 Pitting corrosion
Pitting corrosion results from galvanic action, where the metal surface appears to have pinholes. The pit is the anode while the surrounding surface is the cathode (Jones, 1982). Pitting may occur as a result of one of the following.
i. A change in the acidity of the pit area
4
ii. Differential aeration may also be a contributing factor to the occurrence of pitting corrosion because most solutions are in contact with air and because of convection, transportation of oxygen through the solution leads to areas of high or low oxygen concentration. Therefore, where the metal surface contains the solutions, the variation may cause the area with the higher oxygen concentration to become a cathode while an area of lower oxygen concentration becomes an anode resulting in localized attacks (Moniz, 1986; Szklarska-Smialowska, 1986).
1.2.4 Crevice corrosion
Crevices are present in some equipment. They occur naturally around bolts, rivets etc. They are also created by scratches on metal surfaces. Crevice corrosion absorbs and draws solution toward the reactive area. Crevice corrosion is influenced by the same factors that affect pitting corrosion and is indeed a specific form of pitting corrosion (Fontana, 1986).
1.2.5 Intergranular corrosion
Intergranular corrosion occurs by localized attack at grain boundaries, which behave as anode to the larger surrounding cathode grains (Moore, 1996). Metals usually are not homogeneous. Impurities or alloying elements may segregate into grain boundaries. Heat treatment or localized heating by welding may provoke change in composition localized in or near grain boundaries.
1.2.6 Erosion corrosion
Almost all corrosive media can bring about erosion corrosion and nearly all metals and alloys are susceptible to this except those metals or alloys that are capable of forming hard, dense, adherent and continuous surface film (Staehle, 1989; Moore, 1996). The extent of erosion corrosion increases as the velocity of the corroding medium increases.
5
In some cases, the high velocity increases the supply of oxygen or other gases at the metal surface, which may depolarize the cathodic reaction and consequently increase the corrosion rate (Roberge, 1999; Sastri, 2011).
1.2.7 Cavitation corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation may mechanically destroy any protective layer, causing localized corrosion called cavitation corrosion (Moore, 1996). Similarly when an object such as a propeller rotates in water, the pressure on the trailing surface of the blade fluctuates continually. At some point, very low pressures are produced which create tensile forces high enough to exceed the interatomic binding forces of the liquid.
1.2.8 Interfilm corrosion
Coatings such as paints, conversion coating or metallic coating may lose their adhesion with substrate due to diffusion through the actual coating or to a reaction starting from defects like pinholes or scratches (Morgan, 1984). When this happens, residues of soluble salts, acids or bases will attract water through a paint film because of osmotic effect. The blister filled with water will be formed. Fill-form corrosion is a wormlike delamination of a paint film driven by salt residue and high humidity.
1.2.9 Fretting corrosion
Fretting corrosion is a combination of mechanical wear and atmospheric oxidation which frequently occurs between close fitting metal components (Moore, 1996 and Roberge, 1999). For fretting corrosion to occur, the surface is usually under load and subject to slight relative movement resulting in damage to the contact surface and formation of an oxide debris such as Fe3O4 for iron.
In theory, the eight forms of corrosion are clearly distinct, in practice however, there are corrosion cases that fit into more than one category.
6
1.3 Corrosion Monitoring Techniques
Corrosion measurement is the quantitative way by which the effectiveness of corrosion control and prevention techniques can be evaluated and provides the feedback to enable corrosion control and prevention methods to be optimized. In any corrosion monitoring system, it is common to find two or more of the techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Some of these techniques are discussed below:
1.3.1 Gravimetric technique
The weight loss technique is the simplest, and longest-established, method of estimating corrosion losses in plants and equipment. A weighed sample (coupon) of the metal or alloy under consideration is introduced into a medium, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion products and weighed. The weight loss is converted to an average corrosion rate using proper conversion equations. The basic measurement which is determined from corrosion coupons is weight loss; the weight loss over the period of exposure being expressed as corrosion rate (Oguzie, 2005; Eddy et al., 2010; Olasehinde et al., 2012; Adejo et al., 2012).
The technique is extremely versatile, since weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosion phenomena may be studied. These include, but is not limited to the following:
a) Stress-assisted corrosion
b) Bimetallic (galvanic) attack
7
c) Differential aeration
d) Heat-affected zones
Advantages of weight loss coupons are as follows:
i. The technique is applicable to all environments – gases, liquids, solids/particulate flow.
ii. Visual inspection can be undertaken.
iii. Corrosion deposits can be observed and analyzed.
iv. Weight loss can be readily determined and corrosion rate easily calculated.
v. Localized corrosion can be identified.
vi. Inhibitor performance can be easily assessed.
The disadvantage of the coupon technique is that, if a corrosion upset occurs during the period of exposure, the coupon alone will not be able to identify the time of occurrence of the upset, and depending upon the peak value of the upset and its duration, may not even register a statistically significant increased weight loss (NACE, 1999; Dean, 2003). Therefore, coupon monitoring is most useful in environments where corrosion rates do not significantly change over long time periods. However, they can provide a useful correlation with other techniques such as potentiodynamic polarisation technique (Oguzie et al., 2012a, 2012b).
1.3.2 Gasometric technique
The gasometric assembly is essentially an apparatus which measures the rate of gas evolution during a corrosion reaction. In an acid medium, the volume of hydrogen gas evolved is directly proportional to the rate of corrosion of the metal (Umoren et al., 2009). It consists of a graduated gas burette which is connected to a flask containing paraffin oil. The burette is surrounded with a glass jacket with a water inlet and outlet to
8
regulate the temperature of the gas evolved. A reaction vessel is connected to the gas burette through a delivery tube with a tap for incoming gas and another to expel the gas when the burette is full or at the end of the reaction. The reaction vessel is a three-necked flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last leading to the gas burette (Umoren et al., 2009).
1.3.3 Thermometric technique
The reaction vessel is a well lagged, three-necked round bottom flask consisting of an inlet for the metallic coupon, another one for the thermometer and the last for introducing the test solution.
The flask is well lagged to prevent heat losses. In the thermometric technique, the progress of the corrosion reaction is monitored by determining changes in temperature with time using a thermometer (0 – 100°C) (Eddy and Ebenso, 2008; Obot et al., 2009b).
1.3.4 Potentiodynamic polarisation techniques
Polarisation techniques such as potentiodynamic polarisation, potentiostaircase and cyclic voltammetry are often used for laboratory corrosion testing. These techniques can provide significant useful information regarding the corrosion mechanisms, corrosion rate and susceptibility of specific materials to corrosion in designated environments. Polarisation methods involve changing the potential of the working electrode and monitoring the current which is produced as a function of time or potential. It is probably the most commonly used polarisation testing method for measuring corrosion resistance and is used for a wide variety of functions (Van, 1998; Khaled, 2010a, 2010b).
9
1.3.5 Linear polarisation resistance (LPR)
The polarisation resistance of a material is defined as the slope of the potential-current density (ΔEcorr/Δicorr) curve at the free corrosion potential, yielding the polarisation resistance Rp that can be related (for reactions under activation control) to the corrosion current by the Stern-Geary equation (ASTM, 2001; Dean, 2003) .
1.3.6 Electochemical noise
The non-intrusive use of electrochemical noise (EN) for corrosion monitoring is very attractive; examples are found in aircraft corrosion and gas scrubbing tower monitoring. Fluctuations of potential or current of a corroding metallic specimen are a well known and easily observable phenomenon. The extensive development in the sensitivity of the equipment for studying electrochemical systems has rendered the study of oscillations in electrochemical processes, that translate into measurable electrochemical noise, EN, increasingly accesible. No other technique, electrochemical or otherwise is remotely as sensitive as EN to system changes and upsets (Sastri, 2011) .
1.3.7 Electrochemical impedance spectroscopy (EIS)
Impedance spectroscopy is also called AC impedance or just impedance spectroscopy. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. A small amplitude signal, usually a voltage between 5 to 50mV, is applied to a specimen over a range of frequencies of 0.001 Hz to 100,000Hz. The EIS instrument records the real and imaginary components of the impedance response of the system. Depending upon the shape of the EIS spectrum, a circuit model or circuit description code and initial circuit parameters are assumed and analysed (Oguzie et al., 2012a, 2012b).
10
An important advantage of EIS over other laboratory techniques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. It is a non-destructive technique and so can provide time dependent information about the properties but also about ongoing processes such as corrosion. It is however, expensive and complex data analysis is required for quantification (NACE, 1999; Sastri, 2011).
1.3.8 Galvanic/potential monitoring
The galvanic monitoring technique, also known as Zero Resistance Ammetry (ZRA) is another electrochemical measuring technique. With ZRA probes, two electrodes of dissimilar metals are exposed to the process fluid. When immersed in solution, a natural voltage (potential) difference exits between the electrodes. The current generated due to this potential difference relates to the rate of corrosion occurring on the more active of the electrode couple. Galvanic/potential monitoring is applicable to the following:
i. Bimetallic corrosion
ii. Crevice and pitting attack
iii. Corrosion assisted cracking
iv. Corrosion by highly oxidizing species
v. Weld decay
Galvanic current measurement has found widest applications in water injection systems where dissolved oxygen concentrations are a primary concern. Oxygen leaking into such systems greatly increases galvanic currents and thus the rate of corrosion of steel process components. Galvanic monitoring systems are used to provide an indication that oxygen may be invading injection waters through leaking gaskets or deaeration systems.
11
In any corrosion monitoring system, it is common to find two or more techniques combined to provide a wide base for data gathering. The exact techniques which can be used depend on the actual process fluid, alloy system, and operating parameters. Corrosion monitoring offers an answer to the question of whether more corrosion is occurring today compared to yesterday. Using this information, it is possible to identify the cause of corrosion and quantify its effect. Corrosion monitoring remains a valuable weapon in the fight against corrosion, thereby providing substantial economic benefit to the user (ASTM, 2001) .
1.4 Common Methods of Corrosion Prevention
In most industrial situations, it is virtually impossible to prevent corrosion. The general strategy is to use measures that reduce the corrosion rate to an economically sustainable level. The most important corrosion mitigation procedures are as follows (Sastri, 1998; 2011):
(i). Selection of materials and design against corrosion
(ii). Cathodic protection
(iii) Protective coatings
(iv). Chang of the environment
(v). Addition of inhibitors
1.4.1 Selection of materials and design against corrosion
Materials for a particular working environment (composition, temperature, velocity) are selected taking into account mechanical and physical properties, availability, method of fabrication and overall cost of component or structure. Geometrical configurations that facilitate corrosive conditions should be avoided. These include the following:
a. Features that trap dust, air and water
12
b. Designs with inaccessible areas that cannot be re-protected, e.g., by maintenance painting
c. Designs that lead to heterogeneity in the metal or in the environment
Also, metal-metal or metal-non metallic contacting materials that facilitate corrosion such as bimetallic couples, a metal in contact with absorbent materials that maintain constantly wet conditions and contact with substances that give off corrosive vapours, should be avoided.
1.4.2 Cathodic protection
Metals can be protected cathodically by making the interfacial (metal/solution) potential sufficiently negative by means of either sacrificial anode or impressed current or by making the interfacial potential sufficiently positive to cause passivation (formation of a protective film on the metal). This method is used for metals that passivate in the corrodent under consideration.
1.4.3 Protective coatings
Ideally, a protective coating should provide a complete barrier and exclude the corrosive environment from having contact with the metal which it is designed to protect. This can be achieved by the following techniques:
a. Using inorganic coatings , e.g., vitreous enamel, glasses, ceramics
b. Application of organic coatings, e.g., paints, plastics, greases.
c. Generating metallic coatings that form protective barriers (Ni, Cr) or protect the substrate by sacrificial action (Zn, Al, Cd on steel).
1.4.4 Environment modification and addition of inhibitors
For aqueous corrosion, the environment can be made less agressive by removing constituents or modifying conditions that facilitate corrosion: decrease temperature,
13
decrease velocity, prevent access of water and moisture, remove dissolved O2 , increase pH (for steel) while for atmospheric corrosion, the air is dehumidified and solid particles removed (Roberge, 1999).
Where these methods are not applicable, then chemicals may be added to the environment to interfere with the corrosion process, usually by forming a film of some kind. These chemicals called corrosion inhibitors are substances which, when added in small quantities to a normally corrosive environment, reduce the corrosion rate of the metal, without significantly changing the concentration of corrosive species (Umoren et al., 2009; Eddy et al., 2010; Akalezi et al., 2012).
1.5 Inhibitors
Inhibitors are chemicals that react with a metallic surface giving the surface a certain level of protection. Inhibitors often work by being adsorbed on the metallic surface, protecting the metallic surface by forming a film (Sastri, 2011). Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes as follows:
I. Increasing the anodic or cathodic polarization behaviour (Tafel slopes)
II. Reducing the movement or diffusion of ions to the metallic surface
III. Increasing the electrical resistance of the metallic surface
1.5.1 Classification of inhibitors
Inhibitors have been classified differently by various authors. Some authors prefer to group inhibitors by their chemical functionality as follows (Jones, 1988):
a) Inorganic inhibitors. These are usually crystalline salts such as sodium chromate, phosphate, or molybdate. Only the anions of these compounds are involved in reducing metal corrosion. When zinc is used instead of sodium, the
14
zinc cation can add some beneficial effect. These zinc-added compounds are called mixed-charge inhibitors.
b) Organic anionic.Sodium sulfonates, phosphonates, or mercaptobenzotriazole (MBT) are used commonly in cooling waters and anti-freeze solutions.
c) Organic cationic.In their concentrated forms, these are either liquids or wax-like solids. Their active portions are generally large aliphatic or aromatic compounds with positively charged amine groups.
However, by far the most popular organization scheme consists of regrouping corrosion inhibitors as follows (Hackerman and Snaveley, 1984):
(I). Passivating (anodic) inhibitors
Passivating inhibitors cause a large anodic shift of the corrosion potential, forcing the metallic surface into the passivation range (Enenebeaku, 2011). There are two types of passivating inhibitors viz: (a) oxidizing anions, such as chromate, nitrite, and nitrate, that can passivate steel in the absence of oxygen and (b) the nonoxidizing ions, such as phosphate, tungstate, and molybdate, that require the presence of oxygen to passivate steel. These inhibitors are the most effective and consequently the most widely used (Thomas, 1994).
Chromate-based inhibitors are the least-expensive inhibitors and were used until recently in a variety of applications (e.g., recirculation-cooling systems of internal combustion engines, rectifiers, refrigeration units, and cooling towers). Sodium chromate, typically in concentrations of 0.04 to 0.1%, was used for these applications. At higher temperatures or in fresh water with chloride concentrations above 10 ppm higher concentrations are required. If necessary,
15
sodium hydroxide is added to adjust the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that periodic colorimetric analysis be conducted to prevent this from occurring (Roberge, 1999).
In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason it is essential to monitor the inhibitor concentration.
(II). Cathodic inhibitors
Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas (Oguzie et al., 2012a). Cathodic inhibitors can provide inhibition by three different mechanisms viz: (a) as cathodic poisons, (b) as cathodic precipitates, and (c) as oxygen scavengers. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult.
Other cathodic inhibitors such as calcium, zinc, or magnesium ions may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulphite (Na2SO3).
(III). Organic inhibitors
Both anodic and cathodic effects are sometimes observed in the presence of organic inhibitors, but as a general rule, organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration. Organic
16
inhibitors, usually designated as film-forming, protect the metal by forming a hydrophobic film on the metal surface.Their effectiveness depends on their chemical composition, molecular structure, and affinities for the metal surface. Since film formation is an adsorption process, the temperature and pressure in the system are important factors. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface. Cationic inhibitors, such as amines, or anionic inhibitors, such as sulphonates, will be adsorbed preferentially depending on whether the metal is charged negatively or positively. The strength of the adsorption bond is the dominant factor for soluble organic inhibitors. These materials build up a protective film of adsorbed molecules on the metal surface, which provides a barrier to the dissolution of the metal in the electrolyte. Because the metal surface covered is proportional to the inhibitor concentrations, the concentration of the inhibitor in the medium is critical (Enenebeaku, 2011; Olasehinde et al., 2012; Adejo et al., 2012).
For any specific inhibitor in any given medium there is an optimal concentration. For example, a concentration of 0.05% sodium benzoate or 0.2% sodium cinnamate is effective in water at a pH of 7.5 containing either 17 ppm sodium chloride or 0.5% by weight of ethyl octanol. The corrosion due to ethylene glycol cooling water systems can be controlled by the use of ethanolamine as an inhibitor (Mercer, 1994; Roberge, 1999).
(IV). Precipitation inhibitors
Precipitation-inducing inhibitors are film-forming compounds that have a general action over the metal surface, blocking both anodic and cathodic sites indirectly. Precipitation inhibitors are compounds that cause the formation of
17
precipitates on the surface of the metal, therebyproviding a protective film. Hard water that is high in calcium and magnesium is less corrosive than soft water because of the tendency of the salts in the hard water to precipitate on the surface of the metal and form a protective film.The most common inhibitors of this category are the silicates and the phosphates. Sodium silicate, for example, is used in many domestic water softeners to prevent the occurrence of brownish water. In aerated hotwater systems, sodium silicate protects steel, copper, and brass. However, protection is not always reliable and depends heavily on pH and a saturation index that depends on water composition and temperature. Phosphates also require oxygen for effective inhibition. Silicates and phosphates do not afford the degree of protection provided by chromates and nitrites; however, they are very useful in situations where non toxic additives are required (Mercer, 1994; Roberge, 1999; Sastri, 2011).
(V). Volatile corrosion inhibitors
Volatile corrosion inhibitors (VCIs), also called vapour phase inhibitors(VPIs), are compounds transported in a closed environment to the site of corrosion by volatilization from a source. In boilers, volatile basic compounds, such as morpholine or hydrazine, are transported with steam to prevent corrosion in condenser tubes by neutralizing acidic carbon dioxide or by shifting surface pH toward less acidic and corrosive values. In closed vapour spaces, such as shipping containers, volatile solids such as salts of dicyclohexylamine, cyclohexylamine,and hexamethylene-amine are used. On contact with the metal surface,the vapour of these salts condenses and is hydrolyzed by any moistureto liberate protective ions. It is desirable, for an efficient VCI, to provide inhibition rapidly and to last for long periods. Both qualities depend on the volatility of
18
these compounds and fast action requiring high volatility, whereas enduring
protection requires low volatility (Miksic, 1993; Fiaud, 1994; Roberge, 1999).
1.6 Adsorption Isotherms
Adsorption isotherms are very important in understanding the mechanism of inhibition
of corrosion of metals and alloys. The most frequently used adsorption isotherms are
Langmuir, Freundlich, Temkin, Flory-Huggins and Frumkin isotherms. All these
isotherms can be represented as follows (Oguzie et al., 2012b):
f x kC 2a , exp
1.1
where f(θ, x) is the configuration factor which depends upon the physical model and the
assumptions underlying the derivation of the isotherm, θ is the degree of surface
coverage, C is the inhibitor concentration in the electrolyte, x is the size ratio, a is the
molecular interaction parameter and k is the equilibrium constant of the adsorption
process (Ebenso et al., 2008; Oguzie et al., 2012b).
The heat of adsorption (Qads) of the inhibitor on the surface of the metal can be
calculated using equation 1.2
1
2 1
1 2
1
1
2
2
1
log
1
2.303 log kJmol
T T
T T
Q R ads 1.2
where θ1 and θ2 are the degrees of surface coverage at the temperatures T1 and T2
respectively (Ogoko et al., 2009). At constant pressure, the values of Qads approximate
enthalpy of adsorption (ΔHads) .
19
1.6.1 Langmuir adsorption isotherm
The Langmuir adsorption isotherm assumes monolayer adsorption onto a surface containing a finite number of identical sites and absence of lateral interactions between the adsorbed species. It can be written as follows
where k is the adsorption equilibrium constant and θ is the degree of surface coverage of the inhibitor. By plotting values of C/θ versus values of C, straight line graphs are obtained (Eddy and Ebenso, 2010; Akalezi et al., 2012).
1.6.2 Freundlich isotherm
Freundlich suggested an empirical equation which describes adsorption on heterogenous surfaces. His isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). Freundlich equation is presented as equation 1.5,
where θ = surface coverage.
C = concentration of inhibitor in solution, M.
k = adsorption equilibrium constant.
n = Freundlich isotherm constant ( with 0 < n < 1)
1.6.3 Temkin adsorption isotherm
According to Temkin adsorption isotherm, the degree of surface coverage (θ) is related to the inhibitor’s concentration (C) in the bulk electrolyte according to equation 1.6:
20
where k is the equilibrium constant of adsorption, θ and ‘α’ is the molecular interaction
parameter. Rearranging and taking logarithm of both sides gives equations 1.7 and 1.8
2
2.303 log
2
– 2.303logk C
1.7
or 2 2.303 log k logC 1.8
a plot of θ versus log C gives a linear plot provided the assumptions of Temkin isotherm
are valid (Eddy and Ebenso, 2010; Adejo et al., 2012).
1.6.4 Flory-Huggins adsorption isotherm
The assumptions of Flory-Huggins adsorption isotherm can be expressed as equation
1.9:
log log k x log 1
C
1.9
where ‘x’ is the number of inhibitor molecules occupying one site (or the number of
water molecules replaced by one molecule of the inhibitor). A plot of log(θ/C) versus
log(1- θ) is linear confirming the application of Flory-Huggins (Eddy and Ita, 2010).
1.6.5 El-Awady et al. kinetic-thermodynamic adsorption isotherm
The kinetic-thermodynamic model can be expressed as equation 1.10:
where K is a constant which is related to the adsorption equilibrium constant, k
expressed as equation 1.11:
21
where y is the number of the inhibitor molecules occupying one active site and 1/y = x which is the number of active sites of the surface occupied by one molecule of the inhibitor. It has been found that values of x greater than unity indicate that a given inhibitor molecule will occupy more than one active site. Also, values of y > 1 imply the formation of multilayers of inhibitor on the surface of the metal, while y < 1 indicates that a given inhibitor molecule will occupy more than one active site (Noor, 2009; Li et al., 2010; Adejo et al., 2012).
1.6.6 Frumkin adsorption isotherm
The assumptions of Frumkin isotherm can be expressed as equation 1.12:
where k is the adsorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in the adsorbed layer. When α is positive, it indicates the attractive behaviour of the surface of the metal (Eddy and Odiongenyi, 2010).
The equilibrium constant of adsorption k, of an inhibitor on the surface of a metal is related to the free energy of adsorption ΔGads°as according to equation 1.13:
where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in M.
Generally, ΔGads values with magnitude much less than 40 kJ mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged
22
metal surface (physisorption), whilst those of magnitude in the order of 40 kJ mol-1 and above are associated with charge sharing or transfer from the organic molecules to the metal surface (chemisorption) (Popova et al., 2003; Eddy and Ita, 2010; Oguzie et al., 2012b).
1.7 Purines
A purine is a heterocyclic aromatic organic compound, consisting of a pyrimidine ring fused to an imidazole ring. They are kinds of nitrogen-containing bases (nucleotides) which form the building blocks of nucleic acids. Purines, including substituted purines and their tautomers, are the most widely distributed kind of nitrogen-containing heterocycles in nature. The following purines have been chosen for the present study:
i. Adenine
ii. Guanine
iii. Hypoxanthine
iv. Xanthine
Their chemical structures are presented in Figure 1.1.
1.7.1 Adenine
In older literatures, adenine is called vitamin B4 . It is no longer considered a true vitamin or part of the Vitamin B complex. However, two B vitamins, niacin and riboflavin, bind with adenine to form the essential cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), respectively. Numerous references to its use occur in biochemical literature (Parker et al., 2010). Adenine has been tested for use in cell cultures. Natural sources of adenine include raw unadulterated honey, bee pollen, royal jelly, propolis, most fresh vegetables and fruits. It is believed that all complex carbohydrates contain varying amounts of adenine.
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1.7.2 Guanine
Guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with conjugated double bonds. Guanine is found in integumentary system of many fish such as sturgeon. It is also present in the reflective deposits of the eyes of deep-sea fish and some reptiles such as crocodiles (Fox, 1979; Wilson et al., 1982). In the cosmetics industry, crystalline guanine is used as an additive to various products (e.g., shampoos), where it provides a pearly iridescent effect. It is also used in metallic paints and simulated pearls and plastics. It provides shimmering luster to eye shadow and nail polish.
1.7.3 Hypoxanthine
Hypoxanthine is a naturally occurring purine derivative. It is occasionally found as a constituent of nucleic acids, where it is present in the anticodon of tRNA in the form of its nucleoside inosine. Hypoxanthine is a necessary additive in certain cell, bacteria, and parasite cultures as a substrate and nitrogen source (WWARN, 2012).
1.7.4 Xanthine
Xanthine is a purinebase found in most human body tissues and fluids and in other organisms. A number of stimulants are derived from xanthine, including caffeine and theobromine (Spiller, 1998).
1.8 Statement of the Problem
Due to strict environmental regulations, the continued usage of non environmentally friendly chemical compounds as corrosion inhibitors has faced relentless condemnation. Consequently, large numbers of organic compounds, principally those containing heteroatoms like oxygen, nitrogen or sulphur groups in conjugated systems are being investigated as corrosion inhibitors for the corrosion of different metals in various aggressive media.
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Although some plants extracts have been found to be useful as eco-friendly inhibitors (Oguzie, 2005; Eddy et al., 2009), the actual constituents of the extracts that are responsible for inhibition have been difficult to determine making it difficult to elucidate the specific mechanism for corrosion inhibition. Hence, the challenge for search of corrosion inhibitors, whose actual chemical structures and eco-friendliness have been established are on the increase. Purines are organic compounds with hetero atoms like O and N in their aromatic rings. They are non toxic and can therefore be used as eco-friendly inhibitors against the corrosion of metals in various aggressive media.
1.9 Justification for the Choice of Purines as Corrosion Inhibitors
Organic compounds containing C, N , S and or O in a conjugated system are known to be effective corrosion inhibitors (Eddy, 2008). Purines have hetero atoms like O and N in their aromatic rings. Therefore, they are expected to be good corrosion inhibitors. They are relatively cheap and commercially available. They are non- toxic and can therefore compete with eco-friendly inhibitors. The molecular and electronic structures of the selected purine derivatives have close similarities with those of conventional organic inhibitor molecules hence, they can be investigated as corrosion inhibitors.
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(a) Adenine (b) Guanine
(c) Hypoxanthine (d) Xanthine
Figure 1.1 Chemical structures of (a) Adenine (b) Guanine (c) Hypoxanthine and
(d) Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
Adenine
Guanine
Hypoxanthine
Xanthine
Guanine
Hypoxanthine
Xanthine
2-D structure 3-D structure
Adenine
Guanine
Hypoxanthine
Xanthine
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1.10 Aims of the Research
This research aims at investigating some selected purines as eco-friendly inhibitors for the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 (a low acid concentration) at 303 and 333 K respectively.
1.11 Objectivesof the Research
The objectives of the research are as follows:
a. To carry out a comparative study of the effect of adenine (AD), guanine (GU), hypoxanthine (HYP) and xanthine (XN) on the corrosion of mild steel and aluminium in 0.1 M HCl, H2SO4 and H3PO4 using gravimetric technique at 303 and 333 K respectively.
b. To investigate the adsorptive properties, thermodynamics and kinetic parameters of the purines from weight loss measurements.
c. To establish the effect of each purine derivative at 303 K on the current density and corrosion potentials of mild steel and aluminium in HCl, H2SO4 and H3PO4 at 303 K, using potentiodynamic polarisation measurements.
d. To evaluate the interaction of each purine derivative with the mild steel and aluminium surfaces in HCl, H2SO4 and H3PO4 at 303 K, by electrochemical impedance spectroscopy.
e. To investigate the synergistic effects of iodide ions ( using [KI]= 0.005 M) on the adsorptive behaviour of the selected purines on mild steel and aluminium in the different acid media at 303 K.
f. To carry out quantum chemical calculations in order to get useful theoretical information about the selected purines. Molecular dynamics simulations will be
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employed to understand the interactions of the inhibitors with the Fe (1 1 0) and Al (1 1 0) surfaces.
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