In this thesis, the ethanol extracts of some plant namely: Phyllanthus amarus (PA); Paullina pinnata (PP); Parinari polyandra (PP1); and Boscia senegalensis (BS) were investigated for their corrosion inhibition potential on mild steel and aluminium in HCl (1.0 M) and H2SO4 (0.5 M) respectively using gravimetric and linear polarization methods. The influence of extract concentrations (0.1g/l to 0.5g/l) and temperatures (303K to 323K) on corrosion and corrosion inhibition was assessed. The results obtained showed that PP, PA, BS and PP1 decreased the corrosion rate of mild steel and aluminium in the acid media. The rate generally decreased with increasing extract concentration in the order: PP> PP1> BS> PA and PP> BS> PP1> PA in HCl and H2SO4 respectively for aluminium; PP> PA> PP1> BS and BS> PP> PP1> PA in HCl and H2SO4 respectively for mild steel at 303 K. The corresponding maximum inhibition efficiencies were 73.55% and 40.00% for aluminium in HCl and H2SO4 respectively and 83.52% and 80.29% for mild steel. Inhibition efficiency in all the systems decreased with rise in temperature, suggesting physical adsorption of the extract constituents on the metal surfaces. Scanning electron microscopy (SEM) and infra-red (FTIR) spectroscopy analyses of the metal surfaces in the presence and absence of the inhibitors confirmed the presence of adsorbed protective layers. Linear polarization studies showed that the plant extracts suppressed both the anodic and cathodic half reactions of the corrosion processes, thereby acting as mixed-type inhibitors. Langmuir isotherm was found to be the best in describing the adsorption behaviour of the extract on the surfaces of mild steel and aluminium at room temperature, whereas the adsorption property at elevated temperature was best described by the Freundlich isotherm. Calculated values of free energy of adsorption, , on mild steel in the presence of PP, PA, BS and PP1 are as follows: -9.93 to -10.31 kJ mol-1 and -8.31 to -10.73 kJ mol-1 in HCl at 303 and 333K respectively, and -9.39 to -10.32 kJ mol-1 and -7.68 to -13.58 kJ mol-1 in H2SO4 at 303 and 333K respectively. The calculated values for the corrosion of aluminium in the presence of PP, PA, BS and PP1 follows the order: -9.01 to -9.53 kJ mol-1 and -7.91 to -9.58 kJ mol-1 in HCl at 303 and 333K respectively. Corrosion activation energy (Ea) values for mild steel and aluminium in the acids solutions increased in the presence of the inhibitor and the values were found to be less than 80kJmol-1 supporting the proposed physiosorption of the extract constituents. Density functional theory (DFT) based molecular dynamics simulations were adopted to theoretically describe the interactions of selected extract constituents with the metal surfaces. The computed binding energy values (Ebinding) for the various constituent indicate the adsorption process to be non-covalent (physiosorption).
TABLE OF CONTENTS
Title page ii Declaration iii Certification iv Dedication v Acknowledgements vi Abstract vii Table of Contents viii List of Figures xiii List of Plates xix List of Tables xx List of Appendices xxii Abbreviations and Symbols xxiv CHAPTER ONE 1.0 INTRODUCTION 1 1.1 Justification of Study 3 1.2 Aim and Objectives 4 CHAPTER TWO 2.0 LITERATURE REVIEW 6 2.1 Corrosion 6 2.2 Forms of Corrosion 8 2.2.1 Uniform corrosion 8
2.2.2 Galvanic corrosion 9
2.2.3 Pitting corrosion 10 2.2.4 Crevice corrosion 11 2.2.5 Intergranular corrosion 11 2.2.6 Erosion corrosion 11 2.2.7 Cavitation corrosion 12 2.2.8 Fretting corrosion 12 2.3 Corrosion Monitoring Techniques 12 2.3.1 Gravimetric technique 13 2.3.2 Gasometric technique 14 2.3.3 Thermometric technique 14 2.3.4 Potentiodynamic polarization techniques 15 2.3.5 Linear polarization resistance 15 2.3.6 Electrochemical noise 15 2.3.7 Electrochemical impedance spectroscopy 16 2.3.8 Galvanic monitoring 16 2.4 Common Methods of Corrosion Prevention 17 2.4.1 Selection of materials and design against corrosion 18 2.4.2 Protective coatings 18 2.4.3 Cathodic protection 19 2.4.4 Anodic protection 19 2.4.5 Chemical inhibition 19 2.5 Inhibition of Corrosion 19 2.5.1 Interphase inhibitors 20
2.5.2 Passivating or oxidizing inhibitors 21
2.5.3 Non-oxidizing inhibitors 21 2.5.4 Interface inhibitions 21 2.6 Adsorption Isotherms 22 2.6.1 Langmuir adsorption isotherm 23 2.6.2 Freundlich isotherm 23 2.6.3 Temkin adsorption isotherm 24 2.6.4 Flory-Huggins adsorption isotherm 24 2.6.5 El-Awady adsorption isotherm 24 2.6.6 Frumkin adsorption isotherm 25 2.7 Corrosion Inhibitors 26 2.8 Green Corrosion Inhibitors 26 2.8.1 Green corrosion inhibitors: Plant extract 27 2.8.2 Green corrosion inhibitors: Amino acids 33 2.8.3 Triazoles and benzotriazoles derivativesas corrosion inhibitors 38 2.8.4 Dyes as corrosion inhibitors 40 2.8.5 Schiff bases as corrosion inhibitors 41 CHAPTER THREE 3.0 EXPERIMENTAL 43 3.1 Materials 43 3.2 Methods 44 3.2.1 Plant extraction 44 3.2.2 Chemical analysis 44 3.2.3 Gravimetric method 45
3.2.4 Fourier transform infrared spectrophotometry 47
3.2.5 Scanning electron microscopy 47 3.2.6 Linear polarisation resistance 48 3.3 Computational and Theoretical Consideration 49 3.3.1 Quantum chemical calculations 49 3.3.2 Molecular dynamics simulation 49 CHAPTER FOUR 4.0 RESULTS 51 4.1 Phytochemical Screening and Gas Chromatography Mass Spectrophotometry of the Plants Extract 51 4.2 Weight loss Measurement of Plant Extracts on the Corrosion of Mild steel and Aluminium 51 4.3 Effect of Temperature 68 4.4 Thermodynamic/adsorption Study 98 4.5 Electrochemical Measurement 98 4.6 Infrared Spectroscopic Study 137 4.7 Scanning Electron Microscopy Study 137 4.8 Computational and Theoretical Modelling 161 4.8.1 Quantum chemical calculations 161 4.8.2 Molecular dynamics 168 CHAPTER FIVE 5.0 DISCUSSION 179 5.1 Gas Chromatography – Mass Spectra Analysis 179 5.2 Corrosion Studies 180 5.2.1 Effect of concentration of extracts on the corrosion of mild steel and aluminium 180
5.2.2 Effect of temperature 183 5.2.3 Thermodynamic or adsorption study 185 5.3 Infrared Spectroscopic Study 188 5.4 Scanning Electron Microscopy Study 192 5.5 Linear Polarization Measurement 193 5.6 Computational and Theoretical Modelling 195 5.6.1 Quantum chemical calculations 195 5.6.2 Molecular dynamics simulations 196 CHAPTER SIX 6.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 198 6.1 Summary 198 6.2 Conclusions 199 6.3 Recommendations 200 REFERENCES 202 APPENDICES 217
1.0 INTRODUCTION Industrial development is vital in the history of any developed country. Most industries use various types of metals including their alloy for the construction or fabrication of their plants and other installations. In most cases, contact between the metal and aggressive medium (such as acid, base and salt) is unavoidable (Eddy et al., 2009a, 2009b). Corrosion is defined as an electrochemical process that tends to returns the metal to its original state (Eddy et al., 2010). In view of the above, industrial facilities exposed to corrosion are often protected against corrosion by adopting several options including painting, oiling, cathodic and anodic protections, etc. However, the use of inhibitors has been found to be one of the best options available for the protection of metals against corrosion (Okafor et al., 2007; Oguzie et al., 2006; and Abdallah, 2004a).
Inhibitors are compounds that tend to retard the rate of corrosion of metals by being on the surface of the metal either through the transfer of charge from charge inhibitor molecule to charged metal surface (physical adsorption) or by electron transfer from the inhibitor‟s molecule to the vacant orbital (mostly d-orbital) of the metal (Eddy et al., 2015). Numerous studies have been carried out on the corrosion of metals in different environments and their inhibition and most of the well-known inhibitors suitable for the inhibition of the corrosion of metals in acidic medium are heterocyclic compounds (Eddy et al., 2015, 2010; Elayyoubi et al., 2004; Ita, 2004a, b; Ita and Offiong, 1997; Khavasfar and Iran, 2006; and Okafor et al., 2007). For these compounds, their adsorption on the metal
surface is the initial step of inhibition (El Ashry et al., 2006a, 2006b). The adsorption of inhibitor is linked to the presence of heteroatoms such as N, O, P, and S and long carbon chain length as well as triple bond or aromatic ring in their molecular structure (Emregul et al., 2006; Umoren et al., 2006). Generally, a strong coordination bond causes higher inhibition efficiency. Most inhibitors are organic compounds whose inhibition potentials can be correlated with their chemical structure. However, some of them are toxic, non-degradable and are not eco-friendly (Ita, 2004a). On the other hand green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds. The successful use of naturally occurring substances to inhibit the corrosion of metals in acidic and alkaline environment have been reported by some research groups (Oguzie et al., 2004, 2007; Umoren et al., 2006 and Okafor et al., 2007). Researchers generally agree that most of these plants are green inhibitors because they are biodegradable, less toxic and do not contain heavy metals (Okafor et al.; 2007). In the light of these, several plants extract have been reported and their corrosion inhibition properties are often attributed to their phytochemical constituents (Umoren and Ebenso, 2008).
The active phytochemicals in plants that are effective for corrosion inhibition can be regarded as those with heteroatom(s) in their aromatic or long chain, possession of π- electrons or suitable groups may also facilitate the transfer of charge from the inhibitors‟ molecule to charge metal surface (physical adsorption) or the transfer of electron from the inhibitors molecule to vacant d-orbital of the metal (chemical adsorption). Therefore in
order to identify the active constituent of plant extract involved in corrosion inhibition, explicit knowledge of the chemical structures of its phytochemicals is required. Therefore, the present study is aimed at elucidating the chemical structures of the ethanol extracts of four (4) plants waste and to investigate their corrosion potentials for mild steel and aluminium in acid media. 1.1 Justification of Study In spite of the broad spectrum of naturally/synthesized inhibitors used for the inhibition of the corrosion of metals including mild steel and aluminium, it has been found that most inhibitors are toxic thereby posing a problem to the environment. In order to overcome the challenges poised by toxic inhibitors, there is need to focus future research on the use of eco-friendly corrosion inhibitors. Plant extracts are products of naturally occurring substance, they are cheap and can be readily produced and are readily available in our immediate environment as waste product which cause environmental pollution. Therefore, the present study shall attempt to explore the possibility of using some plant extracts for the inhibition of the corrosion of mild steel and aluminium in HCl and H2SO4 solution. It has also been found that the inhibition potentials of any inhibitor are also affected by electronic and structural parameters. Therefore, quantum chemical studies shall be considered in finding possible relationship between microscopic and macroscopic behaviour of the inhibitors. However, most corrosion inhibitors are not environmentally friendly, hence this study is designed to investigate the corrosion inhibition potential of some plants extract which are environmentally friendly.
1.2 Aim and Objectives The present study is aimed at investigating corrosion inhibition potentials and mechanisms of inhibition of the ethanol extracts of Phyllanthus amarus (PA); Paullina pinnata (PP); Parinari polyandra (PP1); and Boscia senegalensis (BS) on mild steel and aluminium in HCl and H2SO4 solutions at 303K, 313K, 323K and 333K using experimental and theoretical approaches. The aim of the research shall be achieved through the following objectives:
(i) To perform phytochemical screening on the ethanol extract of the plants and identify structures of the active constituents using gas chromatography-mass spectrometry (GCMS).
(ii) To study corrosion rates of the aluminium and mild steel in uninhibited and inhibited acid (HCl and H2SO4) at various temperatures using gravimetric and Linear polarisation techniques.
(iii) To evaluate the inhibition efficiencies of the extracts for the corrosion of mild steel and aluminium in the different media.
(iv) To investigate the effect of temperature and thermodynamic properties of the inhibitors on the metals in the various acid media
(v) To study the adsorption characteristics of the inhibitors by fitting adsorption data into different adsorption isotherms.
(vi) To examined the surface of the metals and corrosion morphology using scanning electron microscopy (SEM).
(vii) To identify the functional groups present in both the extracts as well as corrosion product using Infrared Spectroscopic (FTIR).
(viii) To obtain the electronic structures of the selected constituents of the extracts by means of quantum chemical computations as well as obtain their stable adsorption structures on the different metal surfaces and the corresponding adsorption energies using molecular dynamics simulations.
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