ABSTRACT
Inhibition and adsorption potentials of cysteine, glycine, leucine and alanine have been
investigated using experimental and quantum chemical approaches. The experimental
study was carried out using gravimetric, gasometric, thermometric and FTIR methods of
monitoring corrosion while the quantum chemical study was carried out using semiempirical
and ab-initio methods. The results obtained reveal that various concentrations
of the amino acids inhibited the corrosion of mild steel in solutions of HCl through the
mechanism of physiosorption with activation energy less than 80kJmol-1. The inhibition
potentials of the inhibitors decreased in the order, cysteine >leucine> alanine > glycine
and their adsorption was found to be exothermic, spontaneous and fitted the Langmuir
adsorption model. Computational chemistry results obtained revealed excellent
correlations between quantum chemical parameters (calculated for gas and aqueous
phases PM6, PM3, AM1, RM1 and MNDO Hamiltonians) and experimental inhibition
efficiencies. Correlations between the experimental and theoretical inhibition efficiencies
were also excellent. Application of condensed Fukui and softness functions as well as
relative nucleophilicity/electrophilicity (calculated for B3LYP(6-31G) and MP2(STO-
5G) paturbations) showed that the sites for electrophilic attack are on the amine bonds
(N2-C3) of the amino acids while the sites for nucleophilic attack are on the C-3 atoms of
the amino acids. The HOMO and LUMO diagrams of the amino acids as well as the
electron density diagrams have also been used to support the information obtained from
condensed Fukui function.
TABLE OF CONTENTS
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Abstract vi
Table of content viii
List of tables xii
List of figures xiii
List of appendices xvi
List of abbreviation xxii
CHAPTER ONE
1.1 Background of the study 1
1.2 Different types of Corrosion in metals 4
1.2.1 Uniform Corrosion 4
1.2.2 Galvanic Corrosion 4
1.2.3 Pitting Corrosion 5
1.2.4 Crevice Corrosion 6
1.2.5 Intergranular Corrosion 6
1.2.6 Erosion Corrosion 6
1.2.7 Cavitations Corrosion 6
1.2.8 Interfilm Corrosion 7
8
1.2.9 Fretting Corrosion 7
1.3 Environmental Factor affecting the Corrosion of Iron 7
1.4. Inhibition of Corrosion 11
1.4.1 Interphase Inhibitors 12
1.4.2 Passivating/Oxidizing Inhibitors 12
1.4.3 Non Oxidizing Inhibitors 13
1.4.4 Interface Inhibition 13
1.5 Statement of the problem 14
1.6 Aims and objectives of the Study 15
CHAPTER TWO
2.0 Literature review 19
2.1 Green Corrosion Inhibitors 19
2.1.1Plant extractsas green inhibitor 19
2.1.2 Amino acids as green corrosion inhibitor 24
2.2QSAR in corrosion study 29
2.3 Quantum chemical methods in corrosion inhibiton s’ studies 30
2.4Quantum chemical parameters 33
2.4.1Atomic charges 34
2.4.2Molecular orbital energies 34
2.4.3Dipole moment (l) 35
2.4.4Energy 35
2.5 Semi-empirical methods 36
2.5.1 MNDO (modified neglect of differential overlap) 37
9
2.5.2 AM1 (Austin model 1) 37
2.5.3 PM3 (parameterized model number 3) 38
2.6 Study of corrosion inhibitors by semi-empirical methods 38
2.7 Ab initio and density functional theory (DFT) method 46
2.8 Corrosion inhibitors studied by ab initio and DFT methods 47
CHAPTER THREE
3.0 Materials and Method 60
3.1 Material 60
3.2 Experimental Techniques 60
3.2.1 Gravimetric method 60
3.2.2 Thermometric method 61
3.2.3 Gasometric method 63
3.3 Quantum chemical calculation 65
3.4 Infra red analysis 66
CHAPTER FOUR
4.0 RESULTS AND DISCUSSIONS 67
4.1 Experimental 67
4.1.1 Effect of concentration of amino acids on the corrosion of mild steel 67
4.1.2 Gasometric and thermometric studies 73
4.1.3 Kinetic study 73
4.1.4 Effect of temperature 78
4.1.5 Thermodynamics/adsorption study 80
4.1.6 IR study 84
10
4.2 Quantum chemical study 94
4.2.1 Semi-empirical study 94
4.2.2 Density functional theory (DFT) 108
4.2.2.1 Local selectivity 114
4.2.2.2Quantitative structure activity relation (QSAR) study 126
4.2.2.3Mechanism of inhibition 140
CHAPTER FIVE
5.0 Conclusion and recommendation 143
REFRENCES 144
11
CHAPTER ONE
INTRODUCTION
1.1 Background of the Study
The great expanded industrial applications of mild steel based on their properties
have been recognized by many researchers (Eddy, 2008). However, despite its
importance, the problem associated with corrosion, especially pitting corrosion still
remains a cause of concern to all and sundry. Huge losses of natural and artificial
resources have been noted annually all over the globe as a result of corrosion. In the
petroleum and gas industries, more than half of the reported failures of pipelines are
caused by corrosion and subsequent rupture of the pipe wall. For, example, between 2002
and 2004, the Nigerian National Petroleum Corporation (NNPC) reported 162 cases of
failure due to corrosion (Ajayi, 2003; Eddy, 2008) resulting in large losses of products,
environmental pollution and ecological disaster.
Corrosion of metals has been a major industrial problem that has attracted much
attention and investigations (Abiola et al., 2007; Arora et al., 2007; Ashassi-Sorkhabi et
al., 2005a,2004;Barouni et al., 2008;Clark and Varney, 1987; Eddy, 2009a Kumar, 2008;
Muralidharan et al., 2000; Umoren and Ebenso, 2007,2008; Umoren et al., 2008a,b,
2006a,b). Corrosion is one of the major only means through which metals deteriorate.
Most metals corrode when they are in contact with aggressive medium such as moisture
in the air, either in acids, bases, salts, oils, aggressive metals polishes, and liquid
chemicals. Metals will also corrode when they are exposed to gaseous materials such as
formaldehyde, acid vapour, and sulphur containing gases.
18
Corrosion is the degradation of materials due to interactions with their
environments. It is an electrochemical process which involves the transfer of electrons
between a metal surface and an aqueous solution. When metal atoms are exposed to an
aggressive medium such as water, they can give up electrons, becoming themselves
positively charged ions provided an electrical circuit can be completed. One of the most
familiar corrosion processes is the oxidation of iron (rusting). Iron oxidizes in the
presence of oxygen and aqueous electrolyte solution. Physical strains (scratches, dents,
etc) on the iron are more easily oxidized than other areas. The result is that these regions
are anodic (oxidation occurs) and simultaneously different areas are cathodic regions at
which a reduction occurs. When this occurs, iron atom gives up two electrons to form
Fe2+:
1.1
The electrons released during the abovereaction (equation 1.1) flow through the iron
metal to the cathodic region leading to a reduction which can be expressed as follows
O2 + 2H2 O(l) ¾¾® 4OH- (aq) 1.2
Combination of equation 1.1 and 1.2 gives equation 1.3:
Fe(s) + 1/2O2 (g) + H2 O(l) ¾¾® Fe2+ + 2OH- (aq) 1.3
Common experiences with this process (e.g., car fenders) reveal that Fe2+ is
eventually oxidized further to Fe3+ , in iron (111) oxide compounds (rust) as shown in
equation 1.4 ;
Fe2+ + O2 + 4H2 O(l) ¾¾® 2Fe2 O3(s) + 8H+ (aq) 1.4
All corrosion processes exhibit some common features for example,
thermodynamic principles can be applied to determine which processes can occur and the
19
tendency for the changes to take place (Cardoso et al., 2007; Eddy and Odoemelam,
2008a;El Ashry et al., 2006a,b).
Various studies have been carried out on the inhibition of the corrosion of metals
in different environments and most of the well known inhibitors suitable for the corrosion
of mild steel in acidic medium are organic compounds having heteroatoms in their
aromatic rings or long carbon chains.(Acharya and Upadhyay,2004; Attar and
Scantlebury, 2005; Ebenso et al., 2004a,b; Elayyoubi et al., 2004; Ita, 2004a,b, 2005; Ita
and Offiong, 1997; Khavasfar and Iran, 2006; Okafor, 2004; Rajendran et al., 2000;
Zuchi et al., 1978). For these compounds, their adsorption on the metal is the initial step
of inhibition (Bilgic and Caliskan, 2001; El Ashry et al., 2006a,b; Fang and Li, 2002;
Francis and Mercer, 1990; Li et al., 1999 a,b; Lukovitis et al., 2003, 1998; Quan et al.,
2002; Yurt et al., 2004,2005; Zhenlon et al.,2004). The adsorption of inhibitors is linked
to the presence of heteroatoms such as N, O, P, and S and long carbon chain as well as
triple bonds or aromatic rings in their molecular structure (Ebenso, 2004, 2003a, b;
Ebenso et al., 2005, Emregul et al., 2003; Emregul and Hayvali, 2006; Ehteshanmzade et
al., 2006; Li et al., 1999a, b; Solmaz et al., 2005; Umoren et al., 2006a,b).
Also studies on the use of green corrosion inhibitors have been intensified. Green
corrosion inhibitors are biodegradeable and do not contain heavy metals or other toxic
compounds (Eddy and Ebenso, 2008). The successful use of naturally occurring
substances to inhibit the corrosion of metals in acidic and alkaline environment has been
reported by some group of researchers (Abiola et al., 2007; Abdallah, 2004a,b; Bendahou
et al., 2006).
20
The adsorption bond strength is dependent on the composition of the metal and that
of the corrodent, inhibitor structure and concentration as well as temperature. Despite the
broad spectrum of organic compounds, the final choice of appropriate inhibitors for a
particular application is restricted by several factors which include: toxicity, cost, ease of
availability etc.
1.2 Different Types of Corrosion of Metals
Based on the morphology of attack and the type of the environment the metal is
exposed to, corrosion can be classified into the following types.
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 and as such the thicker the steel coating, the longer
the service life of the metal. Uniform attack is the most common form of corrosion and
causes the greatest destruction of metals on a weight loss basis (Moore, 1996).
Uniform corrosion is often expressed in terms of depth of penetration and the
units often quoted in literature are ipy (inches of penetration per year), ipm (inches of
penetration per month), mg per dm-3 per day and mmyr-1 (millimetres penetration per
year). However, the SI unit is mmyr-1 and the conversion factors are as follow,
mdd = 10 X gm-2d-1 1.5
mmy-1 = 36.52 X mdd X 1/ 1.6
mdd = X ipy X 1/1.44 1.7
where is the density of the metal (in kgm-3)
21
1.2.2 Galvanic corrosion
Galvanic corrosion is a localized corrosion in which metals are be 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.
Therefore a small anode in the presence of a large cathode will corrode more quickly than
the reverse. 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.
1.2.3 Pitting corrosion
This 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;
Moniz, 1986). Pitting may occur as a result of a change in the acidity of the pit area or
differential aeration may also be a contributing factor to the increase in pit 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 anode resulting in a localized attacked.
1.2.4 Crevice corrosion
Crevices are present in all equipments. They occur naturally around bolts, rivets
etc. They are also created by scratches on the metal surface. Crevice corrosion absorbs
22
and draws solution toward the reactive area. Crevice corrosion is influenced by the same
factors affecting pitting corrosion and is indeed a specific form of pitting corrosion.
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 a change in
composition at localized 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 (Moore, 1996). The extent of
erosion corrosion increases as the velocity of the corroding medium increases. 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 (Atkins, 2002).
1.2.7 Cavitations corrosion
Formation and collapse of tiny gas bubbles in a liquid stream called cavitation
may mechanically destroy any protective layer, causing a localized corrosion called
cavitation corrosion (Moore, 1996). Similarly when an object such as propeller rotates in
water, the pressure on the trailing surface of the blade fluctuates continously. At some
23
points, 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
Coating such as paints, conversion coating or metallic coating may lose their
adhesion with substrate due to diffusion through the actual coating or 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). 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.
1.3 Environmental factors affecting the corrosion of iron
Exposure of iron to aerated water at room temperature may make the corrosion
rate to be pH dependent (Okafor, 2004; Evans, 1981). In the range of pH of 4-10, the
corrosion rate of iron is relatively independent of the pH of the environment. In this pH
range, the corrosion rate is governed largely by the rate at which oxygen reacts with
absorbed atomic hydrogen, thereby depolarizing the surface and allowing the reduction
24
reaction to continue. For pH values below 4.0, ferrous oxide (FeO) is soluble. Therefore,
the oxide dissolves as it is formed rather than depositing on the metal surface to form a
film. In the absence of the protective oxide film, the metal surface is in direct contact
with the acid solution, and the corrosion reaction proceeds at a higher rate than at higher
pH values.
Hydrogen is produced in acid solutions below a pH of 4, suggesting that the
corrosion rate no longer depends entirely on depolarization by oxygen, but on a
combination of the two factors (hydrogen evolution and depolarition). For pH values
above 10, the corrosion rate is observed to fall as pH is increased. This is believed to be
due to an increase in the rate of the reaction of oxygen with Fe(OH)3 in the oxide layer
to form the more protective Fe2O3. However, this is not observed in deaerated water at
high temperatures.
Like most other chemical reactions, corrosion rates increase as temperature
increases (Moore, 1996). Temperature and pressure of the medium govern the solubility
of the corrosive species in the fluid, such as oxygen (O2), carbon dioxide (CO2),
chlorides, and hydroxides. A rule of thumb is that the reaction rate doubles for every 100
rise in temperature. This linear increase with temperature does not continue indefinitely
due, in part, to a change in the oxide film.
When iron is exposed to high temperature water, the rate of corrosion of the metal
is observed to decrease with exposure time during the early period of exposure
(Ramachandran et al., 2001). After a few thousand hours, the corrosion rate becomes
relatively constant during the early period of exposure. While the corrosion rate is
decreasing, the oxide film on the surface of the metal grows in thickness. However, the
25
rate at which the film grows tends to decrease with time. The thickness of the oxide film
is soon constant after which film thickness does not change appreciably with further
exposure. As might be expected, relatively constant corrosion rate and oxide film
thicknesses are attained at about the same time.
The velocity of the surrounding fluid (water) may also affect the rate of corrosion
(Okafor, 2004; Soliman and Abdel-Rahman, 2006). When water velocity is extremely
high, the impact of the water tends to remove the protective oxides layer and some of the
metal under it (erosion), thus exposing more metal to corrosion. Water velocities of 30 to
40ft per second are usually said to cause erosion.
An increase in the concentration of oxygen in water to which iron is exposed
increases the corrosion rate. Oxygen promotes corrosion in two ways. First it is a
powerful cathode depolarizer. When an acid gas such as hydrogen sulphide or sulphur
(IV) oxide or carbon (IV) oxide is present, hydrogen gas tends to be formed at the
cathode according to the equation 1.9:
2H+ + 2e- H2 1.9
The associated cathodic reduction is given by equation 1.10:
2H+ + 1/2O2 H2O 1.10
Therefore, cathodic reaction resulting in the attendant corrosion is promoted
because polarization by hydrogen is minimized in the presence of oxygen. Secondly, the
oxygen removes iron by precipitation of iron oxides at the anode and thus prevents
anodic polarization by Fe2+.
The presence of carbon (IV) oxide in the surrounding water can also catalyse the
corrosion of mild steel (Okafor, 2004). According to Kalman et al. (2000), it is evident
26
that the rate of corrosion can be affect by the partial pressure of CO2. Increase in CO2
partial pressure has been observed to increase the concentration of carbonic acid which
is known to the cathodic reaction. Okafor (2004) stated that the CO2 partial pressure
affects the pH of the solution which in turn affects the speciation of species involved in
the corrosion process and that in a film forming condition, increase in CO2 partial
pressure will lead to increase in CO3
2- concentration.
CO2 is a stronger acid than H2S. It combines with hydrogen and trioxocarbonate
ions to form trioxocarbonate (IV) acid according to equation 1. 11:
CO2 + H2O ¾¾® H2CO3 1.11
The acid formed ionizes to hydrogen and trioxocarbonate ions according to equation
1.12 below:
H2CO3¾¾® H+ + HCO3
– 1.12
The ionization constant for the above reaction is K1 = 4.31x 10-7 (Atkins, 2002).
However, the major contribution of CO2 to the corrosion of mild steel is the increase in
acidity by H+ formed in the above reaction and the corrosion product FeCO3(siderite).
The presence of hydrogen sulphide in the surrounding water may also catalyse the
corrosion of mild steel. Hydrogen sulphide is a weak acid but is abundant in oil
producing areas. The acidity or ability of hydrogen sulphide to generate H+ is depicted
by its first ionization constant. Under standard conditions, each mole of hydrogen
sulphide in solution produces only 5.7x 10-8 moles of H+. However, as the H+ is removed
by cathodic reaction, more is formed and hydrogen gas readily appears on steel exposed
to air free water containing H2S. The anion, SH- dissociates further to S2- and H+. The
sulphide anion reacts with iron to form the black FeS corrosion product.
27
Some microorganisms have been found to aid corrosion of mild steel. In an oil
producing environment, the major corrosion caused by bacteria is associated with attack
by hydrogen sulphide that is generated by metabolic processes of certain organism. The
most commonly found sulphide producers are desulfovibrio species such as
desulfovibrio desulfuricanes. They are referred to as sulphate reducers because they
utilize SO4
2- from salty waters and from H2S to a limited extent, sulphide corrosion
results from the hydrogen sulphide generated by species clostridia from organic sulphur
compounds (Eddy, 2008).
1.4 Inhibition of corrosion
According to Hosseini et al. (2007), corrosion can be controlled by the addition of
chemical substances called inhibitors into acid media. By definition, an inhibitor is a
chemical compound which when added in small amounts to the corrosive environment,
alters the cathodic and or anodic reaction and subsequently reduces the corrosion rate.
This implies that inhibitors can be classified as anodic or cathodic depending on the
process (Akpan, 1994). Inhibitors can also be classified as organic or inorganic
depending on their chemical nature, oxidizing or non-oxidizing and their redox
characteristics. However, one of the most widely applied classification systems for
inhibitor is that proposed by Kelly et al. (2003).
• Adsorption inhibitors: These are inhibitors which are adsorbed on the metal
surface and form protective barrier films. Adsorption inhibitors function by
limiting the diffusion of oxygen to the corrosion surface, trapping the metal
ion on the surface, stabilizing the double layer and reducing the rate of
dissolution.
28
• Passivation inhibitors: passivating inhibitors function by inducing and
maintaining a passive film (consisting of metal oxide) on the surface.
• Surface reaction product inhibitors: these are inhibitors that form sparingly
soluble compounds other than an oxide layer. Surface reaction product
inhibitors cannot form a protective oxide layer because they are not oxidizing
agents but interact with metal on the surface to form insoluble compounds
which plug in pores and inhibit corrosion.
1.4.1 Interphase inhibitors
Interphase inhibitors function by facilitating the formation of a 3-D layer which
acts as a barrier between the corroding substrate and the electrolyte. The protective film
could be a solid film on the surface of the metal or a liquid film adjacent to it. The solid
film may be an oxide layer, corrosion product, metallic or non-metallic coating or
inhibitor forming a porous layer or non-porous film (Mercer, 1990; Monika et al., 2005).
However, the liquid film is the electroyte in the interphase which differs from the bulk of
the solution in its chemical or physical properties. Based on this model, the inhibition of
metal corrosion is governed by the properties of the layer such as its porosity and
stability.
1.4.2 Passivating/oxidizing inhibitors
Pourbaix (1981) stated that iron polarizes anodically in the presence of oxidizing
inhibitors and passivates as the open circuit potential shifts to more positive potential. It
is also possible that the inhibitors are reduced and the reduction product is deposited on
the weak points of the films to improve its protective effect.
29
An oxidizing inhibitor functions by inducing and maintaining a passive surface
layer on the metal surface. These inhibitors are effective at certain concentrations (critical
concentration, Ccrit). At concentrations below the critical concentration, corrosion is
enhanced but at very high concentrations of the inhibitor, increase in the anodic current
density is observed.
1.4.3 Non oxidizing inhibitors
Non oxidizing inhibitors are also capable of passivating metals. They include
compounds such as NaOH and the salts of weak acids and strong bases such as Na3PO4,
Na2HPO4, Na2CO3, NaBO3 and Na2B4O7 (Tarvassoli-Salardini, 2004). These substances
are also capable of passivating steel electrochemically.
1.4.4 Interface inhibitions
According to Tavassoli-Salardini (2004), interface inhibition occurs due to a
strong interaction between the corroding metal and the inhibitor. The adsorption of the
inhibitor depends on the potential of the electrode as well as the charge on the adsorbate
molecule. The protective film is a 2-D adsorbate, which can affect the corrosion reaction
by one of the following mechanisms.
i. Geometric blocking: In this case, an inert inhibitor blocks the surface of the metal at a
high degree of coverage and forms a diffusion barrier which restricts the access of
reactants to the metal surface for example, inhibition provided by proteins,
polysaccharides or compounds with hydrocarbon chains.
ii. Deactivation coverage: In this process, instead of the complete coverage of the
metallic surface, only the active sites are covered by an inert adsorbate and the rates of
30
the reactions (cathodic and anodic) are reduced in proportion to the extent of coverage of
the reactive sites.
iii. Reactive coverage: In this case, the adsorbate undergoes electrochemical reaction and
primary or secondary inhibition occurs either by the adsorbate or its reaction product
respectively.
Adsorption inhibition is observed when a bare metal is in contact with the
adsorbate (inhibitor) and this is usually the case in acidic solutions where the condition is
not favourable to oxide formation (Mansfeld, 1987). According to Tarvassolo-Salardini
(2004), majority of the very effective interface inhibitors are organic compounds
containing nitrogen, sulphur and oxygen. It has been found that some interface inhibitors
after initial adsorption onto the metal surface, can be reduced and the reduction product
may also posses some inhibitive properties.
1.5 Statement of the Problem
In spite of the broad spectrum of inhibitors synthesized and used for the inhibition
of the corrosion of metals including mild steel, it has been found that most inhibitors are
toxic thereby posing a problem to the environment. In other to overcome the challenges
and find posed by toxic inhibitors, there is need to investigate eco-friendly corrosion
inhibitors. Amino acids are products of living things. They are non toxic, cheap and can
be readily produced and purified to high quality. Therefore, the present study shall
attempt to explore the possibility of using alanine, glycine, leucine and cysteine for the
inhibition of the corrosion of mild steel in HCl. The chemical structures of these amino
acids are shown in Fig. 1.1. It has been found that the inhibition potentials of any
inhibitor are also affected by electronic and molecular properties of the inhibitor.
31
Therefore, quantum chemical studies shall be used to correlate microscopic and
macroscopic properties of the inhibitors.
1.6 Aims and Objectives of the Study
The aims and objectives of the study are as follows,
(i) To study the inhibition of the corrosion of mild steel in solutions of HCl by
L-cysteine, alanine, glycine and L-leucine using experimental (gravimetric,
gasometric, thermometric and FTIR) and theoretical methods (quantum
chemical methodsand Quantitative structure activity relation). From the
gravimetric, gasometric and thermometric, experiments, the inhibition
efficiencies of the various amino acids used shall be calculated.
(ii) To investigate the effect of temperature and concentration of corrodent and
inhibition efficiencies of the amino acids. The activation energy for the
uninhibited and inhibited corrosion reactions of mild steel shall be calculated
using the Arrhenius equation.
(iii) To study the adsorption characteristics of the inhibitors by fitting adsorption
data into different adsorption isotherms. The heat of adsorption and free
energy of adsorption shall be calculated using the transition state and Gibb
Helmholtz equations respectively.
32
Fig. 1.1: Optimised and chemical structures of L-leucine (LEU), glycine (GLY), Lcysteine
(CYS) and L-alanine (ALA)
Optimised structure Chemical structure
CYS
GLY
LEU
ALA
33
(iv) To examine the kinetics and effect of immersion time on the stability of the
inhibitors. From the kinetic models, the half life of mild steel in HCl solutions
containing various concentrations of the inhibitors shall be calculated.
(v) To carry out quantum chemical calculations for the amino acids using Ultra
Chem 2008, MOPAC. Semi-empirical parameters to be calculated using
MOPAC 2008 shall include the total energy of the molecules (TE), electronic
energy of the molecules (EE), core core repulsion energy (CC), ionization
potentials (IP), electron affinity (EA), dipole moment (μ), electronegativity
(c), chemical potentials (¡), energy of the highest occupied molecular orbital
(EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO), energy
gap (DE), cosmo area (CosAr) and cosmo volume (cosvol). Lowdin and
Muliken charges on the molecules shall be calculated using GAMESS. From
the calculated charges, Fukui and global softness indices (ionization energy,
Fukui functions for electrophilic and nucleophilic attacks, global softness,
global hardness, electronegativity and fraction of electron transfers) for the
various amino acids used shall be calculated. These functions shall be used to
predict the sites for electrophilic and nucleophilic attacks of the inhibitors and
shall also be expected to furnish information needed for the elucidation of the
mechanism of the inhibition process.
(vi) To carry out quantitative structure activity relationship (QSAR) on the various
amino acids studied using the quantum chemical descriptors. The results
obtained from QSAR calculations shall be used to derive equations for the
calculation of theoretical inhibition efficiencies of the amino acids.
34
(vii) From the results obtained from the experimental studies, recommendations
shall be made on the potentials of the various amino acids as inhibitors for the
corrosion of mild steel in HCl. Also from theoretical calculations, the
synthesization of inhibitors that are structurally related to the various amino
acids shall be attempted.
35
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