Green Inhibition of Mild Steel Corrosion in A

GREEN INHIBITION OF MILD STEEL CORROSION IN A

CO2 SATURATED SALINE SOLUTION

Elron Edgar Gomes

A Thesis Presented to the Faculty of the American University of Sharjah College of Engineering in Partial Fulfillment of the Requirements for the Degree of

Master of Science in Chemical Engineering

Sharjah, United Arab Emirates

May 2015

Approval Signatures

We, the undersigned, approve the Master’s Thesis of Elron Edgar Gomes.

Thesis Title: Green Inhibition of Mild Steel Corrosion in a CO2 Saturated Saline Solution Signature Date of signature (dd/mm/yy)

______Dr. Taleb Ibrahim Professor, Department of Chemical Engineering Thesis Advisor

______Dr. Mustafa Khamis Professor, Department of Biology, Chemistry and Environmental Sciences Thesis Committee Member

______Dr. Nabil Abdel Jabbar Professor, Department of Chemical Engineering Thesis Committee Member

______Dr. Mohamed Abou Zour GE Water and Process Technologies Thesis Committee Member

______Dr. Naif Darwish Head, Department of Chemical Engineering

______Dr. Mohamed El-Tarhuni Associate Dean, College of Engineering

______Dr. Leland Blank Dean, College of Engineering

______Dr. Khaled Assaleh Director of Graduate Studies Acknowledgements

I would like to thank and express my sincere appreciation to my advisor Dr. Taleb Ibrahim who was instrumental in helping me throughout all phases of this work as well as providing invaluable guidance and insight during the completion of this thesis. The author is grateful to the Department of Chemical Engineering for offering him the graduate assistantship and funding required to complete this thesis.

I would also like to thank Dr. Mohamed Abou Zour, Dr. Nabil Abdel Jabbar and Dr. Mustafa Khamis for their encouragement and insights throughout the entire work. I would also like to thank Dr. Fathia Mohammed for her insights and moral support, Mr. Ziad Sara and Mr. Thomas Job for their invaluable help with analysis as well as Mr. Ricardo De Jesus for the fabrication of various items needed for this thesis.

Last but definitely not least, I would also like to thank my parents and family for their encouragement and support.

Abstract

Mild steel is extensively used in equipment such as pipelines and machinery by many industries, where it is exposed to corrosive environments. Dissolved carbon dioxide

(CO2) can be found in produced water and results in severe corrosion due to the formation of carbonic acid. CO2 corrosion presents not only an economic loss but also an environmental and safety risk. Most industries use synthetic inhibitors, which are effective in reducing corrosion, but are also toxic and persistent. This has led to stricter regulations and thus, there is a need for alternative inhibitors which can replace them but not exhibit their undesired characteristics. The aqueous extracts of Fig leaves (FLE), Calotropis procera (CPLE) and Eggplant peels (EPPE) were investigated as novel corrosion inhibitors for mild steel in a CO2-saturated 3.5wt% NaCl solution using various electrochemical techniques such as linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and cyclic sweep (CS). The results showed that the corrosion rate is decreasing and inhibition efficiency is increasing as the concentration of inhibitor increased. Corrosion inhibition efficiencies of 90-95% were obtained using low dosage of the green inhibitors. FLE, CPLE and EPPE proved to be effective inhibitors and are compared to the performance of two commercially available inhibitors, A (green inhibitor) and B (synthetic inhibitor). Polarization studies show that FLE, CPLE and EPPE act as mixed inhibitors. The adsorption data was analyzed using various adsorption isotherm models and the results at temperatures of 25, 40, 50 and 70°C have shown that the adsorption behavior of FLE, CPLE and EPPE is best described by the Langmuir adsorption isotherm. The conclusion drawn from this work is that the FLE, CLE and ELE inhibitors proved to be effective inhibitors when compared to Inhibitor A. Inhibitor B proved to be a much more effective inhibitor than the others due to its synergistic mix of active ingredients.

Search terms: corrosion, inhibition, carbon dioxide, salt solution, fig leaves, calotropis procera leaves, eggplant peels

Table of Contents

Abstract ...... 5 List of Figures ...... 11 List of Tables ...... 17 Nomenclature ...... 19 Chapter 1. Introduction ...... 20 Chapter 2. Background ...... 23 2.1 Corrosion...... 23 2.2 Types of inhibitors ...... 24 2.2.1 Anodic corrosion inhibitor...... 26 2.2.2 Cathodic corrosion inhibitors...... 27 2.2.3 Mixed/Adsorption corrosion inhibitors...... 28 2.3 Adsorption isotherms ...... 30 2.3.1 Langmuir isotherm...... 31 2.3.2 Temkin isotherm...... 31 2.3.3 Flory-Huggins isotherm...... 32 2.3.4 Frumkin isotherm...... 32 2.3.5 Gibbs free energy...... 33 Chapter 3. Carbon Dioxide Corrosion ...... 34

3.1 CO2 corrosion mechanism ...... 34

3.2 Parameters affecting CO2 corrosion...... 35

3.2.1 Effect of CO2 partial pressure...... 35 3.2.2 Effect of Temperature...... 37 3.2.3 Effect of Flow Velocity...... 37 3.2.4 Effect of pH...... 38 Chapter 4. Corrosion Inhibitors ...... 39

4.1 Synthetic inhibitors used in CO2 saturated NaCl solutions ...... 40 4.1.1 Imidazoline (IM) based inhibitors...... 42 4.1.2 Carboxylic & naphthenic acids...... 46 4.1.3 Polymeric compounds...... 47

4.2 Green inhibitors used in CO2 saturated NaCl solutions ...... 48

4.2.1 Inhibitors based on plant extracts...... 48 4.2.2 Sulfated fatty acid derivatives...... 49 4.2.2.1 Sulfated fatty acid derived from corn oil...... 49 4.2.2.2 Sulfated fatty acid derived from sunflower oil...... 50 4.2.2.3 Sulfated fatty acid derived from vegetable oils...... 50 4.3 Inhibitors in other corrosive mediums ...... 51 4.3.1 Synthetic inhibitors...... 51 4.3.2 Green inhibitors...... 53 4.4 Investigated inhibitors...... 54 Chapter 5. Experimental Methodology ...... 57 5.1 Preparation ...... 57 5.1.1 Preparation of natural inhibitors...... 57 5.1.2 Preparation of mild steel specimens...... 57 5.1.3 Preparation of test solution...... 57 5.2 Electrochemical tests...... 58 5.2.1 Linear polarization resistance (LPR)...... 58 5.2.2 Electrochemical impedance spectroscopy...... 59 5.2.3 Potentiodynamic sweeps...... 62 5.3 Fourier Transform Infrared Spectrometry...... 64 5.4 Scanning Electron Microscope ...... 65 5.5 Electrochemical Cell Setup ...... 67 Chapter 6. Results and Discussion ...... 70 6.1 Fig leaf extract (FLE)...... 70 6.1.1 Linear polarization resistance results for FLE...... 70 6.1.1.1 Corrosion rate and inhibition efficiency from LPR...... 70 6.1.1.2 LPR parameters...... 72 6.1.2 Electrochemical impedance spectroscopy results for FLE...... 74 6.1.2.1 Corrosion rate from EIS...... 74 6.1.2.2 EIS parameters and plots...... 74 6.1.3 Cyclic sweep results for FLE...... 78 6.1.3.1 Corrosion rate from CS...... 78 6.1.3.2 CS parameters and plots...... 80

6.1.3.3 Difference between test types...... 82 6.1.4 Adsorption isotherms for FLE...... 83 6.1.5 Activation energy of FLE...... 87 6.2 Calotropis procera extract (CPLE)...... 89 6.2.1 Linear polarization resistance results for CPLE...... 89 6.2.1.1 Corrosion rate from LPR...... 89 6.2.1.2 LPR parameters...... 90 6.2.2 Electrochemical impedance spectroscopy results for CPLE...... 92 6.2.2.1 Corrosion rate from EIS...... 92 6.2.2.2 EIS parameters and plots...... 93 6.2.3 Cyclic sweep results for CPLE...... 97 6.2.3.1 Corrosion rate from CS...... 97 6.2.3.2 CS parameters and plots...... 98 6.2.3.3 Difference between test types...... 100 6.2.4 Adsorption isotherms for CPLE...... 101 6.2.5 Activation energy of CPLE...... 106 6.3 Eggplant peel extract (EPPE)...... 107 6.3.1 Linear polarization resistance results for EPPE...... 107 6.3.1.1 Corrosion rate from LPR...... 108 6.3.1.2 LPR parameters...... 109 6.3.2 Electrochemical impedance spectroscopy results for EPPE...... 110 6.3.2.1 Corrosion rate from EIS...... 111 6.3.2.2 EIS parameters and plots...... 112 6.3.3 Cyclic sweep results for EPPE...... 115 6.3.3.1 Corrosion rate from CS...... 115 6.3.3.2 CS parameters and plots...... 117 6.3.3.3 Difference between test types...... 119 6.3.4 Adsorption isotherms for EPPE...... 119 6.3.5 Activation energy of EPPE...... 124 6.4 Commercial inhibitor A ...... 126 6.4.1 Linear polarization resistance results for Inhibitor A...... 126 6.4.1.1 Corrosion rate from LPR...... 126

6.4.1.2 LPR parameters...... 127 6.4.2 Electrochemical impedance spectroscopy results for Inhibitor A...... 129 6.4.2.1 Corrosion rate from EIS...... 129 6.4.2.2 EIS parameters and plots...... 130 6.4.3 Cyclic sweep results for Inhibitor A...... 134 6.4.3.1 Corrosion rate from CS...... 134 6.4.3.2 CS parameters and plots...... 134 6.4.3.3 Difference between test types...... 137 6.4.4 Adsorption isotherms for Inhibitor A...... 138 6.4.5 Activation energy of Inhibitor A...... 143 6.5 Commercial inhibitor B ...... 145 6.5.1 Linear polarization resistance results for Inhibitor B...... 145 6.5.1.1 Corrosion rate from LPR...... 145 6.5.1.2 LPR parameters...... 145 6.5.2 Electrochemical impedance spectroscopy results for Inhibitor B...... 148 6.5.2.1 Corrosion rate from EIS...... 148 6.5.2.2 EIS parameters and plots...... 149 6.5.3 Cyclic sweep results for Inhibitor B...... 152 6.5.3.1 Corrosion rate from CS...... 152 6.5.3.2 CS parameters and plots...... 152 6.5.3.3 Difference between test types...... 155 6.5.4 Adsorption isotherms for Inhibitor B...... 156 6.5.5 Activation energy of Inhibitor B...... 161 6.6 FTIR results...... 163 6.7 SEM results ...... 163 6.8 Summary ...... 165 6.8.1 Effect of temperature...... 165 6.8.2 Comparison between inhibitors...... 166 6.8.3 Summary of thermodynamic results...... 168 6.8.3.1 Gibbs free energy of adsorption...... 168 6.8.3.2 Activation energy...... 170 6.8.3.3 Enthalpy and Entropy...... 170

Chapter 7. Conclusion and Recommendations ...... 173 References ...... 176 Vita...... 189

List of Figures

Figure 1. The investigated inhibitors (a) CPLE, EPPE and FLE (b) Commercial aaaaaaaa inhibitors denoted as Inhibitor A and Inhibitor B...... 21 Figure 2. Effect of anodic inhibitor on the Tafel slope ...... 27 Figure 3. Effect of cathodic inhibitor on the Tafel slope ...... 28 Figure 4. Effect of mixed inhibitor on Tafel slopes ...... 29

Figure 5. Relationship between corrosion rate and CO2 pressure for different oxygen aaaaaaaa pressures...... 36

Figure 6. Relationship between corrosion rate and temperature for different CO 2 aaaaaaaa pressures...... 38 Figure 7. Examples of organic inhibitors...... 41

Figure 8. Effective inhibitors in CO2 staurated brine for carbon steel...... 41 Figure 9. Molecular structure of (a) IM-HOL (b) BIM-NH ...... 44 Figure 10. Molecular structure of RAIM ...... 45 Figure 11. Molecular structures of AEI-11 and AQI-11...... 46 Figure 12. Corn oil-derived inhibitors ...... 49 Figure 13. Molecular structure of SFASS...... 50 Figure 14. Molecular structure of SM...... 51 Figure 15. Equivalent circuit...... 60 Figure 16. Process depicting FTIR analysis ...... 65 Figure 17. Schematic of the Scanning Electron Microscope ...... 66 Figure 18. Connections between the potentiostat, PC and corrosion cell system...... 67 Figure 19. Experimental setup at (a) 25°C and (b) 40°C, 50°C and 70°C (± 2°C) ...... 69

Figure 20. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaa saturated 3.5wt% NaCl solution in the presence of fig leaves extract at aaaaaaaaaa elevated temperatures from the LPR test ...... 71 Figure 21. Effect of FLE concentration on the steady state LPR value obtained ...... 73

Figure 22. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of fig leaves extract at aaaaaaaaaa elevated temperatures from the EIS test...... 75 Figure 23. Nyquist plots for fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) aaaaaaaaaa 70°C from the EIS test ...... 77 11

Figure 24. Theta-frequency plots for fig leaves extract at (a) 25°C (b) 40°C (c) 50°C aaaaaaaaaa and (d) 70°C ...... 78

Figure 25. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of fig leaves extract at aaaaaaaaaa elevated temperatures from the CS test...... 79 Figure 26. CS plots for FLE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C...... 81 Figure 27. Comparison of inhibition efficiency between electrochemical tests at (a) aaaaaaaaaa 25°C (b) 40°C (c) 50°C and (d) 70°C for FLE ...... 82 Figure 28. Langmuir isotherms for adsorption of fig leaves extract at (a) 25°C (b) aaaaaaaaaa 40°C (c) 50°C and (d) 70°C ...... 83 Figure 29. Temkin isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C aaaaaaaaaa (c) 50°C and (d) 70°C ...... 84 Figure 30. F-H isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 85 Figure 31. Frumkin isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C aaaaaaaaaa (c) 50°C and (d) 70°C ...... 86 Figure 32. Arrhenius plot for fig leaves extract ...... 88

Figure 33. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of calotropis procera aaaaaaaaaa leaves extract at elevated temperatures from the LPR test...... 90 Figure 34. Effect of CPLE concentration on the steady state LPR value obtained ...... 92

Figure 35. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of calotropis procera aaaaaaaaaa leaves extract at elevated temperatures from the EIS test ...... 93 Figure 36. Nyquist plots for calotropis procera leaves extract at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C from the EIS test...... 95 Figure 37. Theta-frequency plots for calotropis procera leaves extract at (a) 25°C (b) aaaaaaaaaa 40°C (c) 50°C and (d) 70°C ...... 96

Figure 38. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of calotropis procera aaaaaaaaaa leaves extract at elevated temperatures from the CS test ...... 97 Figure 39. CS plots for CPLE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 100

Figure 40. Comparison of inhibition efficiency between electrochemical tests at (a) aaaaaaaaaa 25°C (b) 40°C (c) 50°C and (d) 70°C for CPLE ...... 101 Figure 41. Langmuir isotherms for adsorption of calotropis procera leaves extract at aaaaaaaaaa (a) 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 102 Figure 42. Temkin isotherms for adsorption of calotropis procera leaves extract at (a) aaaaaaaaaa 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 103 Figure 43. F-H isotherms for adsorption of calotropis procera leaves extract at (a) aaaaaaaaaa 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 104 Figure 44. Frumkin isotherms for adsorption of calotropis procera leaves extract at (a) aaaaaaaaaa 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 105 Figure 45. Arrhenius plot for calotropis procera leaves extract...... 106

Figure 46. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of eggplant leave extract aaaaaaaaaa at elevated temperatures from the LPR test ...... 108 Figure 47. Effect of EPPE concentration on the steady state LPR value ...... 110

Figure 48. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of eggplant peel extract at aaaaaaaaaa elevated temperatures from the EIS test...... 111 Figure 49. Nyquist plots for eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and aaaaaaaaaa (d) 70°C from the EIS test...... 113 Figure 50. Theta-frequency plots for eggplant leave extract at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 114

Figure 51. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of eggplant leave extract aaaaaaaaaa at elevated temperatures from the CS test ...... 116 Figure 52. CS plots for EPPE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C...... 118 Figure 53. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and aaaaaaaaaa (d) 70°C for EPPE ...... 119 Figure 54. Langmuir isotherms for adsorption of eggplant leave extract at (a) 25°C (b) aaaaaaaaaa 40°C (c) 50°C and (d) 70°C ...... 120 Figure 55. Temkin isotherms for adsorption of eggplant leave extract at (a) 25°C (b) aaaaaaaaaa 40°C (c) 50°C and (d) 70°C ...... 121

Figure 56. F-H isotherms for adsorption of eggplant leave extract at (a) 25°C (b) 40°C aaaaaaaaaa (c) 50°C and (d) 70°C ...... 122 Figure 57. Frumkin isotherms for adsorption of eggplant leave extract at (a) 25°C (b) aaaaaaaaaa 40°C (c) 50°C and (d) 70°C ...... 123 Figure 58. Arrhenius plot for eggplant leave extract ...... 125

Figure 59. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated aaaaaaaaaa temperatures from the LPR test...... 127 Figure 60. Effect of Inhibitor A concentration on the steady state LPR value ...... 129

Figure 61. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated aaaaaaaaaa temperatures from the EIS test ...... 130 Figure 62. Nyquist plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C & (d) 70°C aaaaaaaaaa from the EIS test...... 132 Figure 63. Theta-frequency plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) aaaaaaaaaa 70°C...... 133

Figure 64. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated aaaaaaaaaa temperatures from the CS test ...... 135 Figure 65. CS plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 137 Figure 66. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and aaaaaaaaaa (d) 70°C for Inhibitor A ...... 138 Figure 67. Langmuir isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 139 Figure 68. Temkin isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 140 Figure 69. F-H isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) 50°C aaaaaaaaaa and (d) 70°C ...... 141 Figure 70. Frumkin isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 142 Figure 71. Arrhenius plot for Inhibitor A ...... 144

Figure 72. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of Inhibitor B at elevated aaaaaaaaaa temperatures from the LPR test...... 146 Figure 73. Effect of Inhibitor B concentration on the steady state LPR value ...... 147

Figure 74. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO 2 aaaaaaaaaa saturated 3.5wt% NaCl solution in the presence of Inhibitor B at elevated aaaaaaaaaa temperatures from the EIS test ...... 148 Figure 75. Nyquist plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C & (d) 70°C aaaaaaaaaa from the EIS test...... 150 Figure 76.Theta-frequency plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) aaaaaaaaaa 70°C...... 151 Figure 77. Plots obtained from the CS test using Inhibitor B where (a) Corrosion rate aaaaaaaaaa as a function of FLE concentration (b) Inhibition efficiency determined aaaaaaaaaa from the corrosion rate ...... 153 Figure 78. CS plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 155 Figure 79. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and aaaaaaaaaa (d) 70°C for Inhibitor B...... 156 Figure 80. Langmuir isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 157 Figure 81. Temkin isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 158 Figure 82. F-H isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) 50°C aaaaaaaaaa and (d) 70°C ...... 159 Figure 83. Frumkin isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) aaaaaaaaaa 50°C and (d) 70°C ...... 160 Figure 84. Arrhenius plot for Inhibitor B...... 162 Figure 85. Combined FTIR spectra of FLE (a), CPLE (b) and EPPE (c)...... 164 Figure 86. SEM images of mild steel taken at 5um for the following: (a) Unexposed aaaaaaaaaa area (b) Film formed by Inhibitor A (c) Film formed by FLE (d) Film aaaaaaaaaa formed by CPLE and (e) Film formed by EPPE ...... 164 Figure 87. Nyquist plots depicting the effect of temperature for (a) FLE (b) CPLE (c) aaaaaaaaaa EPPE and (d) Inhibitor A ...... 165

Figure 88. CS plots depicting the effect of temperature for (a) FLE (b) CPLE (c) aaaaaaaaaa EPPE and (d) Inhibitor A ...... 166 Figure 89. Nyquist plots for FLE, CPLE, EPPE and Inhibitor A at (a) 25°C (b) 40°C aaaaaaaaaa (c) 50°C and (d) 70°C ...... 167 Figure 90. CS plots depicting the difference between FLE, CPLE, EPPE and Inhibitor aaaaaaaaaa A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C ...... 168 Figure 91. Van’t Hoff plots for (a) FLE (b) CPLE (c) EPPE (d) Inhibitor A and (e) aaaaaaaaaa Inhibitor B ...... 171

List of Tables

Table 1. Parameters obtained from the LPR test for FLE...... 72 Table 2. Impedance parameters obtained using FLE ...... 76 Table 3. Summary of maximum phase angles obtained for FLE ...... 77 Table 4. Tafel parameters obtained from various FLE concentrations at elevated aaaaaaaaa temperatures ...... 80 Table 5. Langmuir thermodynamic results for FLE ...... 87 Table 6. Arrhenius parameters of FLE...... 88 Table 7. Parameters obtained from the LPR test for CPLE ...... 91 Table 8: Impedance parameters obtained using CPLE ...... 94 Table 9. Summary of maximum phase angles obtained for CPLE ...... 96 Table 10. Tafel parameters obtained from various CPLE concentrations at elevated aaaaaaaaa temperatures ...... 99 Table 11: CPLE thermodynamic data from Langmuir isotherm ...... 106 Table 12. Arrhenius parameters of CPLE ...... 107 Table 13. Parameters obtained from the LPR test for EPPE ...... 109 Table 14. Impedance parameters obtained using EPPE...... 112 Table 15. Summary of maximum phase angles obtained for EPPE ...... 115 Table 16. Tafel parameters obtained from various EPPE concentrations at elevated aaaaaaaaa temperatures ...... 117 Table 17. EPPE thermodynamic data from Langmuir isotherm ...... 124 Table 18. Arrhenius parameters of EPPE ...... 125 Table 19. Parameters obtained from the LPR test for Inhibitor A ...... 128 Table 20. Impedance parameters obtained using Inhibitor A ...... 131 Table 21. Summary of maximum phase angles obtained for Inhibitor A...... 133 Table 22. Tafel parameters obtained from various Inhibitor A concentrations ...... 136 Table 23. Thermodynamic data from Langmuir isotherm for Inhibitor A ...... 143 Table 24. Arrhenius parameters of Inhibitor A...... 144 Table 25. Parameters obtained from the LPR test for Inhibitor B ...... 147 Table 26. Impedance parameters obtained using Inhibitor B ...... 149 Table 27. Summary of maximum phase angles obtained for Inhibitor B ...... 151

Table 28. Tafel parameters obtained from various Inhibitor B concentrations at aaaaaaaaa elevated temperatures...... 154 Table 29. Thermodynamic data from Langmuir isotherm for Inhibitor B ...... 161 Table 30. Arrhenius parameters for Inhibitor B...... 162 Table 31. Langmuir thermodynamic results for FLE, CPLE and EPPE ...... 169 Table 32. Langmuir thermodynamic results for Inhibitor A and B ...... 169 Table 33. Activation energy for all inhibitors...... 170 Table 34. Summary of the enthalpy and entropy values for each inhibitor ...... 172

Nomenclature

CPLE – Calotropis procera leaves extract

CR – Corrosion rate

CS – Cyclic sweep

EIS – Electrochemical impedance spectroscopy

EPPE – Eggplant peel extract

FLE – Fig leaves extract

FTIR – Fourier transform infrared

IE – Inhibition efficiency

IM – Imidazoline

LPR – Linear polarization resistance

SEM – Scanning electron microscope

Chapter 1. Introduction

Mild steel is prominently featured in oil and gas industries in the form of sprawling pipelines and various types of equipment. It is also utilized in a wide variety of sectors such as nuclear power, construction, energy, medicine and food [1,2]. Its extensive usage can be attributed to its desirable properties such as strength, malleability and hardness as well its inexpensive cost and ready availability. However, mild steel can be subjected to aggressive and unfavorable environmental conditions, making it susceptible to corrosion and thus resulting in the degradation of its properties [3].

Carbon dioxide (CO2) corrosion is prevalent in various industries, such as both the production and distribution divisions in the oil and gas industry where it is the cause of nearly 60% of all oilfield corrosion problems. Corrosion is a major problem faced by many industries where it presents not only a safety risk [1], but also an economic risk to businesses [4]. It is both directly and indirectly responsible for significant economic losses in industry and, consequently, requires methods for control. Direct costs entail maintenance required due to equipment damage while indirect costs include loss of revenue because of disruptions in production. Corrosion inhibitors are the most common form of control for internal corrosion present in pipelines, storage tanks and other types of equipment. Most inhibitors reduce the corrosion rate through limiting the anodic or cathodic reactions of the corrosion process, or even a combination of the two [5].

Most commercial corrosion inhibitors utilized in industries such as the oil and gas, are multi-component inhibitor systems which consist of nitrogen and sulfur functionalities. Although these inhibitors are stable and provide suitable protection in corrosive environments [6,7], they can be expensive to formulate and can pose a threat to public health and the environment because of its toxicity. Moreover, accompanied by other heavy metals, chromates, and phosphates, multi-component inhibitor systems are persistent and require expensive processes for their removal [8].

Because of the difficulties surrounding commercial corrosion inhibitors, increasing scientific research has been steered towards developing corrosion

20 inhibitors from natural, green products. These natural inhibitors are nontoxic, environmentally friendly and, due to their abundance, are relatively inexpensive to produce [9].

The objective of this work is to investigate the corrosion inhibition provided by Ficus carica (fig leaves), Calotropis procera and Solanum melongena (eggplant peels) and compare this performance with two commercial inhibitors, denoted as A (natural inhibitor) and B (synthetic inhibitor). Electrochemical techniques were used to determine the inhibition efficiency of the selected inhibitors, and the results will expand on the knowledge of natural corrosion inhibitors derived from plant extracts.

Figure 1. The investigated inhibitors (a) CPLE, EPPE and FLE (b) Commercial inhibitors denoted as Inhibitor A and Inhibitor B

Including this introduction, the rest of this thesis is divided into seven sections. The second section will introduce the reader to corrosion, how it occurs and the means of controlling it. The different types of inhibitors as well as the various adsorption isotherms are explored. The third section deals with CO2 corrosion and the mechanism of attack as well as the factors which play a role in determining the extent of attack. The fourth section consists of a detailed literature review on inhibitors that have been used in CO2 saturated salt solutions as well as those that have been used in various other corrosive media. The fifth section discusses the preparation of the selected inhibitors, types of electrochemical tests, equipment used and analysis tools such as Fourier transform infra-red spectroscopy (FTIR) and a scanning electron microscope (SEM). The sixth section presents the results of the electrochemical tests, adsorption isotherms and Arrhenius analysis for each inhibitor. The results are also

21 summarized and discussed at the end of this section. Finally, the conclusions and recommendations for future work are presented in the last section.

Chapter 2. Background

In this section, information is presented about corrosion and its mechanisms. Inhibitors are also discussed and are classified by their mechanisms. Different types of adsorption isotherms are also explored.

2.1 Corrosion

Corrosion is the term used to describe the degradation process a material suffers through the interaction with its surroundings [5]. It is derived from the Latin word “corrosus” which means to be consumed or eaten away [10]. The type of corrosion occurring in the results of this thesis is uniform corrosion. However, corrosion of metal occurs in a variety of forms such as uniform corrosion of the metal surface, localized corrosion, or even along grain boundaries due to differences in corrosion resistance [11]. Moreover, the corrosion attack can be further classified under two main types, namely physiochemical and biological corrosion [11].

Physiochemical corrosion can be categorized as galvanic, pitting, intergranular, stress corrosion cracking or erosion corrosion. Alternatively, biological corrosion is caused by fungi, algae and bacteria which results in metal corrosion as well as wood rot. Generally, the mechanism for biological corrosion involves the alteration of the environment to make it more corrosive. This enables the formation of electrolytic concentration cells on the metal surface and can alter the protective film. Also, it can influence the corrosion redox reaction (both anodic and cathodic) [11].

The corrosion reaction consists of two or more simultaneous reactions where the metal is corroded through an oxidation reaction which releases electrons. Concurrently, a reduction reaction or reactions occurs at the more protected part of the metal which accepts the free electrons [11]. In general, any substance that loses electrons to another is said to be oxidized while a substance that accepts electrons is said to be reduced. Hence, the electrode that is oxidized is said to be the anode while the electrode where reduction occurs is denoted as the cathode [11]. This process requires certain requirements to be met in order to proceed such as the presence of an electrolyte or medium which provides a reservoir for the reduction reaction at the cathode as well as provides solution conductivity, an electrical path for electron flow 23 such as through a metal or between metals and the presence of a chemical potential difference between the adjacent sites of a metal surface.

For example, the corrosion of iron in water begins with the water molecule that disassociates into hydrogen ions and hydroxyl ions in neutral and slightly acidic environments.

+ − 푯ퟐ푶 → 푯 + 푶푯 (1) The immersion of the metal in a solution, such as water, will result in surface ionizations due to the electric charge difference at the metal-liquid interface. This concept can be illustrated by iron which dissolves in water to from ferrous ions.

푭풆 → 푭풆ퟐ+ + ퟐ풆− (2) Ferrous ions which travel away from the surface can be further oxidized into ferric ions.

푭풆ퟐ+ → 푭풆ퟑ+ + 풆− (3) These positively charged ferric ions combine with the negatively charged hydroxyl ions to form the corrosion product.

ퟑ+ − 푭풆 + ퟑ푶푯 → 푭풆(푶푯)ퟑ (4) In order for the corrosion process to continue, the reduction reaction should also occur simultaneously. Since the reduction reaction is dependent on the environment, if the environment is free of oxygen, the reaction proceeds as: 2퐻+ + − 2푒 → 퐻2 . However, when oxygen is present, it will be reduced.

+ − 푶ퟐ + ퟒ푯 + ퟒ풆 → ퟐ푯ퟐ푶 (풂풄풊풅풊풄 풎풆풅풊풂) (5)

− − 푶ퟐ + ퟐ푯ퟐ푶 + ퟒ풆 → ퟒ(푶푯) (풏풆풖풕풓풂풍 풐풓 풂풍풌풂풍풊풏풆 풎풆풅풊풂) (6)

2.2 Types of inhibitors

Corrosion costs industries, such as the oil and gas sectors, billions of dollars annually [12]. The corrosion of metals can be minimized by using corrosion resistant materials, changing or controlling the surrounding environment, and through corrosion preventive techniques such as protective coatings. The selection process involved in determining the appropriate corrosion control technique is dependent on the type of application involved [11]. However, to prevent corrosion occurring 24 internally in equipment such as pipelines, the majority of industries employ inhibitors as the most practical method of combatting the effects of corrosion.

Corrosion inhibitors, as defined by Gopal and Jepson, are chemical compounds or materials whose introduction into an environment results in an effective decrease in the corrosion rate [13]. Characteristically, an effective inhibitor is one that can prevent the transfer of corrosive species and water to and from the metal surface, and impede the reduction and oxidation corrosion reactions at the cathodic and anodic sites [1]. An inhibitor is evaluated based on its inhibition efficiency (IE) which is determined using:

푪푹 − 푪푹 푰푬% = ퟎ 풊 ∗ ퟏퟎퟎ (ퟕ) 푪푹ퟎ

where 퐶푅0 is the corrosion rate of mild steel in the absence of inhibitors and

퐶푅푖 is the corrosion rate of mild steel at a selected concentration of inhibitor

Inhibitor efficiency is not just dependent on the concentration but is also a function of other factors such as temperature, flow regime, fluid composition and partial pressures of corrosive gases (such as CO2 and hydrogen sulfide if present) [2]. On a molecular level, the efficiency is affected by the inhibitor charge density, interaction mechanism with the metal surface, electronic structure of the molecules and the number of adsorption sites [14]. The inhibition efficiency is likely to be high if the inhibitor has a high molecular size and if the adsorption centers have a high electron density [1].

Typically, the inhibitor molecules adsorb on the metal-solution interface resulting in a shift in the potential difference between the metal electrode and solution. This is caused because of the non-uniform distribution of electric charges on the surface. If the potential shift is negligible, then the inhibition is likely to be the result of the geometric blocking of the adsorbed inhibitor molecules on the metal surface [14].

Even though there have been many scientific studies conducted, the majority of knowledge gained from the study of corrosion inhibitors has come from trial and error experiments in laboratories and field investigations. As a consequence, there is a

25 lack of technical information regarding the development and use of inhibitors with regard to theories, rules and equations [2].

The processes by which inhibitors reduce corrosion vary according to the type of inhibitor used. Inhibitors can either operate by altering the environment, creating a film on the metal surface that blocks the interaction between the corrosive material and the metal, or by inhibiting the reduction or oxidation reactions involved in the redox corrosion system [2]. Inhibitors can also be broadly categorized as either those that have an oxidizing effect to enhance the formation of a protective oxide film or those that adsorb onto the surface of the metal forming a barrier that prevents contact with the corrosive media [15].

For example, environmental altering inhibitors such as scavengers, work by removing corrosive ions from the working solution. Hydrazine and sodium sulfate, for example, are common scavenging inhibitors which remove dissolved oxygen [2].

ퟏ 푺풐풅풊풖풎 풔풖풍풑풉풊풕풆 풓풆풂풄풕풊풐풏: 푵풂 푺푶 + 푶 → 푵풂 푺푶 (8) ퟐ ퟑ ퟐ ퟐ ퟐ ퟒ

ퟏ 푯풚풅풓풂풛풊풏풆 풓풆풂풄풕풊풐풏: ퟐ(푯 푵푵푯 )+ 푶 → ퟐ푵푯 + 푯 푶 + 푵 (9) ퟐ ퟐ ퟐ ퟐ ퟑ ퟐ ퟐ

Another type of environment altering inhibitor includes neutralizing inhibitors such as amines, merpholine and ammonia which can reduce the amount of hydrogen ions in the environment [2].

Barrier inhibitors account for the majority of inhibitors used in industry. They diminish the apparent corrosion rate by modifying the surface of the metal through the creation of a layer on the corroding metal surface. Due to this mechanism, barrier inhibitors are also known as film-forming inhibitors [2]. Corrosion inhibitors in this group can be further divided as anodic corrosion inhibitors, cathodic corrosion inhibitors and mixed/adsorption inhibitors [16].

2.2.1 Anodic corrosion inhibitor. These types of inhibitors work by hindering the anodic reactions, thus 2- reducing the corrosion rate. Some examples of anodic inhibitors are nitrite (NO2 ), 2- 3- 2- chromate (CrO4 ), orthophosphate (PO4 ) and molybdate (MoO4 ) [16]. They are typically used in solutions of near neutral pH levels, and where the corrosion products 26

(oxides, salts and hydroxides) are soluble. They are also commonly referred to as

passivation inhibitors [17].

Cathode Potential(E)

Anode

Current (I)

Figure 2. Effect of anodic inhibitor on the Tafel slope [17]

Passivation inhibitors act by shifting the equilibrium of the corrosion process, as seen on the left side of the Pourbaix diagram, depicted in Figure 2. The negative slope signifies the cathode and the positive slope represents the anode. If this shift is in the passivation zone, it will result in the formation of a passivation oxide film, which is transparent and thin, on the anodic sites. This reduces the effective anode area and thus the corrosion rate. The inhibitor depresses the oxidation process by increasing the anode potential. However, the major drawback associated with passivation inhibitors is that it is possible that its introduction can increase the corrosion rate if its concentration is not above the optimum level [16].

2.2.2 Cathodic corrosion inhibitors.

The two main types of cathodic inhibitors are the cathodic poisons and cathodic precipitators [17]. Cathodic poisons hinder the reduction rate of the corrosion reaction because different types of poison can reduce the corrosion rate through different mechanisms. For example, poisons such as selenides and sulfides adsorb on the surface of metals while others such as arsenic and antimony compounds form a metallic layer by being reduced at the cathode. Additionally, compounds such as

27 silicates, phosphates and borates decrease the corrosion rate through the formation of a protective film which limits the transport (by diffusion) of oxygen to the metal surface [17]. In contrast, cathodic precipitators work by increasing the alkalinity at the cathodic sites and selectively precipitating there as insoluble compounds [17]. This results in the establishment of a barrier film on the cathodic sites. Thus, the corrosion rate decreases as the area available for cathodic reactions decreases. Typical cathodic precipitators include polyphosphates, zinc salts, calcium salts and magnesium salts [16].

From Figure 3, it is seen that there is a change in the cathodic Tafel slope as well as a downward shift when an inhibitor is used. This indicates that the inhibitor has influenced the reaction mechanism. If only the slope was affected, the inhibitor works by blocking the active sites [18].

Potential (E)

Cathode without inhibitor

Cathode Anode

Current (I)

Figure 3. Effect of cathodic inhibitor on the Tafel slope [17]

2.2.3 Mixed/Adsorption corrosion inhibitors.

Mixed inhibitors are organic compounds which account for 80% of all inhibitors which do not fall under the anodic or cathodic type. They act by inhibiting corrosion at the metal-solution interface through the adsorption of organic molecules [4].These organic molecules are adsorbed onto the metal surface where they form a stable bond, thus reducing the apparent corrosion rate as the surface adsorption process progresses to completion [2]. This type of inhibition occurs by lowering the 28 amount of surface sites available, resulting in a decrease in the rate of corrosion reactions [4]. Due to this mode of inhibition, mixed inhibitors are also commonly known as adsorption inhibitors. Factors which influence the adsorption process are the surface charge of the metal, the type of electrolyte, and the inhibitor [17]. The efficiency of the film formed by the inhibitor is dependent on its contact time and its concentration [2]. The effectiveness of the inhibitor depends on the coverage of the metal surface and the extent to which it is adsorbed [17].

Figure 4. Effect of mixed inhibitor on Tafel slopes [17]

Examples of adsorption inhibitors include carboxyls (R-COOH), amines (R- - NH2), benzoate (C6H5COO ), sulphonates and phosponates (R-PO3H2) [16]. Other types of barrier-forming inhibitors include volatile corrosion inhibitors (VCI), surfactants and oil and water based inhibitors.

VCIs are typically used in closed spaces such as package bags to reduce corrosion. The vapours of the VCI condense on the metal surface as microscopic crystals. These crystals then dissolve the moisture film present on the surface, displacing the water molecules, and forms a monomolecular transparent protection film. Commonly used VCIs include dicyclohexylamine, guanidine and cyclohexylamine [16].

Surfactants are made up of organic compounds that contain an amphiphilic (hydrophilic and lipophilic) molecule. The molecules construct a polar hydrophilic

29 group known as a head and this is attached to a nonpolar hydrophobic group known as the tail. As a result, surfactants can dissolve in both polar and non-polar solvents [1]. The inhibition process of the surfactants begins when their surface concentration becomes high enough on the surface to initiate the interaction between the surfactant’s hydrocarbon chains through the Van der Waals forces. These form an organized structure, known as a hemimcelle, which blocks the surface of the solid metal and thus reduces the corrosion reactions. However, the adsorption of surfactants on the metal can affect the corrosion resisting properties of the metal [19,20]. Surfactants are typically known for their high inhibition, low toxicity, easy formulation and low price [1,19,20]. They modify the surface and interfacial free energies through preferential adsorption onto the surface or interface in a system [22].

Oil and solvent-based corrosion inhibitors are a type of barrier inhibitor which act by preventing the interaction between water and the metal surface through the formation of coatings. This type of inhibitor produces a heavy film which, based on the specific product, could be soft, oily, colored, transparent or semi-hard, and has strong water-rejecting properties. On the other hand, water based inhibitors reduce corrosion by modifying the characteristics of the metal surface, thus reducing its susceptibility to oxidation and corrosion formation. The thin formations of chemical films are transparent when dry and, in batch systems, are rarely removed in subsequent operations. However, since they do not completely stop the interaction between water and the metal surface, they are not as effective as the oil-based inhibitors in applications such as outside storage [23].

2.3 Adsorption isotherms

The adsorption of the organic molecule can occur through various mechanisms such as physical/electrostatic adsorption and chemisorption [17], as well as through back bonding and organometallic complex formation. Adsorbed inhibitors can work through the geometric blocking effect of its compounds where it reduces the reaction area on the metal surface. It can also inhibit corrosion through the electrocatalytic effect of its compounds or its reaction product where it alters the average activation energy barriers of the anodic and cathodic reactions [24,25]. Physisorption is an adsorption mechanism which occurs by utilizing the electrostatic attraction between

30 the metal surface and the inhibitor whereby the opposite charges result in the adsorption of the inhibitor onto the metal surface. Inhibitors which are physically adsorbed interact quickly but can also be removed easily from the surface. Moreover, an increase in temperature will result in the desorption of the adsorbed inhibitor molecules [17]. Chemisorption is another mechanism of adsorption where the charge is either shared or transferred between the metal surface and the inhibitor molecules. Moreover, adsorption and inhibition increase when temperature increases [17]. For example, ionic inhibitors such as quaternary ammonium chloride adsorb through electrostatic adsorption, while non-ionic inhibitors such as imidazoline adsorb through chemisorption [26-28].

In this section, four different adsorption isotherms are discussed. Isotherms describe adsorption using the relationship between the amount of adsorbate (inhibitor molecules) which is adsorbed on the adsorbent (surface of mild steel). The adsorption isotherms explored are the Langmuir isotherm, Temkin isotherm, Flory-Huggins (FH) isotherm and Frumkin isotherm.

2.3.1 Langmuir isotherm.

The Langmuir isotherm is given by

퐂 ퟏ = + 퐂 (10) ∅ 퐊 where C in the inhibitor concentration, ∅ is the degree of surface covered by the inhibitor and K is the adsorption equilibrium constant.

A plot of C versus C yields a linear relationship where the slope is unity and ∅ the constant 1 [29]. The inhibitor is said to be strongly adsorbed if the adsorption 퐾 equilibrium constant is large. The slope is near unity because each inhibitor molecule is adsorbed on an individual active site on the metal surface [30].

2.3.2 Temkin isotherm.

The Temkin isotherm is given by

ퟐ.ퟑퟎퟑ 퐥퐨퐠퐊 ퟐ.ퟑퟎퟑ 퐥퐨퐠퐂 ∅ = − − (11) ퟐ퐚 ퟐ퐚 where C in the inhibitor concentration, ∅ is the degree of surface covered by the inhibitor, K is the adsorption equilibrium constant and a is the attractive parameter.

A plot of ∅ versus log C yields a linear relationship where the slope is − 2.303 2푎 and the intercept is − 2.303 푙표푔퐾 . The values of “a” should be negative to prove that 2푎 repulsion exists in the adsorption layer [29].

2.3.3 Flory-Huggins isotherm.

The Flory-Huggins isotherm is given by

퐥퐨퐠( ∅/퐂) = 퐥퐨퐠 퐊 + 퐱 퐥퐨퐠(ퟏ − ∅) (12) where C is the inhibitor concentration, ∅ is the degree of surface covered by the inhibitor, K is the adsorption equilibrium constant and x is the size parameter.

A plot of log( ∅/퐶) versus 푙표푔(1 − ∅) yields a linear relationship where x is the slope and log K is the intercept. The values of x should be positive since this means that the adsorbed species can be described as bulky due to being able to displace water molecules from the surface of mild steel [29].

2.3.4 Frumkin isotherm.

The Frumkin isotherm is given by

log( C ∗ ∅ ) = 2.303 log K + 2 ∝ ∅ (13) 1−∅

where C in the inhibitor concentration, ∅ is the degree of surface covered by the inhibitor, K is the adsorption-desorption constant and ∝ describes the interaction in the adsorbed layer (lateral interaction term).

A plot of log( 퐶 ∗ ∅ ) versus ∅ yields a linear relationship where 2 ∝ is the 1−∅ slope and 2.303logK is the intercept. The value of ∝ should be positive as this indicates that the inhibitor is attracted to the mild steel surface [29].

2.3.5 Gibbs free energy.

The equilibrium adsorption constant (K) is determined from the adsorption 0 isotherm and can be used to find the standard Gibbs free energy of adsorption (∆Gads) using:

0 −∆퐺 ( 푎푑푠) 푲 = 풆 푹푻 (14)

where T is the temperature in Kelvins and R is the universal gas constant [31].

Chapter 3. Carbon Dioxide Corrosion

Carbon dioxide (CO2) corrosion not only leads to an economic loss but also poses a safety and environmental pollution risk [24]. In the oil and gas production sector, dissolved CO2 can be found to vary between trace levels to up to 50% where it forms carbonic acid (H2CO3). Although it is a weak acid, it was found that for the same pH level, H2CO3 resulted in greater steel corrosion than hydrochloric acid [32,33]. Its existence is due to both a natural presence in the hydrocarbons as well as to anthropogenic additions, such as through secondary enhanced oil recovery processes [4]. One example is where CO2 is injected into oil wells to reduce the oil’s viscosity and enhance production. However, actions like this may cause severe CO2 corrosion [34] which is also known as sweet corrosion. CO2 corrosion can also result in a type of chalky fracture which is exclusive to this type of corrosion [35]. CO2 may not be corrosive to metals and alloys in dry conditions but it can be corrosive when present in solutions such as produced water. The produced CO2 saturated brine poses a considerable threat to low alloy steel process equipment [18,36].

3.1 CO2 corrosion mechanism

De Waard and Milliams described the basic mechanisms involved in CO2 corrosion in four steps in their work for Shell Research [37]. The first step is the dissolution of CO2 into its reactive species in an aqueous solution, followed by the transport of these reactants from the bulk solution to the metal surface. Thirdly, electrochemical reactions take place at the anodic and cathodic sites on the surface and, finally, the corrosion reaction products are transported away to the bulk solution [18].

Carbon dioxide which is dissolved in salt water results in the formation of weak carbonic acid through hydration. It disassociates to form hydrogen, bicarbonate and carbonate ions, seen in equations 16 to 18, all of which contribute to the cathodic reactions required in the corrosion process [18,24].

푪푶ퟐ (퐠) → 푪푶ퟐ(퐚퐪) (15)

푪푶ퟐ (퐚퐪)+ 푯ퟐ퐎 → 푯ퟐ푪푶ퟑ (풂풒) (16)

+ − 푯ퟐ푪푶ퟑ (풂풒) ⇿ 푯 + 푯푪푶ퟑ (17) 34

− + ퟐ− 푯푪푶ퟑ ⇿ 푯 + 푪푶ퟑ (18) Carbonic acid provides a reservoir of hydrogen ions and thus hydrogen evolution/hydrogen ion reduction is assumed to be the dominant cathodic reaction.

− − Cathodic reactions 푹풆풅풖풄풕풊풐풏: ퟐ푯ퟐ푪푶ퟑ + ퟐ풆 → ퟐ푯푪푶ퟑ + 푯ퟐ (19) − + 푹풆품풆풏풆풓풂풕풊풐풏: ퟐ푯푪푶ퟑ + ퟐ푯 → ퟐ푯ퟐ 푪푶ퟑ (20)

+ − 푶풗풆풓풂풍풍: ퟐ푯 + ퟐ풆 → 푯ퟐ (21)

Anodic reaction 푭풆 → 푭풆ퟐ+ + ퟐ풆− (22)

This corrosion process causes iron ions to be produced and hydrogen ions to be depleted at the surface of the metal. This leads to the formation of concentration gradients which in turn contributes to the diffusion of species to and from the metal surface. However, these species concentrations affect the rate of the electrochemical reactions and, subsequently, a relationship can be observed between the electrochemical and the diffusion processes. In turbulent flow, the diffusion of species to and from the surface is improved, thus resulting in increasing the corrosion rate. Protective films can slow down the diffusion process and thus can reduce the corrosion rate [18,38].

3.2 Parameters affecting CO2 corrosion

The CO2 corrosion rate is dependent on several parameters such as temperature, pH, flow velocity, and partial pressure of carbon dioxide [18]. It is also affected by other factors such as solution chemistry, oil-water ratio and the type of corrosion product film formed [19,39].

3.2.1 Effect of CO2 partial pressure.

According to Dewaard and Milliams, the relationship between the CO2 partial pressure and corrosion rate is given by equation 23 at temperatures between 15°C and 60°C.

ퟎ.ퟔퟕ ( ) 푪풐풓풓풐풔풊풐풏 풓풂풕풆 = 푪풐풏풔풕풂풏풕 ∗ 푷푪푶ퟐ (23)

where 푷 is the partial pressure of CO2. The exponent (0.67) varies across 푪푶ퟐ sources in the literature where it is typically between 0.5 to 0.8 [18].

However, the corrosion rate does not increase solely by increasing the CO2 partial pressure [40]. It also depends on the surrounding environmental conditions.

Without scale protection, higher CO2 partial pressures results in lowering the pH and increasing the rate of carbonic acid reduction, which can lead to increasing the corrosion rate. In a study by Wang, it was observed that the anodic reaction was unaffected by an increase in the CO2 partial pressure from 3 to 20 bars in the presence of a large carbonic acid reservoir which ensured a high cathodic limiting current density [18].

Figure 5. Relationship between corrosion rate and CO2 pressure for different oxygen pressures [18]

Figure 5 demonstrates that the corrosion rate depends on the competing carbon dioxide and oxygen partial pressures in the bulk solution. The dotted vertical line indicates the difference in pH where towards the left it varies between 9.01 and 7 while towards the right it becomes more acidic with increasing CO2 pressure. CO2 acidifies the solution and increases the corrosion rate. However, the presence of protective films, high CO2 partial pressures and a pH lower than 5.2 can result in reducing the corrosion rate. This is due to a lower availability of cathodic sites and thus results in the accumulation of carbonate and bicarbonate ion concentration [18].

Oxygen reduction forms alkalis which decrease the corrosion rate. For a given experiment, the oxygen reduction current density is not dependent on pH, but is limited by its diffusion into the solution [32].

3.2.2 Effect of Temperature.

When fixing the CO2 partial pressure, ramping up the temperature causes an increase in the corrosion rate up to a certain temperature point. After this point, increases in the temperature will result in the formation of protective surface films which reduce the corrosion rate [18]. Temperature can also affect the physical properties of the inhibitor film, the adsorption process, and the structure of the inhibitor compound. At higher temperatures, some inhibitors may have a lower solubility in brine than at room temperature. This can even cause the polymerization of inhibitors which can lead to plugging [18].

At temperatures less than 60°C, the CO2 corrosion rate is dependent on salt concentration, temperature, pH, CO2 partial pressure and the metallurgy of the steel [18]. Without the use of inhibitors, temperatures below 60°C will create ineffective films, due to the high solubility of FeCO3, and thus low precipitation rates which ultimately results in a porous structure and poor adhesion with the surface. Alternatively, at temperatures between 60°C and 150°C, there is a change in the surface electrochemistry, thus making the corrosion rate dependent on system hydrodynamics. The corrosion product film created at this temperature is strong since its solubility becomes lower [18].

For each CO2 pressure, the corrosion rate increases as temperature increases as seen in Figure 6 [32]. There is a positive relationship between the temperature and the corrosion rate. This can be attributed to the diffusion of oxygen through the cathodic layers of the protective oxide film which is facilitated at higher temperatures [11].

3.2.3 Effect of Flow Velocity.

In the absence of inhibitors, turbulent flow can bring about an increase in the transport rate of species to and from the metal surface which can increase the corrosion rate. The type of flow can pose problems such as mechanically removing the formed inhibition films or by influencing their formation [18,38]. 37

Flow accelerated corrosion (FAC) is a significant issue in pipeline degradation in environments containing CO2 [41]. X65 steel was tested in CO2 saturated formation water. The flow field developed causes the steel electrode to experience different flow velocities and shear stresses at different locations [42,43]. With increasing radial distance, the flow velocity and shear stress decreases which results in the corrosion rate decreasing away from the center of the pipeline [41]. Different impact angles such as those in an elbow bend can affect the corrosion rate [41].

Figure 6. Relationship between corrosion rate and temperature for different CO2 pressures [18]

3.2.4 Effect of pH.

Most inhibitors are pH selective where their efficiency depends on the inhibitor structure and composition, the metal that needs protection and the active species in the solution [44]. The pH can affect the solubility of the corrosion products as well as the properties of the protective deposits and can thus dictate the rate of corrosion. Typically, as the pH decreases, the corrosion rate increases. However, this relationship is not always true since field observations show that even when the pH is greater than 7, there can still be acidic conditions present under deposits (due to the carbonic acid). This can lead to the occurrence of local modes of fracture [45].

Chapter 4. Corrosion Inhibitors

This section will primarily focus on the inhibitors that are used to reduce the corrosion of mild steel, and other low alloy steels, in CO2 saturated saline mediums as well as various other corrosive mediums such as hydrochloric acid, sulfuric acid and seawater. The inhibitors will also be distinguished between being synthetic or natural inhibitors.

The use of corrosion inhibitors is critical in mitigating CO2 corrosion inside equipment such as pipelines and storage containers. In the early days, sodium ferrocyanide and sodium arsenite were used as inorganic inhibitors to hinder CO2 corrosion in oil wells. However, due to its unsatisfactory performance, organic chemical formulations involving amines were developed instead [46]. Heterocyclic organic compounds which contain nitrogen, phosphorous, oxygen and sulfur atoms are typically used as effective corrosion inhibitors alongside organic surfactants [47- 51]. Examples of current corrosion inhibitors in the petroleum industry include amines, which can be primary, secondary, or tertiary (aliphatic and heterocyclic). Examples include, but are not limited, to pyridines, imidazolines, amine alcohols and triazines [6,7]. Ketones and esters are also known as inhibitors of CO2 corrosion when their concentration and molecular structure are optimal but they are rarely used [45]. A summary of corrosion inhibitors, used by the oil and gas industry, has been reported by Finsgar [52].

Most synthetic inhibitors are effective corrosion inhibitors that typically consist of one or more active ingredients, additives and other solvents [12]. However, they have been found to be toxic and persistent. Chromates, phosphates, silicates and heavy metals are other examples of film-forming inhibitors which have a negative environmental effect due to their persistence and toxicity. Moreover, their removal is expensive and complicated [53]. The use of these hazardous inhibitors has to be reduced in order to meet evolving environmental restrictions which are also getting firmer. Thus there is a need for alternative inhibitors which can replace them but not have any of their undesired characteristics [44].

Green inhibitors do not contain toxic components such as heavy metals and are biodegradable, inexpensive to formulate, renewable and easily available. Recently, there has been an increasing trend in employing readily available, natural products such as the extracts of leaves and fruits in corrosion inhibitors in an effort to create environmentally-friendly inhibitors, generally referred to as green inhibitors [5]. Natural or green inhibitors make use of the natural compounds found in fruits and vegetables to inhibit the corrosion of metals. The natural compounds such as minerals, vitamins, and phenolic compounds can be harnessed from extracts of various plants such as khillah seeds, neem leaves, passion fruits, cashews, African bush peppers and many more [9,30]. Buchweishaija [15] reports on a vast number of plant extracts that can be used as corrosion inhibitors in a variety of different solutions. Moreover, numerous plant extracts also have a history of previous application in traditional fields such as pharmaceuticals and biofuels [15].

Plant extracts contain organic compounds which consists of polar atoms such as oxygen, phosphorus, sulfur and nitrogen. These polar atoms can all provide inhibition by adsorbing onto the metal surface, thus forming a protective film Although there has been significant research performed on plant extracts that could potentially be used as corrosion inhibitors, little research has been conducted on the adsorption mechanisms or the identification of the active ingredient of the plant extract [54].

4.1 Synthetic inhibitors used in CO2 saturated NaCl solutions

The inhibitors from Figure 7 were tested using carbon steel in a sodium chloride solution which was saturated with CO2. For each organic inhibitor, the use of increasing concentration resulted in obtaining lower currents due to the inhibitor’s effect on the anodic and cathodic processes. The change in the cathodic reaction was observed to be more significant than the anodic reaction, which indicates that they acted as cathodic inhibitors [25].

The corrosion inhibitors seen in Figure 8, were tested on carbon steel immersed in a CO2 saturated 3% NaCl brine. The results of the experiment showed that the use of these inhibitors brought about a decrease in the overall current densities [47]. 40

Two triazole derivatives (Itraconazole and Fluconazole compounds) were used to inhibit the corrosion of API 5L-B carbon steel in CO2 saturated 3.5% NaCl solutions. These two compounds act as mixed type inhibitors whose efficiency increases with increasing concentration [46].

Figure 7. Examples of organic inhibitors [25]

Figure 8. Effective inhibitors in CO2 staurated brine for carbon steel [47]

The two generic corrosion inhibitor packages, quaternary ammonium chloride (QAC) and fatty amino, were tested based on their performance in water wetting and

CO2 corrosion in an oil-water phase system [55]. The environment created for this test

41 involved using deionized water and NaCl to create a brine solution which was then purged with CO2 until it was saturated and then adjusted to a pH value of 5.0. Conclusive results showed that QAC was not able to efficiently inhibit corrosion when present in concentrations below 20 ppm, but when present between 60 to 100 ppm, this resulted in a 90% inhibition efficiency in the brine phase [55]. Comparatively, the amino acid package was able to achieve 85% inhibition efficiency at only 2 ppm in the brine phase. This percentage shoots up to over 90% when concentrations of 5 ppm or higher are used. Moreover, the amino acid package was able to reach steady corrosion inhibition more quickly than the QAC inhibitor [55].

4.1.1 Imidazoline (IM) based inhibitors.

Organic inhibitors such as imidazoline based inhibitors are widely employed due to their availability and effectiveness. These inhibitors are known for their high inhibition ability in acidic media [24,56,57,58]. Examples of imidazoline based corrosion inhibitors used in CO2 environments include hydroxyethyl, aminoethyl, and amidoethyl imidazolines [59] and N,N-di(poly oxy ethylene) [60].

A typical imidazoline molecule is composed of “a nitrogen-containing five member ring, a pendant side chain with a hydrophilic active functional group (R1) and a long hydrophobic hydrocarbon chain (R2)” [33]. Studies point out that the hydrophilic R1 group improves corrosion inhibition while others observe that the R2 length is critical in influencing the inhibitive behavior of the IM [33,45]. IM inhibitors have a larger adsorption energy than water molecules which allows them to preferentially adsorb on the metal surface in aqueous mediums. The imidazoline ring which contains its reactive sites adsorbs on the metal through the formation of coordinate and back-donating bonds with the metal atoms on the surface [33,61].

Coconut oil was used to modify a commercial IM corrosion inhibitor and was tested in a 3% NaCl solution saturated with CO2. The results showed that this modification resulted in reducing the corrosion rate by 85%. However, the commercial IM inhibitor by itself was able to reduce corrosion by 400%. Small doses of this combination can still be used as a corrosion inhibitor [62].

Another type of commercial IM-based inhibitor was used to test two carbon steels with different microstructures in a 5 wt% NaCl solution saturated with CO2 at 40°C with a pH of 6. This inhibitor provided great efficiencies for both ferritic- pearlite and tempered martensite microstructures at just 50ppm [51,63].

2-undecyl-1-sodium ethanoate-imidazoline salt (2M2) and thiourea (TU) was used in experiments to inhibit the corrosion of N80 mild steel. The environment was a

CO2 saturated 3 wt% NaCl solution at 25°C, 1 bar and a pH of 4. The inhibition efficiency was found to increase with 2M2 concentration and decrease with increasing TU concentration. This led to the selection of an optimum balance between the two inhibitors. The compounds act as a mixed-type inhibitor where they adsorb to form a film on the metal surface [58].

An imidazoline derivative, used in a study [24], was synthesized from diethylenetrianmine, thiourea and oleic acid. This imidazoline derivative was found, by potentiodynamic polarization and EIS measurements, to inhibit corrosion of API

X65 steel in a CO2 saturated 5% NaCl (made with deionized water) environment [24]. X65 steel is a large diameter pipe typically used in the oil and gas industry due to its high strength, weldability and toughness [64]. By decreasing the polarization current density, the inhibitor inhibits both cathodic and anodic reactions thus classifying it as a mixed inhibitor. It also affects the kinetics of the anodic and cathodic reaction as well as increases the activation energy of the corrosion reaction. Its inhibition efficiency increases with higher concentration. Moreover, the inhibition film grows with higher immersion times and this can increase the inhibition efficiency [24].

A rotating cylinder electrode (RCE) was used to simulate turbulent conditions in a CO2-saturated 3% NaCl solution at 80°C with a pH value of 4. EIS and LPR measurements were performed on 1-(2-hidroxyethyl)-2(heptadec-8-enyl)-imidazoline and 1-(2-aminoethyl)- 2(heptadec-8-enyl)-bis-imidazoline which are referred to as IM-HOL and BIM-NH respectively [33]. These inhibitors, seen in Figure 9, were determined to enhance corrosion protection by forming a more compact inhibitor layer. Their molecular structure allowed lower concentrations such as 5 ppm to be able to produce up to 94% inhibition efficiency. Both inhibitors act as either an anodic

43 or a mixed type inhibitor due to the anodic shift in the corrosion potential. This may be due to the blocking of active sites [33].

Imidazoline (IM) compounds such as oleic imidazoline are among the most effective corrosion inhibitors used in oil field applications [33,65,66,67]. The corrosion protection offered by using oleic imidazoline and phosphate were found to be enhanced after direct contact between the working electrode and the oil phase. They promote oil wetting for both clean steel surfaces and when covered with a corrosion product film (FeCO3). However, even if there is no direct contact with the oil phase, its presence on top of the brine phase still improves the inhibitor’s performance [45].

Figure 9. Molecular structure of (a) IM-HOL (b) BIM-NH [33]

A 100 ppm dose of benzimidazole was used to protect carbon steel from corrosion in a deoxygenated, 40°C NaCl solution which is saturated with CO2 at a pH of 6. It was determined not to be a useful inhibitor for the experimental conditions in which it was studied [56]. Quench and tempered (Q&T) carbon steel samples were found to have a better corrosion resistance than the annealed samples in the absence of inhibitors. However, when benzimidazole was used, the corrosion resistance of the annealed sample was improved while the Q&T sample decreased [56]. 44

2-undecyl-1-ethylamino imidazoline (2UEI) can be used as an inhibitor for

N80 mild steel in 3% NaCl solutions saturated with CO2. The inhibition efficiency provided can be increased by increasing its concentration as well as the temperature. It was also found that the presence of iodide ions can enhance the inhibition efficiency of 2UEI. The inhibition mechanism employed in the absence of corrosion products is the blocking of active sites. The inhibition mechanism employed when in the presence of corrosion products is the geometric blocking effect [67].

An amido-imidazoline derivative can inhibit corrosion of API 5L X52 steel in

3 wt% NaCl solution which is saturated in CO2. It acts as a mixed-type inhibitor whose inhibition efficiency can be enhanced through the addition of iodide ions because of its synergistic effect, where iodide ions and IM molecules are co-adsorbed [68].

Rosin amide imidazoline (RAIM) was used as a corrosion inhibitor for N80 and P110 carbon steels subjected to CO2-saturated synthetic formation water. The inhibition efficiency increases with increased RAIM concentration as well as temperature. It acts as a mixed-type inhibitor where a chemically adsorbed RAIM film is formed on the metal surface through its polycentric adsorption sites [69]. The structure of RAIM can be seen in Figure 10.

Figure 10. Molecular structure of RAIM [69]

2-undecyl-1-aminoethyl imidazoline (AEI-11) and 2-undecyl-1-aminoethyl-1- hydroxyethyl quaternary imidazoline (AQI-11) were used as corrosion inhibitors for

N80 mild steel in a 3% NaCl solution saturated with CO2. Their structures can be

45 seen in Figure 11. AQI-11 was found to be more effective than AEI-11 due to its stronger electrostatic interaction as well as the additional adsorption sites provided by oxygen atoms on the surface. In flow systems, the inhibitor performance is degraded due to mechanical abrasion [66].

N80 mild steel in a CO2 saturated 3% NaCl solution can be protected using by 2-undecyl-1-1-ethylamino-1-ethylcarboxyl quaternary imidazoline (CQI). Its inhibition efficiency can be increased through increasing the concentration of CQI where it chemically adsorbs on the metal surface. It can even be enhanced through the addition of halide ions where there is a combination of chemical and coulombic attraction [70].

Figure 11. Molecular structures of AEI-11 and AQI-11 [66]

Carboxyamindo imidazoline is used as a corrosion inhibitor for X-70 pipeline steel in saltwater saturated with CO2 at 50°C. A concentration of 8.1E-05 mol/L was found to offer the best corrosion inhibition where anything greater would result in desorption [71].

4.1.2 Carboxylic & naphthenic acids.

Linear polarization resistance tests showed that a 10 ppm dose of nitrogen- containing derivatives and oleic and isomer mono carboxylic acids, resulted in a corrosion rate of less than 0.1 mm/yr. Therefore, it can be concluded that these inhibitors can be used in corrosive environments which contain CO2. As a result, they are classified as a mixed typed inhibitor since they decrease the rates of the anodic and cathodic reactions [72,73]. 46

A 10 ppm concentration of carboxylic acid N-coco-amine-1-proprionic acid

(C14H29)N(CH3)(C2H4COOH) was used to inhibit the corrosion of mild steel in a 5%

NaCl solution subjected to CO2 [74]. Previous work performed on a similar inhibitor possessing two carboxylic groups was found to behave anodically and this was also discovered to be the case with the present inhibitor [74,75].

A study conducted by Abbasov [19] reported that the sodium and potassium salt of naphthenic acids can be used to inhibit the corrosion of carbon steel in a CO 2- saturated, magnetically-stirred 1% NaCl solution prepared with distilled water [19]. The LPR corrosion rate and potentiodynamic polarization curves indicated that the salts acted like mixed inhibitors. These salts can be classified as surfactants and were synthesized through the liquid phase oxidation process of the Baku crude oil’s naphtha fraction. The two inhibitors were composed as R-COONa (Inhibitor I) and R- COOK (Inhibitor II) [19]. Increased concentrations of the inhibitor resulted in increasing the efficiency except for inhibitor II whose efficiency reached a maximum of 99.48% at 100 ppm. The inhibitors have a high equilibrium adsorption constant which shows their ability for high adsorption [19].

Out of all the carboxylic acids studied, Lauric acid was determined to be the most effective inhibitor of CO2 corrosion. This acid is volatile in spite of its molecule consisting of a relatively long alkyl fragment (C11H23) [76].

4.1.3 Polymeric compounds.

Polymeric-based inhibitors can be used to protect metal surfaces exposed to an acidic saline environment saturated with CO2. Examples include polyacrylic acid, polyaspartic acid and its salts, and polymaleic acid. Other corrosion inhibitors can include 2-ethylhexanoic acid and its alkali salts, sodium borates, benzotriazoles, triazoles, hydrocarbyl, and octanoic acid [9].

Moreover, polyspartic acid and its salts possess dispersancy properties for calcium carbonate and phosphate as well as the sulfates of calcium and barium, and can even inhibit scale formation. Depending on the size of the environment in question, low amounts of polyspartic acid (10 ppm) are adequate enough to provide

CO2 corrosion inhibition. Furthermore, polyspartic acid can even be used in CO2- containing saline mediums to inhibit corrosion of ferrous metals [8].

There are many review articles on organic inhibitors which have been used in the oil industry such as the one reported by Palou et al. [12]. In this article, Palou summarized a list of organic inhibitors and low-toxicity natural products including natural polymers [12].

4.2 Green inhibitors used in CO2 saturated NaCl solutions

Green inhibitors are those that can utilize plant extracts, bio-chemicals, ionic liquids or green inorganics or biodegradable organics [53]. Plant extracts have been used as corrosion inhibitors as early as the Middle Ages, where extracts such as yeast and bran were used in the pickling of metals by blacksmiths. Bran in particular was used as an inhibitor for iron corrosion in acidic mediums in very early reports. The first corrosion inhibition patent registered was for a plant product (molasses and vegetable oils) used to protect steel in acids [53].

The extracts from plants can be obtained easily using cheap solvents such as water or ethanol. Ethanol solvents are used when the extracts have a low solubility in water. The extracts obtained can contain various natural ingredients such as tannins, pigments, steroids, flavoids, flavones and essential oils. These extracts are conjugated aromatic structures with long aliphatic chains. The drawbacks of using green inhibitors derived from plant extracts are their low stability and the fact that they are easily biodegradable which means they have a shorter life span. However, it is possible to minimize this by using biocides [12].

4.2.1 Inhibitors based on plant extracts.

The leaf extracts of Gingko biloba (GBE) were used to inhibit carbon steel corrosion in CO2-saturated 3.5wt% NaCl solution. The extract was determined to consist of urea, bifalvones and other molecules rich in heteroatoms (N, S and O). The carbon steels used were C110, P110SS, N80 and J55 steel where a maximum of 96% inhibition efficiency was observed for J55 steel [34,77].

The seed extract of Momordica charantia was found to be good inhibitor for

J55 steel in a CO2-saturated 3.5wt% NaCl solution. The extract consists of glycosides, oils, fatty acids and saponins which are rich in heteroatoms such as nitrogen and oxygen. It was also found to act like a mixed-type inhibitor [78].

4.2.2 Sulfated fatty acid derivatives.

4.2.2.1 Sulfated fatty acid derived from corn oil. Corn oil is reacted with diethanolamine to produce the fatty acid diethanolamine amide from which sulfating syntheses are performed to create five novel surfactants of the composition R-CH-(OSO3M)-CON-(CH2-CH2-OH)2. The M can represent Na, K, NH4, –NH-CH2-CH2-OH and –N-(CH2-CH2-OH)2 [21,79].

These surfactants were used as corrosion inhibitors for mild steel in CO2- saturated NaCl solutions. They all act as mixed-type inhibitors where the inhibition efficiency increases with the concentration. The maximum efficiency observed was 99.4% for 75 ppm of Inhibitor II [79]. They are more effective than the ethanolamine salts of fatty acids [21].

Figure 12. Corn oil-derived inhibitors [79]

4.2.2.2 Sulfated fatty acid derived from sunflower oil.

A synthesized sulfated fatty acid sodium salt (SFASS) is used as an anionic surfactant inhibitor to protect carbon steel alloy in a CO2-saturated 1.0% NaCl solution. It was created from sunflower oil which underwent some processes to produce fatty acids which were then used as the base materials for the sulfonating process [80]. SFASS is a green, mixed-type inhibitor whose inhibition efficiency increases with concentration as well as temperature. Using 100 ppm of SFASS enabled an efficiency of 99.5% at 50°C and its structure can be seen in Figure 13 [80].

4.2.2.3 Sulfated fatty acid derived from vegetable oils.

Monoethanolamine sulfated fatty acid (SM) is a type of complex surfactant that was synthesized from four types of vegetable oils such as sunflower oil (SM 1), cottonseed oil (SM 2), corn oil (SM 3) and palm oil (SM 4). The oil is processed to obtain fatty acid sodium salt which undergoes further processing to extract the fatty acids which are then used to perform the sulfating syntheses to get the final product of SM whose structure can be seen in Figure 14. The SM is both easily produced with high purity and relatively inexpensive [81]. These surfactants were tested in CO2- saturated 1.0% NaCl solution and it was found that increasing its concentration from 10 to 100 ppm resulted in the decrease of the corrosion rate [81].

Figure 13. Molecular structure of SFASS [80]

The results showed that inhibition by SM 3 was higher than that of the other oils and this can be related to its molecular weight where SM 3 was the highest. The difference in fatty acid compositions for the four oils also contributes to this where the SM strength follows the trend: 3>1>2>4 [81].

Figure 14. Molecular structure of SM [81]

4.3 Inhibitors in other corrosive mediums The inhibitors discussed in this section were all used in a medium that was not a CO2-saturated salt solution. They have been divided into synthetic and green inhibitors which have been used in corrosive media such as HCl, H2SO4, HNO3 and sea water. It is possible that these inhibitors are good enough to be utilized in similar acidic mediums such as a CO2-saturated salt solution.

4.3.1 Synthetic inhibitors.

In an acidic solution, mild steel can be protected through the use of a bipyrazolic type carboxylate. The inhibitor compound’s carboxyl group bonds with the iron ions on the steel surface through complexation reactions, thus forming a coordination bond between the organic compounds adhering to the steel surface. Different types of steel will carry different surface excess charges when subjected to an acidic salt solution. This may affect the inhibitor’s adsorption behavior and thus its corrosion-inhibiting efficiency [82].

Nitrogen and sulphur-containing compounds, thioaldehydhes, acetylenic compounds, and aldehydes and alkaloids such as quinine, nicotine, papverine and strychnine can be used as inhibitors in acidic mediums [9]. Other examples include 1- cinnamylidine-3-thiocarbohydrazide and the reaction product of aldehyde and thiol or amine functionalized ring structure [83]. A combination of cinnamaldehyde and organo-sulfur can be used as an effective acid corrosion inhibitor for petroleum and water wells which also reduces the occurrence of pitting [9].

12-aminododecanoic acid is a mixed-type inhibitor which can be used to protect carbon steel in CO2-saturated HCl mediums [84]. 2-(2-alkyl-4,5-dihydro-1H-

51 imidazol-1-yl)ethanol which is a type of biodiesel-based IM can be used as a corrosion inhibitor for mild steel in HCl [62]. K1384, an amine-fatty acid corrosion inhibitor was tested in a 3% NaCl solution whose pH was adjusted to 4.5 using HCl. An inhibition efficiency of 99% was obtained using 10ppm of K1384 where the inhibition is due to physisorption [85]. 4-amino-5-phenyl-4H-1,2,4-trizole-3-thiol are used to inhibit corrosion in an HCl medium [86].

A thermally-produced polyaspartate (synthetic polypeptide built from 20 aspartic acid residues) can be used as a corrosion inhibitor for mild steel in synthetic sea water. However, it can only achieve up to 30% inhibition [8]. The addition of either 75 ppm of thiourea or 50 ppm of aluminum can result in enhancing the cathodic protection of steel in 3.5% NaCl solution and seawater by 50% and a combination can result in almost 90% enhancement [87].

N coco-amine-2-proprionic acid, which is a tertiary amine which contains two carboxylic acid groups, can be used to inhibit corrosion of mild steel in a 5% NaCl solution saturated with CO at a pH of 6.5. On a clean sample, high levels of inhibition efficiency were observed using a 10 ppm concentration. If the mild steel has been pre- corroded, this efficiency is lower but still greater than 90% [75].

Organic carboxylate compounds can be used to inhibit the corrosion of steel according to [88-90]. In neutral environments, carboxylate and carboxylic acid have been observed to be effective corrosion inhibitors. The adsorption mechanism of these compounds such as carboxylic acid is through chemisorption. In saline conditions (NaCl solution), mild steel can be protected using cerium carboxylate to inhibit corrosion [82]. Other organic chemicals such as pyrrole, pyridine and isopylamine were used successfully to protect carbon steel in an agitated saline medium [91].

Sodium tripolyphosphate (SH) and sodium hexametaphosphate (SH) have been used as mild steel corrosion inhibitors in the presence of zinc acetate where ST adsorbs through chemisorption while SH adsorbs through physisorption [92]. Mercaptopyrimidine has been found to be a nonpoisonous inhibitor which can obtain inhibition efficiencies up to 99% when used in low concentrations [40]. Chromate, phosphate, nitrite and benzoate are other inhibitors that have been used in neutral media [9].

4.3.2 Green inhibitors.

Green inhibitors such as the extracts of poinciana pulcherrima, cassia occidentlis, papaia, datura stramonium seeds, auforpio turkiale sap and calotropis procera all showed corrosion inhibition in 1N HCl. This efficiency decreases in higher concentrations of the acid due to hydrolysis of the plant protein content [54].

Three parts of a watermelon were used to create green inhibitors to be used to protect mild steel in HCl. These parts were the rind extract (WRD), peel extract (WPE) and seed extract (WSE). It was found that the WSE was stronger than WRE and WPE. It was determined that the inhibition effect was dependent on the extract’s concentration. The inhibition mechanism was the adsorption of the extract components onto the mild steel through physisorption which followed the Langmuir isotherm. Cyclic sweep tests revealed that they worked as mixed-type inhibitors [93].

The extract of Khillah seeds (Ammi visnaga) can inhibit the corrosion of steel in an HCl medium through the interaction of iron cations and khellin [94]. The extract of potato peels (PPE) has been used as a mixed-type inhibitor to reduce the corrosion of mild steel in a 2M HCl solution, reaching efficiencies of up to 90% [95].

Corrosion of carbon steel in sea water can be inhibited using an aqueous extract of banana peel (BPE) whose main constituent is bananadine [(3Z,7Z,10Z)-1- oxa-6-azacyclododeca-3,7,10-triene]. Its use in addition to 15 ppm of Zn2+ can result in 98% inhibition efficiency. It acts as a mixed-type inhibitor [96].

The aqueous extract of fenugreek leaf (AEFL) and fenugreel seed (AEFS) was studied for mild steel in sulfuric acid and HCl. AEFS was found to be stronger than AEFL in inhibiting corrosion due to the higher concentration of phytochemical constituents [93]. Okafor conducted experiments on mild steel corrosion in HCl and sulfuric acids using leaves (LV), seeds (LS) and various combinations (LVSD) of the two extracts of phyllanthis amarus [93]. Amaranthus cordatus has been found to inhibit mild steel corrosion in sulfuric acid and sodium chloride solutions. Allamanda blanchetii was found to be an effective inhibitor in sulfuric acid and citric acid [97].

Ginger extract has been used to inhibit steel corrosion in sulfide-contaminated NaCl solution. It was found that it acted as a mixed-type inhibitor whose inhibition

53 efficiency increases with concentration [98]. Spearmint plant extract was used to provide inhibition for low carbon steel in a 3.5% NaCl solution [99]. Gum exudates from acacia seyal var seyal, acacia drepanolobium and acacia sengal trees have been used to protect mild steel in fresh water. They are mixed-typed inhibitors which reduce the anodic current density [44].

Green inhibitors formulated from the extracts of piper guinensis, carica papya leaves, gum arabic, rosmarinus officinalis oil and neem leaves showed corrosion inhibition of mild steel in sulfuric acid. Numerous other types of green inhibitors are listed in review articles such as those seen in Rani et al. [54] and Negm et al. [53] where green inhibitors are reported to be used in various corrosive media. There have also been studies investigating the use of drugs as corrosion inhibitors since these heterocyclic compounds are environmentally-friendly and do not contain toxic components such as heavy metals. Thus, they can possibly compete with plant extracts.

4.4 Investigated inhibitors

The inhibitors discussed in the earlier sections demonstrate that there is a large variety of inhibitors that can be used to reduce corrosion. Most industries use synthetic inhibitors which are effective but also hazardous to the environment. In response to this, there have been many studies conducted on finding alternative inhibitors such as those derived from plant extracts. It is observed that these green inhibitors have been used in a variety of different corrosive environments to reduce the effects of corrosion.

However, there have not been many studies regarding green inhibitors tested in CO2-saturated NaCl solutions. Specifically, there has not been much work in studying green inhibitors derived from plant extracts (such as those in Section 4.2.1) where the inhibitor is formulated using reflux as the extraction method. Thus, this thesis investigates three green inhibitors which are derived from fig leaves, calotropis procera and eggplant peels and investigates how they perform in a CO2-saturated 3.5wt% NaCl solution.

The inhibitor based on fig leaves extract (FLE) were used successfully by Ibrahim and Abou Zour to inhibit mild steel corrosion in HCl [100]. The aqueous Fig leaves extract contains phenylpropanoid compounds such as polyphenols rutin, isoschaftoside, isoquercetin and chlorogenic acid [101].

The extracts of calotropis procera (CPLE) were found to be an effective inhibitor of mild steel corrosion in sulfuric acid [54]. It was also determined that the inhibition efficiency increased with temperature due to its adsorption type being chemisorption. The CPLE inhibitor acted like an anodic type inhibitor where SEM and UV analysis revealed that a complex was formed between the CPLE phytoconstituents and the mild steel. The active ingredients of CPLE are thought to be alkaloids, cellulose and proteins [102]. The aqueous Calotropis procera extract contains tyranton (C6H12O2), 1-pentadecene (C15H30), 1-heptadecene (C17H34) as well as triterpenoids. There is also evidence of the presence of long chain fatty acids and amides, carbonyls , flavonoids and saponins [103,104].

Extracts from eggplant peels (EPPE) have been reported to be a good mild steel corrosion inhibitor in HCl where its inhibition efficiency increases with concentration [105]. The peel extract of the eggplant contains the highest phenolic and anthocyanins content as well as falvonols [106]. The major class of phenolic compounds in eggplant is the esters of hydroxycinnamic acids where chlorogenic acid is the predominant constituent [107].

Thus, due to the positive results of FLE, CPLE and EPPE in their reported acidic mediums, they have been selected to investigate the inhibition of mild steel in a

CO2 saturated 3.5wt% NaCl solution, at 25°C, 40°C, 50°C and 70°C. Their performance will be compared with two commercial inhibitors denoted as Inhibitor A (natural inhibitor) and Inhibitor B (synthetic inhibitor).

Inhibitors A and B are commercially available products from a major chemical solution provider that are designed to support oil and gas industry applications with corrosion inhibition within production streams, produced water and water injection applications. Both these two formulations have proven performance in the various industrial applications and are commercially available at around 3.5 to 4 dollars per kilogram. 55

Inhibitor A is an organic solution of active polysaccharides (10-30% activity) formulated in an aqueous solvent package to serve as an environmentally-friendly corrosion inhibitor which does not contain nitrogen. It can be used in downstream applications in the refining process stream. Inhibitor B is a synergistic mix of corrosion inhibition active ingredients (10-30% activity) of ammonium salts with salted imidazoline which both contain nitrogen atoms. It is formulated in an aqueous solution to give a wide range of protection under variable oilfield production and operating conditions. The nitrogen and sulfur atoms present in this inhibitor allow it to provide superior performance but it also adversely affects the environment if disposed of in areas such as oceans, disposal wells and evaporative ponds.

Chapter 5. Experimental Methodology

This section will cover the preparation of the natural inhibitors and mild steel samples. An overview of the tests performed such linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and cyclic sweep (CS) tests will also be discussed. Moreover, techniques such as Fourier transform infrared (FTIR) and scanning electron microscope (SEM) analysis will be presented.

5.1 Preparation

5.1.1 Preparation of natural inhibitors.

The natural inhibitors were all prepared in the same manner. Fig leaves/Calotropis procera/Eggplant peels were rinsed with doubled distilled water (DDW) and left to air-dry overnight. They were then further dried in a fluidized bed heater at 80°C for 2 hours to reduce their water content. The dried leaves were then crushed into smaller pieces and refluxed in 500 mL of DDW for 2 hours to prepare the inhibitor solution. This was then filtered by vacuum filtration to obtain the final stock solution which was then stored in a glass container. The concentration of this solution was determined by drying a sample volume and comparing it to the residue weight. Thus, different concentrations of inhibitor can be prepared from the stock solution through dilution.

5.1.2 Preparation of mild steel specimens.

Rectangular mild steel panels were procured from PRO TEST PANELS LTD. and then cut into circular pieces. The mild steel panels have a composition of 0.037 C, 0.0090 P, 0.0140 S, 0.151 Mn, 0.001Si, 0.026 Sa, 0.0028 N, 0.017 Cr, 0.001 Mo, 0.028 Ni, 0.028 Al, 0.0020 As, 0.0029 Bo, 0.001 Co, 0.019 Cu, 0.001 Nb, 0.004 Sn, 0.001 Ti, 0.001 V, 0.0002 Ca, 0.069 CEV, and Fe makes up the balance.

5.1.3 Preparation of test solution.

The mild steel specimen was placed in an inert holder which was immersed in a 700 mL test solution prepared using the required volume of stock inhibitor needed

57 for a specific concentration, with the remaining volume made up of 3.5 wt% sodium chloride solution.

5.2 Electrochemical tests

A GILL AC potentiostat from ACM Instruments was used to perform electrochemical tests. The electrochemical measurements conducted include linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and potentiodynamic sweeps or cyclic sweep polarizations (CS).

5.2.1 Linear polarization resistance (LPR).

LPR is an effective electrochemical technique used to measure corrosion rapidly and has been used in industry for more than half a century [108]. A small potential is applied between the electrode, and the resulting current is measured. This potential does not affect the natural corrosion process. Thus, the LPR technique can be used to assess the corrosion rate non-destructively.

The applied potential causes the electrode to corrode at a rapid rate and produce a current due to the metal ions which pass into the solution. Thus, a low polarization resistance indicates a high corrosion rate [108]. The purpose of this test is to measure the polarization resistance (Rp) which indicates the oxidation resistance of a metal. The polarization resistance can be determined using the Stern-Geary equation [109]:

풃풂 ∗ 풃풄 푹풑 = (ퟐퟒ) ퟐ. ퟑ (푰풄풐풓풓)(풃풂+ 풃풄)

where Rp is the polarization resistance, Icorr is the corrosion current and ba and bc are the anodic and cathodic Tafel slopes.

The LPR test is conducted using a scan from -10mV to +10mV. Each scan is taken every five minutes for twenty two measurements to build a long-term LPR graph. The inhibition efficiency using the LPR values is determined by:

푳푷푹 − 푳푷푹 푰푬% = 풊 풃풍풂풏풌 ∗ ퟏퟎퟎ (ퟐퟓ) 푳푷푹풊

where 푳푷푹풊 is the value at the selected inhibitor concentration and 푳푷푹풃풍풂풏풌 is the value obtained in the absence of inhibitors.

5.2.2 Electrochemical impedance spectroscopy.

The EIS or AC Impedance test analyses the resistive and capacitive characteristics of a corrosion cell using a scan of test frequencies. The solution resistance (Rs), polarization resistance (Rct) and double layer capacitance (Cdl) can be determined using curve fitting software from the data obtained from the EIS test.

In the last decade, EIS has been widely used as a powerful tool to characterize the corrosion process and the performance of protective pretreatments and organic coatings. It is a non-destructible electrochemical technique which can be used to determine the corrosion kinetics of applications such as the accelerated aging of a coated system [110]. EIS is used in the corrosion study of protective films and scale analysis to provide information about corrosion and the protection mechanism. It is also used in monitoring the inhibitor film persistency [24].

In this experiment, the EIS test was conducted using a start frequency of 0.1Hz, finish frequency of 1000Hz and amplitude of 20mV peak to peak. The impedance response of an electrochemical cell should be linear since it indicates that the response is independent of the perturbation amplitude. A very small amplitude results in a high signal-to-noise ratio while a large value of amplitude can result in a non-linear response. Thus, a small amplitude signal, such as 10mV, superimposed on the interface’s DC potential offers many advantages. For example, different types of perturbing signals result in the same quantity and information obtained [77].

The Nyquist plots can be likened to a circuit which consists of a classic parallel capacitor (Cdl) and a resistor (Rct) which are connected in series to Rsol. Rsol is the resistance of the solution, Rct is the resistance-to-charge transfer and Cdl is the double layer capacitance. The plot consists of Z'' against Z' where Z'' is the imaginary impedance and Z' is the real impedance.

An equivalent circuit model is used to fit the impedance spectra for the investigated inhibitors in a CO2-saturated 3.5wt% NaCl solution. In this equivalent circuit (Figure 15), CPE is a constant phase element. CPE represents the double layer 59 capacitance which can be seen as a non-homogeneity or rough, porous surface in the system. Thus, the CPE can be used to fit the data obtained accurately and is determined using:

푹풄풕 풁푪푷푬 = 푹풔 + 풏 (ퟐퟔ) ퟏ + 푹풄풕풀(풋풘)

풏흅 풋( ) 풏흅 풏흅 (풋풘)풏 = 풘풏 풆 ퟐ = 풘풏 (퐜퐨퐬 + 풋 퐬퐢퐧 ) (ퟐퟕ) ퟐ ퟐ

Figure 15. Equivalent circuit [77]

′ After simplification using 푍퐶푃퐸 = 푍 + 푗푍′′, the equation of the depressed semi-circle which is obtained from the Nyquist plots is:

푹 ퟐ 풏흅 ퟐ 푹 풏흅 (풁′ − 푹 − 풄풕) + (−풁풏 + 푹 퐜퐨퐭 ) = ( 풄풕 퐜퐬퐜 ) (ퟐퟖ) 풔 ퟐ 풄풕 ퟐ ퟐ ퟐ

푅푐푡 푛휋 Cdl is obtained from the center at (푅 + ,푅 cot ), where the x-coordinate is the 푠 2 푐푡 2 2 푛 2 maximum in the Nyquist plot (Z’). This occurs when 1 − Rct (Y 푤푚푎푥 ) = 1 and thus:

ퟏ 풘풏 = (ퟐퟗ) 푹풄풕 풀

The capacitance represented by CPE is expressed as:

ퟏ ퟏ 풏 ( ) 풀 ퟏ ퟏ 푹 풀 ퟏ−풏 풁 = = = 풄풕 = 풀 (푹 풀) 풏 (ퟑퟎ) 푪푷푬 풘 풏 푹 ퟏ 푹 풀 풄풕 풎풂풙 풄풕 ퟏ 풏 풄풕 푹풄풕 ( ) 푹풄풕 풀

60 where Y is the CPE constant related to the capacity of the double layer

−1 푛 −2 (Ω 푠 푐푚 ), j is the square root of -1, 푤푚푎푥 is the angular frequency for Z’’ in the Nyquist plot and n is the degree of roughness or heterogeneity of the surface. The constant n for the CPE constant is 0 for resistance (Y=R), -1 for inductance (Y=L), +1 for capacitance (Y=C) and 0.5 for Warburg impedance (Y=W).

풏 풀푪푷푬 = 풀풐(풋풘) (ퟑퟏ)

ퟏ −ퟏ 풁푪푷푬 = ((풋 ∗ ퟐ흅풇풎풂풙)풏) (ퟑퟐ) 풀풐 where 푌퐶푃퐸 is the admittance and 푓푚푎푥 is the AC frequency at maximum.

Thus Cdl can be determined using:

풀 풘풏−ퟏ 푪풅풍 = 풏흅 (ퟑퟑ) 퐬퐢퐧 ퟐ

The increase in the Cdl value can be attributed to the increase in the electrical double layer thickness or the decrease of the local dielectric constant. Cdl can also be expressed using the Helmholtz model:

휺 휺 푪 = ퟎ 푺 (ퟑퟒ) 풅풍 풅

-12 -1 where 휀0 is the permittivity of is free space (8.854E Fm ), 휀 is the local dielectric constant of the medium, d is the protective layer thickness and S is the surface area of the electrode [77].

The relaxation time (Ʈ) is the period of time needed for the charge distribution to return to the equilibrium point after an electrical disturbance. If there is no disturbance introduced to replace the Cdl, then the relaxation time is given as:

ퟏ 훕 = = 푪풅풍 푹풄풕 (ퟑퟓ) ퟐ흅풇풎풂풙

A large charge transfer resistance indicates that the system is corroding more slowly. Inhibitor molecules adsorb onto the surface of the metal forming a barrier. The double layer between the electrolyte and metal surface is considered an electrical capacitor. The inhibitor molecules remove water molecules and other ions that were 61 adsorbed on the metal surface, thus reducing its electrical capacity. The inhibitor molecules must have a lower dielectric constant to be able to displace the water molecules which have a higher dielectric constant.

Rct affects the corrosion rate of mild steel and thus the inhibition efficiency is calculated by:

푹풄풕 − 푹풄풕 푰푬% = 풊 풃풍풂풏풌 ∗ ퟏퟎퟎ (ퟑퟔ) 푹풄풕풊 where 푹 is the value corresponding to the selected inhibitor concentration and 풄풕풊 푹 is the value obtained in the case without the inhibitor. 풄풕풃풍풂풏풌

5.2.3 Potentiodynamic sweeps. The corrosion behavior of metals and alloys can be evaluated using potentiodynamic polarization curves. This method is also known as potential sweep, cyclic voltammetry or linear sweep voltammetry. This widely used technique in electrochemistry measures the current that develops from varying the electrode potential between a selected range. It can identify potential regions where there might be electrode activity [111].

The cyclic sweep test was conducted using a range from -150mV to +150mV with a sweep rate of 60mV/min. The sweep rate affects the linearity and smoothness of the polarization curves. The cathodic portion of the Tafel plot was scanned from - 150mV to 0 while the anodic portion was scanned from 0 to +150mV.

The corrosion process was modelled using the assumption that the rates of the cathodic and anodic mechanisms are determined by the kinetics of electron transfer reaction at the metal surface. The Tafel equation is given by:

ퟐ.ퟑ(푬−푬ퟎ) 휷 푰 = 푰ퟎ 풆 (ퟑퟕ) where I is the resultant current from the reaction, 퐼0 is the exchange current (reaction dependent constant), E is the electrode potential, 퐸0 is the equilibrium potential which is constant for a given reaction and β is the reaction Tafel constant [77].

In a corrosion system, there are two opposing reactions which are anodic and cathodic. The Tafel equation for these two reactions can be obtained to produce the Butler-Volmer equation:

ퟐ.ퟑ(푬−푬ퟎ푪) ퟐ.ퟑ(푬−푬ퟎ푪) 훃 훃 푰 = 푰풂 + 푰풄 = 푰풄풐풓풓 풆 풂 − 풆 풄 (ퟑퟖ)

where 퐼푐표푟푟 is the corrosion current (in amperes), βa is the anodic beta Tafel,

βc is the cathodic beta Tafel constant and Eoc is the corrosion potential

Consider the equation:

풏푭 풏푭 −휷 휼풓풙풏 ퟏ−휷 휼풓풙풏 푰풓풙풏 = 푰ퟎ (풆 푹푻 − 풆 푹푻 ) (ퟑퟗ)

When 휂푟푥푛 is cathodic (negative), the second term in the above equation becomes negligible and thus:

풏푭 −휷 휼풓풙풏 푰풓풙풏 = 푰풄 = 푰ퟎ (풆 푹푻 ) (ퟒퟎ)

푰풄 or 휼풓풙풏 = 휼풄 = 풃풄 퐥퐨퐠 ( ) (ퟒퟏ) 푰ퟎ where Ic is the cathodic current and bc is the cathodic Tafel constant and is given by:

푹푻 풃 = −ퟐ.ퟑퟎퟑ (ퟒퟐ) 풄 휷풏푭

However, when 휂푟푥푛 is positive (anodic), the first term of the equation 39 becomes negligible and thus:

풏푭 ퟏ−휷 휼풓풙풏 푰풓풙풏 = 푰풂 = −푰ퟎ (풆 푹푻 ) (ퟒퟑ)

푰풂 or 휼풂 = 풃풂 퐥퐨퐠( ) (ퟒퟒ) 푰ퟎ where ba is given by:

푹푻 풃 = −ퟐ.ퟑퟎퟑ (ퟒퟓ) 풂 휷풏푭

The extrapolation of the Tafel straight line determines the corrosion current density (Icorr) [77]. The inhibition efficiency can be calculated using:

푰풄풐풓풓 − 푰풄풐풓풓 푰푬% = 풃풍풂풏풌 풊 ∗ ퟏퟎퟎ (ퟒퟔ) 푰풄풐풓풓풃풍풂풏풌

5.3 Fourier Transform infrared spectrometry Fourier transform infrared (FTIR) is used to identify the functional groups of the investigated extracts of FLE, CPLE and ELE. FTIR is a type of infrared (IR) spectroscopy which is used to identify unknown materials. It is able to determine the quantity as well as quality of components in a sample. The FTIR spectrometer used was the Spectrum One Model and was manufactured by Perkin Elmer. The number of scans taken per sample was 10.

The sample to be analyzed must be created using a series of steps. The first step is to dry a small volume of the inhibitor solution to obtain the solid residue. This residue is mixed with KBr powder and ground using a miniature mortar. The resultant powder mix is then inserted into a device where it is compressed at a high pressure to form a solid circular disk. This disk will be used as the sample for FTIR analysis. Initially, a background spectrum (without the sample) is taken in order for there to be a relative scale for absorption intensity. Ultimately, this leads to obtaining a spectrum which has all the instrument characteristics removed, thus making the spectrums obtained from samples only contain their spectral features [112].

In the FTIR test, IR energy is emitted from a glowing black body source where it passes through an aperture which limits the amount of energy that is exposed to the sample and detector. The emitted beam enters the interferometer before passing through the sample. The beam is manipulated through spectral encoding thus resulting in an interferogram signal. This beam then hits the sample surface where it is transmitted through or reflected. The specific frequencies of energy are absorbed depending on the unique characteristics of the sample in this step. The beam passes through the sample and reaches the detector where the special interferogram signal is measured. The measured signal is digitized and sent to the PC where the Fourier transformation occurs. Finally, the resultant IR spectrum is obtained and can be

64 further manipulated using various tools, such as smoothening, and can be interpreted. This process can be seen in Figure 16.

The spectrum obtained using FTIR consists of absorption peaks which are associated with the frequency of vibration between the bonds of the unknown component in the sample. This can be likened to the molecular fingerprint of the sample due to the fact that no two unique molecular structures produce the same IR spectrum. The size of the peaks also indicate the amount of the component present in the sample and thus FTIR is said to be an excellent tool for quantitative analysis.

Figure 16. Process depicting FTIR analysis [112]

5.4 Scanning electron microscope The scanning electron microscope (SEM) is able to generate a variety of signals, using focused beams of high energy electrons, at the surface of solid samples. SEM is able to use the signals from the sample-electron interactions to obtain information such as the texture (external morphology), chemical composition, orientation of components as well as the crystalline structure of the sample. The VEGA series 3 SEM model was used to study the samples. The purpose of SEM analysis is to ascertain the presence of the inhibitor on the mild steel surface.

SEM is used extensively in the study of solid materials, especially in terms of characterization. Typically, the data is obtained from the sample surface using a selected area (ranging from 1 cm to 5 microns) which is represented as a 2D image. The magnification can range from 20X to 30,000X with a spatial resolution of 50 to 100 nm. Moreover, SEM analysis is considered non-destructive since the sample- electron interactions do not result in volume loss of the sample [113].

The apparatus inside the SEM can be seen in Figure 17. The electrons emitted in a SEM contain kinetic energy which is dissipated when it interacts with the surface of the sample. Secondary electrons produce the SEM image along with backscattered electrons. The secondary electrons show the morphology and topography of the sample while the backscattered electrons are used to illustrate the contrast in composition in multiphase samples. Diffracted backscattered electrons are used to determine the crystal structure and orientation of components in the sample [113].

Figure 17. Schematic of the Scanning Electron Microscope [113]

5.5 Electrochemical cell setup A three electrode cell system is employed to measure the corrosion rate of the mild steel sample which is immersed in the CO2-saturated 3.5wt% NaCl solution. This system utilizes a saturated calomel electrode as the reference, a platinum electrode as the auxiliary and a metal specimen held in a non-conductive holder as the working electrode. These electrodes are connected to the Gill AC potentiostat that is connected to the PC and this can be seen in Figure 18.

Figure 18. Connections between the potentiostat, PC and corrosion cell system [21]

The working electrode (WE) is the metal specimen whose exposed surface area of 1 cm2 will corrode. The reference electrode (RE) acts as a stable reference for which the applied potential can be measured accurately. The reference electrode must be chosen depending on the temperature and type of environment involved. In a chlorine-free environment, the use of a saturated calomel electrode which contains potassium chloride is a poor choice since the chloride ions will leak out of the glass frit which separates the solution from the reference electrode [109]. The auxiliary 67 electrode (AE) provides the applied current and should thus be corrosion-resistant. Therefore, materials such as platinum will ensure it will not corrode during the experiment and alter the solution chemistry thus affecting the experimental results. Moreover, the size of the counter electrode should be scaled with the amount of current required [109].

The cell body is fabricated from Teflon and has a removable cover which can be fixed to the cell body using six clamps. The cell cover consists of five openings for the WE, RE, AE and the CO2 inlet and outlet tubes. Electrode clamps are attached in these openings and hold the required electrodes and tubes in tandem with external clamps from a stand. Moreover, the electrode clamps tighten around the electrodes and tubes, thus making the system closed. The experimental setup can be seen in Figure 19.

The inside of the cell is rinsed with DDW before the required volume of salt water is poured into the cell using a graduated cylinder. The required volume of the inhibitor, measured using a syringe, is then added to this solution. The WE is then prepared and cleaned along with the mild steel sample while wearing clean gloves. The RE, AE, as well as the cell cover openings are also rinsed with DDW. The electrodes are then immersed into the test solution where the electrode clamps on the cell cover are used to secure the RE and AE and make sure their tips are level with the middle of the WE. The bubbler, used to release CO2 into the solution, is cleaned with

DDW before it is immersed in the test solution. The CO2 inlet and outlet connections are then made from the cell cover. The necessary connections from the potentiostat to its respective electrodes are then made after which the software from the PC is opened and the desired test settings are made.

After the experiment run is over, the spent test solution is disposed into a sink and the cell is rinsed with tap water followed by DDW. The cell is then be rinsed with 0.1M HCl and 0.1M NaOH to ensure no traces of inhibitor are left. The cell is then rinsed with DDW once more before the next experiment.

Figure 19. Experimental setup at (a) 25°C and (b) 40°C, 50°C and 70°C (± 2°C)

Chapter 6. Results and Discussion

The results of the electrochemical tests for each of the selected inhibitors are presented in this section. The results for each inhibitor are divided into separate sections which contain the results from the LPR, EIS and CS tests. Moreover, from the results obtained, different types of adsorption isotherms are fitted and the free energy of adsorption, enthalpy and entropy is determined. The activation energy is also found using the Arrhenius plot. Furthermore, the FTIR and SEM results are also presented at the end of this section.

6.1 Fig leaf extract (FLE) Five concentrations of FLE were used to investigate the corrosion inhibition of mild steel in a CO2-saturated 3.5wt% NaCl solution. The results from the experiments performed at 25°C, 40°C, 50°C and 70°C are presented in this section. There are five sections, each which respectively show the LPR results, the EIS results, the CS results, the adsorption isotherms and the Arrhenius plot for FLE.

6.1.1 Linear polarization resistance results for FLE. The potential, corrosion current density, LPR value, time and corrosion rate is measured at every scan. A steady state is usually achieved after 90 minutes for each FLE concentration. The results of using the LPR method for the FLE inhibitor at 25°C, 40°C, 50°C and 70°C (± 2°C) are presented in this section.

6.1.1.1 Corrosion rate and inhibition efficiency from LPR. The corrosion rates of mild steel can be seen in Figure 20 where it is observed that increasing amounts of FLE inhibitor brought about a decrease in the corrosion rate. The inhibition efficiency, which is determined using the corrosion current density, is seen to increase with FLE concentration.

It is seen from Figure 20(a) that the addition of FLE concentration causes the corrosion rate to decrease, when the temperature is fixed. The addition of a small amount of FLE, such as 25 ppm, results in a drastic drop in the corrosion rate of mild steel. Further additions of the inhibitor causes the corrosion rate to fall further until it gradually reaches 0.054 mm/yr at 200 ppm of FLE. It is also noticed that when the 70 temperature is increased, the corrosion rate, for a fixed concentration, increases. It is observed that the addition of FLE inhibitor at elevated temperatures still results in reducing the corrosion rate.

3.0 (a) 25°C 2.5 40°C

50°C 70°C 2.0

1.5

1.0 Corrosion (mm/yr)rate

0.5

0.0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 20. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of fig leaves extract at elevated temperatures from the LPR test

From Figure 20(b), it can be said that increasing the temperature results in reducing the inhibition efficiency (IE). It is seen from the 25°C and 40°C cases that 71 the inhibition efficiency is not significantly affected by the temperature increase. This is not the case for IE at 50°C and 70°C, where IE becomes lower. However, the general trend observed is that the IE increases with FLE concentration and decreases at elevated temperatures. The highest inhibition efficiency observed is 95% when 200ppm of FLE is used at 40°C. The IE curve starts to plateau at a maximum value at low concentrations and thus indicates that FLE is a good inhibitor.

6.1.1.2 LPR parameters. The LPR test provides insights about the LPR value, corrosion potential and corrosion current density for each FLE experiment.

Table 1. Parameters obtained from the LPR test for FLE

T C (ppm) LPR (ohm.cm²) Icorr (mA/cm²) Potential (mV) IE% 0 243.5 0.0662 -710.2 - 25 1287.4 0.0156 -651.8 81 50 2917.9 0.0066 -639.2 92 25°C 100 3675.8 0.0055 -640.0 93 150 3746.9 0.0054 -636.9 94 200 4017.2 0.0047 -633.2 94 0 178.3 0.1435 -719.8 - 25 486.8 0.0441 -698.5 63 50 967.2 0.0172 -678.0 82 40°C 100 2464.1 0.0075 -657.9 93 150 1598.4 0.0126 -664.4 89 200 2587.4 0.0078 -652.8 93 0 132.7 0.2040 -722.9 - 25 270.9 0.0628 -704.6 51 50 483.6 0.0535 -698.7 73 50°C 100 632.1 0.0326 -684.3 79 150 789.1 0.0328 -680.3 83 200 910.1 0.0235 -676.6 85 0 128.1 0.2052 -739.0 - 25 177.0 0.1309 -737.6 28 50 274.6 0.0844 -732.0 53 70°C 100 327.2 0.0662 -727.4 61 150 478.3 0.0482 -720.2 73 200 559.1 0.0403 -721.5 77

It is seen from Table 1 that increasing the FLE concentration results in the increase of the polarization resistance. The LPR value is the long-term polarization resistance that is caused by the protective film formed over the mild steel. The protective film is formed through the adsorption of the FLE molecules onto the mild steel surface. Increased values of LPR result in a lower current density that passes through the solution. This is confirmed by the decrease of the corrosion current which passes through the solution as the LPR increases. Moreover, the addition of FLE also tends to lead to a decrease in the corrosion potential. The positive shift in the corrosion potential is indicative of a lower corrosive attack. These trends signify that the addition of FLE inhibitors does reduce the corrosion rate.

The IE seen in Table 1 is based on the LPR value where it is observed that the IE increases with FLE concentration. This is similar to the trends of the IE based on the corrosion current density.

25°C 4000 40°C 50°C 70°C

3000

2000

(ohm.cm²) LPR 1000

0 0 50 100 150 200 C oncentration (ppm)

Figure 21. Effect of FLE concentration on the steady state LPR value obtained

The LPR values obtained for each FLE concentration at 25°C, 40°C, 50°C and 70°C (± 2°C) can be seen in Figure 21. This graph clearly illustrates that increasing the temperature brings about a decrease in the LPR value for a specific FLE concentration. This can be seen by the downward shift of the plot from 25°C to 70°C. 73

It is also seen that as the FLE concentration increases, so does the polarization resistance of the metal. It is noticed that the case of 150ppm of FLE at 40°C does not follow the observed trend. This anomaly could be due to the desorption of FLE molecules from the mild steel surface [100].

6.1.2 Electrochemical impedance spectroscopy results for FLE. The impedance test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of FLE. The test was conducted at 25°C, 40°C, 50°C and 70°C (± 2°C).

6.1.2.1 Corrosion rate from EIS. The corrosion rates of mild steel can be seen in Figure 22. It can be seen that increasing amounts of FLE inhibitor brought about a decrease in the corrosion rate. This is similar to the corrosion rate determined using the LPR method. The inhibition efficiency is determined using the corrosion current density.

It is observed that the plots in Figure 22 are similar to the plots in Figure 20. This confirms that the EIS and LPR tests are similar. It is again noticed that the case of 150 ppm of FLE at 40°C is the only anomaly in the observed trend in both LPR and EIS tests. Although it is within the margin of error, this anomaly could be due to desorption or a possible experimental error.

6.1.2.2 EIS parameters and plots. This section consists of the impedance parameters obtained as well as the Nyquist and Theta-frequency plots which depict different concentrations of FLE at 25°C, 40°C, 50°C and 70°C, respectively.

The results from Table 2 confirm that the addition of FLE results in the increase of Rct and decrease of Cdl. This is an indication of the decrease in formation of the anodic process controlling intermediates from the dissolution of mild steel. The

Cdl values may have decreased due to the protective film formed by FLE on the mild steel surface. These molecules are believed to have replaced the water molecules on the mild steel surface and thus decrease the diffusion rate of the reactant elements.

25°C 4 (a)

40°C 50°C 3 70°C

1 Corrosion (mm/yr)rate

0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 Concentration (ppm)

Figure 22. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of fig leaves extract at elevated temperatures from the EIS test

Figure 23 illustrates that the addition of FLE results in the increase of the diameter of the semi-circle in the Nyquist plots. This indicates the strengthening of the film formed by FLE. This trend is observed at all temperatures. This film is also thought to suppress the kinetics of the cathodic and anodic reactions [105].

The phase angle is plotted against the log of frequency as seen in Figure 24. These plots are useful since they can define a domain of pure capacitive behavior. At high frequencies, the phase angle decreases to almost zero and this is characteristic of a response to resistive behavior. At intermediate frequencies, the maximum phase angles obtained at various concentrations of FLE can be seen in Table 3.

Table 2. Impedance parameters obtained using FLE

풎푭 Cdl ( ) T C (ppm) Rsol (ohms.cm²) Rct (ohms.cm²) 풄풎ퟐ IE%

0 16.53 221.2 367.60 - 25 17.03 1618.0 43.47 86 50 15.95 3217.0 37.82 93 25°C 100 16.61 3988.0 36.20 94 150 16.02 3789.0 40.35 94 200 16.49 4376.0 43.76 95 0 14.53 138.6 231.60 - 25 13.95 501.2 90.49 72 50 14.27 1058.0 49.21 87 40°C 100 15.13 2644.0 39.29 95 150 15.22 1630.0 52.51 91 200 15.03 2888.0 51.98 95 0 11.24 105.2 321.60 - 25 11.40 246.2 136.30 59 50 12.85 437.3 86.92 77 50°C 100 11.76 616.7 75.02 84 150 12.37 761.5 65.83 87 200 11.33 901.3 61.88 89 0 8.80 84.1 306.80 - 25 8.33 159.1 228.20 47 50 8.96 258.8 162.30 68 70°C 100 9.67 286.0 152.60 71 150 8.79 440.3 121.60 81 200 8.60 511.7 118.70 84

Ideally, the phase angle should be 90° which is indicative of capacitive behavior. The difference between the maximum phase angle and 90° is thought to be due to deviations that arise from ideal capacitive behavior. The phase angle gradually approaches the ideal capacitive value due to the slowing down of the dissolution rate with time. Thus, in the presence of inhibitors, it is observed that the phase angle becomes closer to 90° through a faster attainment of a steady state [77].

2000 1200 Blank Blank (a) FLE-25ppm (b) FLE-25ppm

FLE-50ppm 1000 FLE-50ppm

1500 FLE-100ppm FLE-100ppm FLE-150ppm 800 FLE-150ppm FLE-200ppm FLE-200ppm 1000 600

Z'' Z'' (ohm.cm²) 400 Z'' Z'' (ohm.cm²) 500 200

0 0 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 Z' (ohm.cm²) Z' (ohm.cm²) 400 200 Blank (d) Blank (c) FLE-25ppm FLE-25ppm

FLE-50ppm FLE-50ppm

300 FLE-100ppm 150 FLE-100ppm FLE-150ppm FLE-150ppm FLE-200ppm FLE-200ppm

200 100 Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 100 50

0 0 0 200 400 600 800 1000 1200 0 200 400 600 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 23. Nyquist plots for fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C from the EIS test

Table 3. Summary of maximum phase angles obtained for FLE Phase angle (°)

C (ppm) 25°C 40°C 50°C 70°C

0 47.26 45.36 43.19 44.05

25 70.71 57.54 52.85 48.94

50 75.46 66.74 59.17 53.17

100 74.43 72.10 63.11 53.16

150 73.81 69.25 65.12 59.54

200 73.91 71.22 67.41 60.62

90 90 Blank Blank 80 25ppm (a) 80 25ppm (b) 70 50ppm 70 50ppm 100ppm 100ppm

60 60

150ppm 150ppm 50 200ppm 50 200ppm 40 40

30 30

Phase angle Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency 90 90 Blank (c) Blank (d) 80 25ppm 80 25ppm 70 50ppm 70 50ppm 100ppm 100ppm

60 150ppm 60

150ppm 50 200ppm 50 200ppm 40 40

30 30

Phase Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency

Figure 24. Theta-frequency plots for fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

6.1.3 Cyclic sweep results for FLE. The cyclic sweep test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of FLE.

6.1.3.1 Corrosion rate from CS. The corrosion rates of mild steel can be seen in Figure 25, where it is observed that increasing amounts of FLE inhibitor brought about a decrease in the corrosion rate. This trend is similar to the corrosion rate determined using the LPR and EIS method. It is seen from Figure 25(a) that the addition of FLE concentration causes the corrosion rate to decrease when the temperature is fixed. However, when the 78 temperature is increased, the corrosion rate, for a fixed concentration, increases. This is similar to the corrosion plots obtained from the LPR and EIS tests with the only difference being the magnitude of the observed values.

From Figure 25(b), the increase in temperature results in reducing the inhibition efficiency (IE). It is only for the 50ppm case at 25°C where the inhibition efficiency is lower than at 40°C. However, the general trend observed is that the IE increases with FLE concentration and decreases at elevated temperatures. This is similar to the trends observed in the LPR and EIS results.

7 (a) 25°C 6 40°C

50°C 5 70°C

2 Corrosion (mm/r)rate

0 0 50 100 150 200 Concentration (ppm)

100 (b)

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 25. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of fig leaves extract at elevated temperatures from the CS test

6.1.3.2 CS parameters and plots. The corrosion current value is observed to decrease with FLE concentration

(Table 4). It is observed that there is a greater change in the cathodic Tafel slope (bc) than the anodic Tafel slope (ba). This might indicate that FLE does not affect the anodic corrosion process [114].

Table 4. Tafel parameters obtained from various FLE concentrations at elevated temperatures

T C (ppm) Ecorr ba (mV) bc (mV) Icorr IE% 0 -711.0 58.0 702.2 0.2514 - 25 -651.4 55.2 203.0 0.0446 82 50 -638.0 61.2 131.0 0.0420 83 25°C 100 -635.7 65.9 155.3 0.0264 89 150 -635.4 67.8 144.5 0.0142 94 200 -632.8 60.7 150.2 0.0137 95 0 -721.4 65.8 559.3 0.3841 - 25 -699.9 60.7 265.5 0.0756 80 50 -680.3 82.4 204.5 0.0517 87 40°C 100 -660.7 62.0 135.0 0.0493 87 150 -664.9 66.9 150.3 0.0486 87 200 -654.6 62.5 131.9 0.0297 92 0 -725.5 66.7 570.0 0.4811 - 25 -713.0 65.0 370.7 0.1807 62 50 -699.7 77.2 255.8 0.1126 77 50°C 100 -690.4 76.0 199.9 0.1028 79 150 -681.4 79.3 195.6 0.0979 80 200 -681.0 80.7 196.8 0.0895 81 0 -741.8 70.3 433.1 0.4852 - 25 -739.7 60.2 464.6 0.2771 43 50 -734.2 59.7 422.0 0.1835 62 70°C 100 -730.2 58.7 329.8 0.1490 69 150 -723.5 65.3 280.3 0.1236 75 200 -722.8 66.2 238.9 0.0924 81

The cyclic sweep test was performed to measure the change in the cathodic and anodic behaviour of the mild steel. This respectively controls hydrogen reduction and metal oxidation. It is observed in the plots from Figure 26 that there was a clear

80 shift in both the anodic and cathodic Tafel curves with addition of FLE and this resulted in the decrease of the current density and thus corrosion rate as well. The anodic shift is more noticeable than the cathodic shift from the blank case. This indicates that the metal oxidation rate decreased. Thus, the adsorbed film decreases the metal dissolution at a higher rate than hydrogen reduction.

-500 -500 (a) (b) -550 -550

-600

-600 -650 -650 -700 -700

-750 Blank -750 Blank Potential (mV) FLE-25ppm Potential (mV) FLE-25ppm -800 FLE-50ppm -800 FLE-50ppm FLE-100ppm FLE-100ppm -850 FLE-150ppm -850 FLE-150ppm FLE-200ppm FLE-200ppm -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2) -500 -550 (c) (d) -550 -600

-600

-650 -650 -700 -700 -750 -750

Blank Blank

Potential (mV) Potential (mV) FLE-25ppm -800 FLE-25ppm -800 FLE-50ppm FLE-50ppm FLE-100ppm -850 FLE-100ppm -850 FLE-150ppm FLE-150ppm FLE-200ppm FLE-200ppm -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2)

Figure 26. CS plots for FLE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The corrosion potential shift from the blank (uninhibited) case to the various Tafel curves that correspond to the FLE concentrations is seen to be less than 85 mV at all temperatures. It is known that an inhibitor that is able to displace the corrosion potential more than 85mV, with respect to the blank case, can be classified as an

81 anodic or cathodic inhibitor. Thus, it can be said that FLE acts as a mixed inhibitor [114].

6.1.3.3 Difference between test types. The inhibition efficiency determined from the corrosion current density obtained from the LPR, EIS and CS tests are compared at different temperatures. The IE obtained from the LPR, EIS and CS methods follow the same trend at their respective temperatures. The difference between the IE of each test is observed to be minimal.

100 (a) 100 (b)

80 80

60 60

IE% IE% 40 40 LPR LPR 20 EIS 20 EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm) 100 100 (c) (d) 80 80

60 60

IE% IE% 40 40

20 LPR 20 LPR EIS EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm)

Figure 27. Comparison of inhibition efficiency between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C for FLE

6.1.4 Adsorption isotherms for FLE. The data collected from the FLE experiments were used to plot four types of adsorption isotherms which were discussed in this section. The adsorption isotherms which were tested are Langmuir, Temkin, Florry-Huggins (F-H) and Frumkin. The surface coverage used in these adsorption isotherms is determined using the inhibition efficiency obtained from the EIS test. The inhibition effect is determined by the surface coverage of the inhibitor since the inhibition process is dependent on its adsorption. The IE should be directly proportional to the degree of surface coverage.

250 250 (a) (b)

200 200

150 150

C/ C/ 100 100

y = 1.043x + 2.1822 50 50 y = 1.0185x + 7.3914 R² = 0.9999 R² = 0.9985 0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

250 (c) 250 (d)

200 200

150

Ø 150

C/ C/ 100 100 y = 1.0759x + 25.733 50 y = 1.0544x + 14.37 50 R² = 0.9956 R² = 0.9997 0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

Figure 28. Langmuir isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

1.10 (a) (b) 0.96 1.00

0.92 0.90

Ø Ø 0.80 0.88 0.70 y = 0.0854x + 0.7624 y = 0.234x + 0.4334 R² = 0.7709 R² = 0.8283 0.84 0.60 1 1.5 2 2.5 1 1.5 2 2.5 Log C Log C

0.90 0.90 (c) (d) 0.80 0.80

0.70

Ø 0.70 Ø 0.60

0.60 0.50 y = 0.3164x + 0.1839 y = 0.3806x - 0.0293 R² = 0.9215 R² = 0.9374 0.50 0.40 1 1.5 2 2.5 1 1.5 2 2.5 Log C Log C

Figure 29. Temkin isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

-1.3 (a) -1.5 (b) -1.5

-1.7

-1.7 /C)

/C) -1.9 Ø

-1.9Ø

log( log( -2.1 -2.1

-2.3 -2.3 y = 1.8272x + 0.1758 y = 0.918x - 1.0335 R² = 0.8132 R² = 0.7913 -2.5 -2.5 -1.4 -1.2 -1 -0.8 -1.4 -1.2 -1 -0.8 -0.6 -0.4 Log (1-Ø) Log (1-Ø)

-1.4 -1.6 (c) (d) -1.6

-1.8

-2.0

/C)

/C) Ø

-2.0 Ø log( -2.2log( -2.2 -2.4 -2.4 y = 1.3141x - 1.0672 y = 1.3031x - 1.3488 R² = 0.9637 R² = 0.9137 -2.6 -2.6 -1 -0.8 -0.6 -0.4 -0.2 -1 -0.8 -0.6 -0.4 -0.2 0 Log (1-Ø) Log (1-Ø)

Figure 30. F-H isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

4.0 3.6 (a) (b)

3.5

)) ))

3.2Ø

Ø -

- 3.0

/(1

/(1 Ø 2.8 Ø

2.5 log log (C* 2.4 log (C* y = 14.348x - 10.243 2.0 y = 7.2847x - 3.5408 R² = 0.8797 R² = 0.9313 2.0 1.5 0.84 0.88 0.92 0.96 Ø 0.7 0.8 Ø 0.9 1

3.4 3.5 (c) (d)

3.0 3.0

))

Ø -

- 2.6 2.5

/(1

Ø Ø

2.2 2.0

log log (C* log log (C* 1.8 1.5 y = 5.3328x - 1.6768 y = 4.5187x - 0.8494 R² = 0.9562 R² = 0.9733 1.4 1.0 0.5 0.6 0.7 0.8 0.9 1 0.4 0.5 0.6 0.7 0.8 0.9 Ø Ø

Figure 31. Frumkin isotherms for adsorption of fig leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The Langmuir, Temkin, Florry-Huggins and Frumkin isotherms can be respectively seen in Figure 28, Figure 29, Figure 30 and Figure 31.Based on the results from the four isotherms used, the Langmuir isotherm was determined to be the best one that is able to fit the results from the FLE experiments. The residuals from the Langmuir plots are very low at all temperatures compared to the residuals in other isotherms which are slightly higher.

In the Langmuir isotherm, the degree of surface coverage is plotted against the concentration of FLE to obtain a linear relationship as seen from Figure 28. It is clearly seen that the adsorption of FLE inhibitor onto the surface of mild steel follows the Langmuir isotherm. This can be confirmed by the slope being close to unity for all temperatures in Figure 28 and is in accordance with equation 10. The residuals

86 obtained when fitting the points are also close to one, thus indicating that the Langmuir isotherm is well-fit.

It can be seen from Table 5 that the equilibrium adsorption constant (K) decreases with the increase of temperature which indicates the weakening of the adsorbed FLE film. The value of K also gives information about the ratio of amount adsorbed to adsorptive concentration. The higher the adsorption constant, the stronger is the bond between the inhibitor molecules and the mild steel surface. The values of

0 ∆퐺푎푑푠 are positive for FLE at all temperatures studied where it was found to increase 0 with temperature. Positive values of ∆퐺푎푑푠 are a result of values of K being lower than one. This indicates that the amount of FLE inhibitor in the solution is more than the amount of FLE molecules that is adsorbed on the surface of mild steel.

Table 5. Langmuir thermodynamic results for FLE

∆퐺0 ( 풌푱 ) T (± 2°C) K 푎푑푠 풎풐풍

25 0.458 1.93

40 0.135 5.21

50 0.070 7.16

70 0.039 9.27

6.1.5 Activation energy of FLE. The corrosion rate used in the Arrhenius plot for FLE is obtained from the cyclic sweep test and can be seen in Figure 32. The purpose of this plot is to determine how the FLE inhibitor affects the activation energy of the corrosion process. From the Arrhenius plot, the Arrhenius constant and activation energy can be determined.

퐸 ln (퐶표푟푟표푠푖표푛 푟푎푡푒) = ln 퐴 − 푎 (ퟒퟕ) 푅푇

where A is the Arrhenius constant, Ea is the activation energy and R is the universal gas constant. The values of A were obtained from the y-intercept of the

Arrhenius plot for FLE while Ea was obtained using the slope.

2.5

2.0

1.5

1.0

0.5

0.0

Blank y = -1511.2x + 6.2458

-0.5 R² = 0.8159 ln (CR) ln FLE-25ppm y = -4366.8x + 13.986 -1.0 R² = 0.9465 -1.5 FLE-50ppm y = -3580.3x + 11.186 R² = 0.9244 -2.0 FLE-100ppm y = -4083.6x + 12.563 R² = 0.9524 -2.5 FLE-150ppm y = -4975.2x + 15.144 R² = 0.8865 -3.0 y = -4632.8x + 13.841 FLE-200ppm R² = 0.8482 -3.5 0.00270 0.00280 0.00290 0.00300 0.00310 0.00320 0.00330 0.00340 1/T Figure 32. Arrhenius plot for fig leaves extract

Table 6. Arrhenius parameters of FLE 퐄퐚 Ea ( 풌푱 ) ln A 퐑 풎풐풍 Blank 6.25 1511.2 12.56

25ppm 13.99 4366.8 36.31

50ppm 11.19 3580.3 29.77

100ppm 12.56 4083.6 33.95

150ppm 15.14 4975.2 41.36

200ppm 13.84 4632.8 38.52

Table 6 summarizes the results for all concentrations of FLE. Inspection of the table reveals that the introduction of FLE causes an increase in the activation energy from the blank case of 12.56 kJ/mol. The activation energy is the amount required for the corrosion reaction to proceed. Thus, the increase in magnitude of Ea, with FLE concentration, indicates that it is more difficult for the corrosion reaction to proceed.

Higher values of Ea are a good indication that FLE provides strong inhibitive action through raising the energy barrier of the corrosion process. The increase of the Ea can be attributed to the presence of the FLE extract which increases the thickness of the double layer [115]. The overall trend for FLE is that as the concentration increases, the activation energy increases. However, the concentrations of 25 ppm and 200 ppm do not follow this trend and this may have occurred due to a stronger adsorption at 25ppm and slight desorption of the inhibitor at 200 ppm. The increase of activation energy with FLE concentration indicates that the molecular adsorption on the mild steel surface is physical [100].

6.2 Calotropis procera extract (CPLE)

6.2.1 Linear polarization resistance results for CPLE.

CPLE was used to investigate the corrosion inhibition of mild steel in a CO2- saturated 3.5wt% NaCl solution. The results are presented in this section at 25°C, 40°C, 50°C and 70°C using five different concentrations. There are five sections, each which respectively present the LPR results, EIS results, CS results, adsorption isotherms and the activation parameters of the inhibition process.

6.2.1.1 Corrosion rate from LPR. The corrosion rate of mild steel is observed to decrease with the addition of CPLE inhibitor at each temperature and this is seen in Figure 33(a). The IE, which is based on the corrosion current density, can be seen in Figure 33(b) where the general trend is that the IE increases with CPLE concentration at all temperatures. It is also observed that the IE of CPLE increases with temperature (up to 50°C). It is only at 70°C that the IE becomes lower than at 25°C and this could be due to the weakening of the inhibitor film on the surface of mild steel. The highest inhibition efficiency was 93% for 150 ppm of CPLE at 50°C.

3 (a) 25°C

40°C

50°C

2 70°C

1 Corrosion (mm/yr)rate

0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 33. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of calotropis procera leaves extract at elevated temperatures from the LPR test

6.2.1.2 LPR parameters. The LPR test provides insights about the LPR value, corrosion potential and current density for each CPLE experiment.

It is seen from Table 7 that the addition of CPLE inhibitor results in the increase of the polarization resistance. The LPR value is the long term polarization resistance that is caused by the protective film formed over the mild steel. The protective film is formed through the adsorption of the CPLE molecules onto the mild 90 steel surface. A higher LPR value indicates that a lower current density passes through the solution and this is confirmed by the decrease of the corrosion current

(Icorr) with LPR value as seen in Table 7. Moreover, the addition of CPLE also tends to decrease the corrosion potential. These trends indicate that the addition of CPLE inhibitors does reduce the corrosion rate.

Table 7. Parameters obtained from the LPR test for CPLE

T C (ppm) LPR (ohm.cm²) Icorr (mA/cm²) Potential (mV) IE% 0 243.5 0.0661 -710.2 - 25 616.2 0.0378 -669.9 60 50 865.6 0.0208 -655.7 72 25°C 100 1086.8 0.0167 -646.0 78 150 1762.5 0.0099 -646.9 86 200 2058.7 0.0076 -647.8 88 0 178.3 0.1435 -719.8 - 25 491.8 0.0385 -683.0 64 50 550.3 0.0413 -680.2 68 40°C 100 732.2 0.0273 -679.3 76 150 819.5 0.0216 -677.9 78 200 1317.8 0.0134 -669.6 86 0 132.7 0.2040 -722.9 - 25 317.6 0.0716 -711.5 58 50 429.5 0.0501 -705.4 69 50°C 100 595.7 0.0362 -696.7 78 150 886.9 0.0163 -684.6 85 200 1028.4 0.0190 -691.7 87 0 128.1 0.2052 -739.0 - 25 134.6 0.1566 -743.4 5 50 160.3 0.1149 -740.0 20 70°C 100 201.1 0.1015 -740.2 36 150 240.6 0.0763 -737.6 47 200 276.4 0.0664 -735.6 54

The steady state LPR value obtained for each CPLE concentration at 25°C, 40°C, 50°C and 70°C can be seen in Figure 34. It clearly illustrates that LPR increases with

91 concentration but decreases with temperature. This decrease is portrayed by the downward shift of the curve from 25°C to 70°C.

25°C 2000 40°C 50°C

1500 70°C

1000 LPR (ohm.cm²) LPR

500

0 0 50 100 150 200 Concentration (ppm)

Figure 34. Effect of CPLE concentration on the steady state LPR value obtained

6.2.2 Electrochemical impedance spectroscopy results for CPLE. The impedance test was used to investigate the inhibition of the corrosion of mild steel in a CO2-saturated 3.5wt% NaCl solution, using CPLE.

6.2.2.1 Corrosion rate from EIS. The corrosion rate of mild steel is observed to decrease with CPLE concentration in Figure 35(a), while the inhibition efficiency increases as seen in Figure 35(b). This is similar to the corrosion rate determined using the LPR method. It is noticed that at higher concentrations of CPLE, the increase in the system temperature does not affect the IE obtained. It is only at 70°C where the IE becomes diminished. The IE curves for each temperature begin to plateau at low concentrations of CPLE and this is good indicator that CPLE is a good inhibitor.

It is noticed that the plots in Figure 35 are similar to the plots in Figure 33. The same anomaly of 150ppm at 50°C is also witnessed for the EIS test. This confirms the validity of the results obtained from the LPR and EIS tests.

(a) 25°C 4

40°C

50°C 3 70°C

Corrosion (mm/yr)rate 1

0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 35. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of calotropis procera leaves extract at elevated temperatures from the EIS test

6.2.2.2 EIS parameters and plots. This section consists of the impedance parameters obtained as well as Nyquist and Theta-frequency plots which depict different concentrations of CPLE at 25°C, 40°C, 50°C and 70°C, respectively.

The results from Table 8 confirm that the addition of CPLE results in the increase of Rct and decrease of Cdl. This is an indication of the decrease in formation of the anodic process controlling intermediates from the dissolution of mild steel. The 93

Cdl values may have decreased due to the protective film formed by CPLE on the mild steel surface. This trend occurs due to the adsorption of the inhibitor molecules onto the mild steel surface. These molecules are believed to have replaced the water molecules on the mild steel surface and thus decrease the diffusion rate of the reactant elements [105].

Table 8: Impedance parameters obtained using CPLE

풎푭 Cdl ( ) T C (ppm) Rsol (ohms.cm²) Rct (ohms.cm²) 풄풎ퟐ IE%

0 16.53 221.20 367.60 - 25 17.12 822.30 125.00 73 50 18.48 1120.00 62.39 80 25°C 100 16.43 1513.00 60.18 85 150 17.20 2170.00 43.41 90 200 16.69 2611.00 38.11 92 0 14.53 138.60 231.60 - 25 15.72 598.10 57.20 77 50 14.93 628.40 61.26 78 40°C 100 14.60 837.60 50.02 83 150 14.97 920.90 53.29 85 200 14.91 1611.00 47.24 91 0 11.29 101.20 302.80 - 25 10.73 289.80 97.01 65 50 11.83 402.90 80.93 75 50°C 100 12.97 588.00 63.00 83 150 12.97 948.10 55.41 89 200 11.94 1047.00 49.41 90 0 8.80 84.10 306.80 - 25 7.59 107.90 257.20 22 50 7.82 128.80 185.20 35 70°C 100 8.28 186.60 181.50 55 150 7.80 215.90 173.70 61 200 8.92 246.80 207.30 66

Figure 36 illustrates that the addition of CPLE results in the increase of the diameter of the semi-circle in the Nyquist plots. This indicates that the strength of the film, formed by CPLE, is increasing. This trend is observed at all temperatures. This film is also thought to suppress the kinetics of the cathodic and anodic reactions [105]. 94

1400 800 Blank (a) Blank (b) 1200 CPLE-25ppm 700 CPLE-25ppm CPLE-50ppm CPLE-50ppm

600 CPLE-100ppm

1000 CPLE-100ppm CPLE-150ppm 500 800 CPLE-150ppm CPLE-200ppm CPLE-200ppm 400 600

300

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 400 200 200 100 0 0 0 1000 2000 3000 0 500 1000 1500 2000 Z' (ohm.cm²) Z' (ohm.cm²) 500 100 Blank Blank (c) CPLE-25ppm (d) CPLE-25ppm 400 CPLE-50ppm 80 CPLE-50ppm

CPLE-100ppm CPLE-100ppm

CPLE-150ppm CPLE-150ppm 300 CPLE-200ppm 60 CPLE-200ppm

200 40

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 100 20

0 0 0 200 400 600 800 1000 1200 0 50 100 150 200 250 300 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 36. Nyquist plots for calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C from the EIS test

The phase angle is plotted against the log of frequency as seen in Figure 37. These plots are useful since they can define a domain of pure capacitive behavior. At high frequencies, the phase angle decreases to almost zero and this is characteristic of a response to resistive behavior. At intermediate frequencies, the maximum phase angles obtained at various concentrations of CPLE can be seen in Table 9.

Ideally, the phase angle should be 90° which is indicative of capacitive behavior. The difference between the maximum phase angle and 90° is thought to be due to deviations that arise from ideal capacitive behavior. The phase angle gradually approaches the ideal capacitive value due to the slowing down of dissolution rate with time. Thus, in the presence of the CPLE inhibitor, it is observed that the phase angle becomes closer to 90° through a faster attainment of steady state [77].

90 90 Blank (a) Blank (b) 80 25ppm 80 25ppm 70 50ppm 70 50ppm

100ppm 100ppm

60 60 50 150ppm 50 150ppm 200ppm 200ppm 40 40

30 30 Phase Phase angle 20 Phase angle 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency 90 90 Blank (c) Blank (d) 80 25ppm 80 25ppm 70 50ppm 70 50ppm 100ppm

100ppm 60 60 50 150ppm 50 150ppm 200ppm 200ppm 40 40

30 30

Phase angle Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency

Figure 37. Theta-frequency plots for calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

Table 9. Summary of maximum phase angles obtained for CPLE Phase angle (°)

C (ppm) 25°C 40°C 50°C 70°C

0 47.26 45.36 43.19 44.05

25 62.63 62.40 57.57 47.81

50 65.81 62.00 60.42 50.47

100 67.22 65.10 63.16 53.02

150 70.97 66.00 69.00 53.87

200 72.43 71.00 69.65 53.28

6.2.3 Cyclic sweep results for CPLE. The cyclic sweep test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of CPLE. The test was conducted at 25°C, 40°C, 50°C and 70°C.

6.2.3.1 Corrosion rate from CS.

7 (a) 25°C 6

40°C 5 50°C

4 70°C

2 Corrosion (mm/yr)rate

0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 38. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of calotropis procera leaves extract at elevated temperatures from the CS test

It is seen from Figure 38(a) that the addition of CPLE results in the decrease of the corrosion rate of mild steel, when the temperature is fixed. When the temperature is increased, the corrosion rate, for a fixed concentration, increases. This is similar to the corrosion plots obtained from the LPR and EIS tests with the only difference being the magnitude of the observed values.

From Figure 38(b), it is observed that the IE is not affected by temperature up to 50°C for concentrations of CPLE greater than 50ppm. It is only at 70°C that the IE becomes diminished. This is similar to the IE trends observed in the LPR and EIS tests.

6.2.3.2 CS parameters and plots. The effect of CPLE on the electrochemical behavior of mild steel was studied using the CS test.

It is observed that the corrosion current (Icorr) decreases with CPLE concentration as seen in Table 10. The change in the cathodic Tafel slope (bc) is greater than the anodic Tafel slope (ba) and this might indicate that CPLE does not affect the anodic corrosion process [114]. It is also seen that the shift in the corrosion potential becomes more positive with CPLE concentration. This is indicative of inhibitor behavior in CO2-saturated brines [4].

It is observed from Figure 39, that there was a clear shift in both the anodic and cathodic Tafel slopes with addition of CPLE and this resulted in the decrease of the corrosion rate. The anodic shift is more noticeable than the cathodic shift from the blank case. This indicates that the metal oxidation rate has decreased. Thus, the adsorbed film decreases the metal dissolution at a higher rate than hydrogen reduction. The corrosion potential shift from the blank (uninhibited) case to the various Tafel curves that correspond to the CPLE concentrations is seen to be less than 85 mV at all temperatures. The lowest difference is seen at 70°C. Based on these observations, CPLE is classified as a mixed inhibitor [114].

Table 10. Tafel parameters obtained from various CPLE concentrations at elevated temperatures

T C (ppm) Ecorr ba (mV) bc (mV) Icorr IE% 0 -711.0 58.0 702.2 0.2514 - 25 -667.6 59.3 272.0 0.0743 70 50 -655.5 51.1 216.3 0.0476 81 25°C 100 -649.8 50.5 240.4 0.0415 83 150 -649.9 50.8 187.5 0.0260 90 200 -650.0 46.6 159.4 0.0198 92 0 -721.4 65.8 559.3 0.3841 - 25 -684.9 66.4 245.0 0.0847 78 50 -681.6 68.4 253.7 0.0640 83 40°C 100 -681.9 58.4 214.5 0.0402 90 150 -679.4 49.7 191.7 0.0321 92 200 -668.5 54.8 158.8 0.0280 93 0 -725.5 66.7 570.0 0.4811 - 25 -715.1 61.3 355.0 0.2204 54 50 -709.9 60.4 272.7 0.1305 73 50°C 100 -699.2 63.8 221.7 0.0698 85 150 -687.9 44.3 133.3 0.0607 87 200 -693.1 61.9 163.7 0.0522 89 0 -741.8 70.3 433.1 0.4852 - 25 -745.8 54.6 432.6 0.3665 24 50 -741.7 47.6 381.9 0.3054 37 70°C 100 -741.6 52.4 449.7 0.3081 37 150 -738.0 51.0 245.3 0.2773 43 200 -739.6 56.5 310.9 0.2534 48

-500 -500 (a) (b) -550 -550 -600

-600

-650 -650 -700 -700 Blank -750 -750 Blank

Potential (mV) CPLE-25ppm Potential (mV) -800 CPLE-50ppm -800 CPLE-25ppm CPLE-100ppm -850 CPLE-150ppm -850 CPLE-50ppm CPLE-200ppm -900 -900 0.00001 0.001 0.1 10 Current (mA/cm2) 0.00001 0.001Current (mA/cm0.12) 10 -500 -500 (c) (d) -550 -550

-600 -600

-650 -650

-700 -700

-750 Blank -750 Blank Potential (mV) CPLE-25ppm Potential (mV) CPLE-25ppm -800 CPLE-50ppm -800 CPLE-50ppm CPLE-100ppm CPLE-100ppm -850 CPLE-150ppm -850 CPLE-150ppm CPLE-200ppm CPLE-200ppm -900 -900 0.00001 0.001 0.1 10 0.0001 0.001 0.01 0.1 1 10 Current (mA/cm2) Current (mA/cm2)

Figure 39. CS plots for CPLE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

6.2.3.3 Difference between test types. The inhibition efficiency, determined using the corrosion current density, from the LPR, EIS and CS tests are compared at different temperatures. It can be seen from Figure 40 that the inhibition efficiency of the LPR, EIS and CS methods follows the same trend at their respective temperatures. It is only in plot (d) that a noticeable difference in the IE between the three tests is witnessed.

100

100 (a) 100 (b)

80 80

60 60

IE% IE% 40 40 CS CS 20 LT 20 LT EIS EIS 0 0 0 50 100 150 200 0 50 100 150 200 C (ppm) C (ppm) 80 100 (c) (d)

80 60

IE% IE% 40 CS CS 20 20 LT LT EIS EIS 0 0 0 50 100 150 200 0 50 100 150 200 C (ppm) C (ppm)

Figure 40. Comparison of inhibition efficiency between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C for CPLE

6.2.4 Adsorption isotherms for CPLE. The data collected from the CPLE experiments are used to plot four types of adsorption isotherms which are Langmuir, Temkin, Florry-Huggins and Frumkin. The best-fit adsorption isotherm will be chosen to represent CPLE. The surface coverage used in these adsorption isotherms is determined using the inhibition efficiency obtained from the EIS test.

101

250 250 (a) (b)

200 200

150 150

Ø C/ 100 100C/

50 y = 1.0501x + 9.576 50 y = 1.0734x + 9.6814 R² = 0.9995 R² = 0.9962 0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

250 350 (c) (d) 200 300

250

150 Ø

Ø 200 C/ 100C/ 150 y = 1.072x + 85.147 50 y = 1.0361x + 14.269 100 R² = 0.9936 R² = 0.9993 0 50 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm) Figure 41. Langmuir isotherms for adsorption of calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

102

0.97 0.93 (a) (b) 0.92 0.88

0.87

0.83 Ø Ø 0.82

0.78 0.77 y = 0.204x + 0.4495 0.73 y = 0.1499x + 0.542 R² = 0.9954 0.72 R² = 0.8762 0.68 0.67 1 1.5 2 2.5 1 1.5 2 2.5 Log C Log C 0.80 0.93 (c) (d) 0.88 0.60

0.83

0.78 Ø Ø 0.40 0.73

0.68 y = 0.2875x + 0.2542 0.20 y = 0.5052x - 0.49 0.63 R² = 0.991 R² = 0.9893

0.58 0.00 1 1.5 2 2.5 1 1.5 2 2.5 Log C Log C Figure 42. Temkin isotherms for adsorption of calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

103

-1.4 (a) -1.4 (b)

-1.8 -1.8

/C)

Ø log(

log( -2.2 -2.2

y = 1.5819x - 0.6715 y = 1.7035x - 0.6477 R² = 0.9801 R² = 0.753 -2.6 -2.6 -1.2 -1 -0.8 -0.6 -0.4 -1.2 -1 -0.8 -0.6 -0.4 Log (1-Ø) Log (1-Ø)

-1.4

-1.8

-2.2

/C)

log( log( -2.2 -2.4 y = 1.2758x - 1.0407 y = 1.112x - 1.9319 R² = 0.9751 R² = 0.9615 -2.6 -2.6 -1.2 -1 -0.8 -0.6 -0.4 -0.5 -0.4 -0.3 -0.2 -0.1 0 Log (1-Ø) Log (1-Ø)

Figure 43. F-H isotherms for adsorption of calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

104

3.5 3.5

(a) (b)

3.1 3.1

))

- -

2.7 2.7

/(1

Ø Ø

2.3 2.3 log log (C* log log (C* y = 9.2611x - 5.054 1.9 y = 8.1036x - 4.1356 1.9 R² = 0.9611 R² = 0.9939 1.5 1.5 0.7 0.75 0.8 0.85 0.9 0.95 0.75 0.8 0.85 0.9 0.95 Ø Ø

3.5 3.0 (c) (d)

3.1 2.5

)) ))

Ø Ø -

- 2.7 2.0

/(1 /(1

Ø Ø

2.3 1.5

log log (C* log (C* 1.9 y = 6.2237x - 2.4302 1.0 y = 3.8362x + 0.0327 R² = 0.9918 R² = 0.9961 1.5 0.5 0.6 0.7 0.8 0.9 1 0.2 0.3 0.4 0.5 0.6 0.7 Ø Ø Figure 44. Frumkin isotherms for adsorption of calotropis procera leaves extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The Langmuir, Temkin, Florry-Huggins and Frumkin isotherms can be respectively seen in Figure 41, Figure 42, Figure 43 and Figure 44. Based on the four isotherms studied, it is observed that Langmuir is the best isotherm that is able to fit the results of the CPLE experiments due to it having the lowest residuals.

It can be seen from Table 11 that the equilibrium adsorption constant (K) decreases with the increase of temperature which indicates the weakening of the adsorbed CPLE film. The higher the adsorption constant, the stronger is the bond 0 between the inhibitor molecules and the mild steel surface. The values of ∆퐺푎푑푠 are positive for CPLE at all temperatures studied where it was found to increase with

0 temperature. Positive values of ∆퐺푎푑푠 are a result of values of K being lower than one. This indicates that the amount of CPLE inhibitor in the solution is more than the amount of CPLE molecules that is adsorbed on the surface of mild steel.

105

Table 11: CPLE thermodynamic data from Langmuir isotherm

∆퐺0 ( 풌푱 ) T (± 2°C) K 푎푑푠 풎풐풍

25 0.104 5.60

40 0.103 5.91

50 0.070 7.14

70 0.012 12.68

6.2.5 Activation energy of CPLE. The corrosion rate used in the Arrhenius plot for CPLE is obtained from the cyclic sweep test. The purpose of this plot is to determine how the CPLE inhibitor affects the activation energy required for corrosion. The plot can be seen in Figure 45, and from it the activation energy is determined.

2.2

1.2

0.2 Blank y = -1511.2x + 6.2458

R² = 0.8159 ln (CR) ln CLE-25ppm y = -3934.8x + 12.905 -0.8 R² = 0.8911 CLE-50ppm y = -4390.9x + 13.979 R² = 0.9533

CLE-100ppm y = -4677x + 14.572 R² = 0.8126 -1.8 CLE-150ppm y = -5488.3x + 16.885 R² = 0.8933 CLE-200ppm y = -5872.4x + 17.928 R² = 0.9178 -2.8 0.00275 0.00285 0.00295 0.00305 0.00315 0.00325 0.00335 1/T Figure 45. Arrhenius plot for calotropis procera leaves extract 106

Table 12. Arrhenius parameters of CPLE 퐄퐚 Ea ( 풌푱 ) ln A 퐑 풎풐풍 Blank 6.246 1511.2 12.56

25ppm 12.905 3934.8 32.71

50ppm 13.979 4390.9 36.51

100ppm 14.572 4677.0 38.88

150ppm 16.885 5488.3 45.63

200ppm 17.928 5872.4 48.82

It is observed from Table 12 that increasing the concentration of CPLE in a

CO2-saturated 3.5wt% NaCl solution, results in an increase in the activation energy. The increased magnitude of the activation energy from the uninhibited/blank case indicates the degree of difficulty for the corrosion reaction to occur. Higher values of

Ea are a good indication that CPLE provides strong inhibitive action through raising the energy barrier of the corrosion process. The increase of the Ea can be attributed to the presence of the CPLE extract which increases the thickness of the double layer [115].

6.3 Eggplant peel extract (EPPE)

6.3.1 Linear polarization resistance results for EPPE.

EPPE is used to investigate the corrosion inhibition of mild steel in a CO2- saturated 3.5wt% NaCl solution. The results are presented in this section at 25°C, 40°C, 50°C and 70°C using five different concentrations of 25 ppm, 50 ppm, 100 ppm, 150 ppm and 200 ppm of EPPE. The five sub-sections of this section will respectively show the LPR results, the EIS results, the CS results, the adsorption isotherms and the activation parameters of the inhibition process.

107

6.3.1.1 Corrosion rate from LPR.

3.0 (a) 25°C 2.5 40°C 50°C 2.0 70°C

1.5

1.0 Corrosion (mm/yr)rate 0.5

0.0 0 50 100 150 200 Concentration (ppm) 100 (b)

IE% 40

20 25°C 40°C 50°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 46. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of eggplant leave extract at elevated temperatures from the LPR test

It is observed from Figure 46(a) that when the EPPE concentration increases, the corrosion rate decreases and this is observed for all temperatures. When the concentration is fixed and the temperature is increased, the corrosion rate was found to increase. The general trend observed in Figure 46(b) is that the IE increases with EPPE concentration at all temperatures. The IE is determined using the corrosion current density. At EPPE concentrations greater than 50 ppm, it is observed that the 108

IE at 50°C becomes greater than the IE at 40°C. The highest IE is seen at 25°C while the lowest is at 70°C.

6.3.1.2 LPR parameters. The LPR test provides insights about the LPR value, corrosion potential and corrosion current density for each EPPE experiment. The LPR value is the long-term polarization resistance that is caused by the protective film formed over the mild steel. The protective film is formed through the adsorption of the EPPE molecules onto the mild steel surface. Increased values of LPR result in a lower current density that passes through the solution.

Table 13. Parameters obtained from the LPR test for EPPE

T C (ppm) LPR (ohm.cm²) Icorr (mA/cm²) Potential (mV) IE% 0 243.5 0.0662 -710.2 - 25 492.5 0.0439 -669.5 51 50 848.1 0.0232 -650.0 71 25°C 100 1074.9 0.0164 -653.6 77 150 1730.1 0.0105 -651.8 86 200 2267.2 0.0080 -655.2 89 0 178.3 0.1435 -719.8 - 25 316.0 0.0804 -708.3 44 50 395.9 0.0609 -701.8 55 40°C 100 412.8 0.0557 -701.7 57 150 484.7 0.0457 -702.9 63 200 507.9 0.0466 -703.3 65 0 132.7 0.2040 -722.9 - 25 236.6 0.1009 -718.0 44 50 258.5 0.0866 -714.7 49 50°C 100 280.8 0.0723 -710.7 53 150 372.7 0.0543 -705.0 64 200 396.7 0.0510 -700.5 67 0 128.1 0.2052 -739.0 - 25 123.4 0.1624 -742.5 -4 50 146.2 0.1406 -744.0 12 70°C 100 163.9 0.1315 -743.5 22 150 171.6 0.1093 -742.9 25 200 213.2 0.0940 -742.9 40

109

It is observed from Table 13 that the LPR value increases with EPPE concentration while the Icorr decreases. An anomaly is witnessed for 25ppm of EPPE at 70°C where a negative IE is obtained. This is due to a lower value of LPR than the uninhibited case. However, subsequent additions of EPPE result in an increase of the LPR value obtained.

A plot of LPR obtained for each EPPE concentration at 25°C, 40°C, 50°C and 70°C can be seen in Figure 47. It clearly illustrates the relationship between the LPR obtained for each EPPE concentration. Moreover, the LPR decreases when the temperature is increased and this is seen by the downward shift of the LPR curve from 25°C to 70°C.

2500 25°C

2000 40°C 50°C

70°C 1500

1000 LPR (ohm.cm²) LPR

500

0 0 50 100 150 200 Concentration (ppm)

Figure 47. Effect of EPPE concentration on the steady state LPR value

6.3.2 Electrochemical impedance spectroscopy results for EPPE. The impedance test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of EPPE.

110

6.3.2.1 Corrosion rate from EIS. The corrosion rates obtained by the EIS test are similar to the corrosion rates determined using the LPR method and the observed trend is similar to the results of FLE and CPLE. The corrosion rate of mild steel is observed to decrease with the addition of the EPPE inhibitor. This also results in the inhibition efficiency increasing. These results are represented graphically in Figure 48.

(a) 25°C 4.0

40°C

) 50°C yr 3.0 70°C

2.0

Corrosion (mm/rate 1.0

0.0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 48. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of eggplant peel extract at elevated temperatures from the EIS test

111

It is clear that the IE increases with EPPE concentration for each temperature. The IE is also seen to decrease with temperature for a fixed concentration of EPPE. It is noticed that the IE curve for 50°C becomes higher than the 40°C case at concentrations greater than 150ppm of EPPE. This is similar to the IE plots from the LPR and EIS tests.

6.3.2.2 EIS parameters and plots. This section consists of the impedance parameters and the Nyquist plots from the EIS test. These depict different concentrations of EPPE at 25°C, 40°C, 50°C and 70°C respectively.

Table 14. Impedance parameters obtained using EPPE

풎푭 Cdl ( ) T C (ppm) Rsol (ohms.cm²) Rct (ohms.cm²) 풄풎ퟐ IE%

0 16.53 221.20 367.60 - 25 17.75 634.30 125.00 65 50 18.24 1068.00 78.35 79 25°C 100 18.37 1284.00 5.39 83 150 19.55 2120.00 40.98 90 200 19.26 2790.00 35.05 92 0 14.53 138.60 231.60 - 25 14.08 299.50 135.50 54 50 13.87 367.30 102.70 62 40°C 100 14.39 378.80 124.30 63 150 14.33 434.20 105.20 68 200 13.74 458.80 98.17 70 0 11.29 101.20 302.80 - 25 12.31 189.50 122.10 47 50 11.76 209.00 125.50 52 50°C 100 10.73 246.80 115.50 59 150 11.56 314.90 95.33 68 200 11.28 351.00 80.56 71 0 8.80 84.10 306.80 - 25 7.73 91.46 227.50 8 50 7.53 100.00 184.80 16 70°C 100 8.25 128.90 157.00 35 150 8.06 131.00 139.10 36 200 8.04 167.70 125.20 50 112

The results in Table 14 confirm that the addition of EPPE results in the increase of Rct and decrease of Cdl. This is an indication of the decrease in formation of the anodic process controlling intermediates from the dissolution of mild steel. It was observed that the Cdl values did not always decrease with EPPE concentration. It was also found that the increase of temperature leads to a decrease in Rct for a fixed concentration. This can quantitatively be seen from the tables mentioned. The observed trend can be explained by the adsorption of the inhibitor molecules onto the mild steel surface where it replaces the water molecules on the surface and thus decreases the diffusion rate of the reactant elements.

1400 200 (a) Blank (b) Blank 1200 EPPE-25ppm EPPE-25ppm EPPE-50ppm EPPE-50ppm

EPPE-100ppm 150 EPPE-100ppm

1000 EPPE-150ppm EPPE-150ppm 800 EPPE-200ppm EPPE-200ppm 100

600 Z'' Z'' (ohm.cm²) 400 Z'' (ohm.cm²) 50 200

0 0 0 1000 2000 3000 0 100 200 300 400 500 600 Z' (ohm.cm²) Z' (ohm.cm²) 160 80 (c) Blank (d) Blank 140 EPPE-25ppm 70 EPPE-25ppm EPPE-50ppm EPPE-50ppm

120 EPPE-100ppm 60 EPPE-100ppm

EPPE-150ppm EPPE-150ppm 100 EPPE-200ppm 50 EPPE-200ppm 80 40

60 30

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 40 20 20 10 0 0 0 100 200 300 400 0 50 100 150 200 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 49. Nyquist plots for eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C from the EIS test

113

The plots in Figure 49 illustrate the trend that the addition of EPPE results in the increase of the diameter of the semi-circle in the Nyquist plots. This indicates that the strength of the film, formed by EPPE, is increasing [105]. The observed trend is consistent at all temperatures.

90 90 Blank (a) Blank 80 25ppm 80 25ppm (b) 70 50ppm 70 50ppm

100ppm 100ppm

60 60 150ppm 150ppm 50 200ppm 50 200ppm 40 40

30 30

100ppm 100ppm

60 150ppm 60 150ppm 50 200ppm 50 200ppm 40 40

30 30

Phase Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency

Figure 50. Theta-frequency plots for eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The phase angle is plotted against the log of frequency as seen in Figure 50. These plots are useful since they can define a domain of pure capacitive behavior. At high frequencies, the phase angle decreases to almost zero and this is characteristic of a response to resistive behavior. At intermediate frequencies, the maximum phase angles obtained at various concentrations of EPPE can be seen in Table 15.

114

Ideally, the phase angle should be 90° which is indicative of capacitive behavior. The difference between the maximum phase angle and 90° is thought to be due to deviations that arise from ideal capacitive behavior. The phase angle gradually approaches the ideal capacitive value due to the slowing down of dissolution rate with time. Thus, in the presence of the EPPE inhibitor, it is observed that phase angle becomes closer to 90° through a faster attainment of steady state [77].

Table 15. Summary of maximum phase angles obtained for EPPE Phase angle (°)

C (ppm) 25°C 40°C 50°C 70°C

0 47.26 45.36 43.19 44.05

25 57.19 51.58 50.14 42.30

50 63.33 55.25 50.62 46.74

100 66.22 53.99 53.88 48.77

150 70.53 54.86 56.54 50.90

200 72.17 56.60 58.20 53.32

6.3.3 Cyclic sweep results for EPPE. The cyclic sweep test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of EPPE.

6.3.3.1 Corrosion rate from CS. Figure 51(a) shows that the addition of EPPE results in the decrease of the corrosion rate of mild steel for each temperature investigated. For a fixed concentration, the corrosion rate is observed to increase with temperature. This is similar to the corrosion plots obtained from the LPR and EIS tests with the only difference being the magnitude of the observed values. From Figure 51(b), the increase in temperature results in reducing the inhibition efficiency (IE). The general trend observed is that the IE increases with EPPE concentration and decreases at

115 elevated temperatures. This similar to the trends observed in the LPR and EIS tests. It is observed that the IE is not affected significantly by increasing the temperature from 40°C to 50°C.

7.0 (a) 25°C 6.0 40°C

50°C

5.0 70°C

4.0

3.0

2.0 Corrosion (mm/yr)rate

1.0

0.0 0 50 100 150 200 Concentration (ppm)

25°C (b) 100 40°C 50°C 70°C 80

60 IE%

0 0 50 100 150 200 C oncentration (ppm)

Figure 51. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of eggplant leave extract at elevated temperatures from the CS test

116

6.3.3.2 CS parameters and plots. The corrosion current is observed to decrease with EPPE concentration in

Table 16. It is seen that there is a greater change in the cathodic Tafel slope (bc) than the anodic Tafel slope (ba). This is similar to observations of the FLE and CPLE Tafel parameters.

Table 16. Tafel parameters obtained from various EPPE concentrations at elevated temperatures

T C (ppm) Ecorr ba (mV) bc (mV) Icorr IE% 0 -711.0 58.0 702.2 0.2514 - 25 -668.5 58.9 319.9 0.0902 64 50 -650.8 55.9 238.2 0.0569 77 25°C 100 -657.0 49.8 219.6 0.0322 87 150 -652.7 54.6 176.0 0.0222 91 200 -656.8 48.8 158.8 0.0158 94 0 -721.4 65.8 559.3 0.3841 - 25 -709.9 65.5 540.4 0.2053 47 50 -703.8 65.8 290.6 0.1277 67 40°C 100 -704.4 60.8 315.8 0.1147 70 150 -704.7 67.9 287.6 0.1112 71 200 -705.4 67.3 283.0 0.0956 75 0 -725.5 66.7 570.0 0.4811 - 25 -721.2 62.4 460.6 0.2602 46 50 -718.0 62.2 423.8 0.2346 51 50°C 100 -714.6 58.7 369.6 0.1924 60 150 -708.1 57.6 241.7 0.1412 71 200 -704.2 62.7 232.6 0.1264 74 0 -741.8 70.3 433.1 0.4852 - 25 -745.8 53.5 409.2 0.4796 1 50 -747.4 53.6 589.9 0.4472 8 70°C 100 -742.7 55.0 499.7 0.4089 16 150 -744.5 47.1 510.7 0.3426 29 200 -744.2 51.3 456.2 0.3366 31

It is observed from Figure 52 that there was a clear shift in both the anodic and cathodic Tafel slopes with addition of EPPE and this resulted in the decrease of the corrosion rate. The anodic shift is more noticeable than the cathodic shift from the 117 blank case at 25°C, 40°C and 50°C. This indicates that the metal oxidation rate decreased. Thus, the adsorbed film decreases the metal dissolution at a higher rate than hydrogen reduction. However, at 70°C, the cathodic shift is observed to be more prominent. The corrosion potential shift from the blank (uninhibited) case to the various Tafel curves that correspond to the CPLE concentrations is seen to be less than 85 mV at all temperatures. The lowest difference is seen at 70°C and this is similar to the CS plots of FLE and CPLE at 70°C. Based on these observations, EPPE can be regarded as a mixed inhibitor [114].

-500 -500 (a) (b) -550 -550

-600 -600

-650 -650

-700 -700

-750 -750 Blank Potential (mV) Potential (mV) Blank EPPE-25ppm -800 EPPE-25ppm -800 EPPE-50ppm EPPE-50ppm EPPE-100ppm EPPE-100ppm -850 EPPE-150ppm -850 EPPE-150ppm EPPE-200ppm EPPE-200ppm -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2) -500 -500 (c) (d) -550 -550

-600 -600

-650 -650

-700 -700

-750 Blank -750 Blank

Potential (mV) Potential (mV) EPPE-25ppm EPPE-25ppm -800 EPPE-50ppm -800 EPPE-50ppm EPPE-100ppm EPPE-100ppm -850 EPPE-150ppm -850 EPPE-150ppm EPPE-200ppm EPPE-200ppm -900 -900 0.0001 0.001 0.01 0.1 1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2)

Figure 52. CS plots for EPPE at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

118

6.3.3.3 Difference between test types. The IE calculated using the corrosion rates obtained from the LPR, EIS and CS tests are compared at different temperatures as seen in Figure 53. The IE of the LPR, EIS and CS tests follows the same trend at their respective temperatures. It is only in plot (d) that a noticeable difference in the IE between the three tests can be seen.

100 (a) 80 (b) 80 60

IE% IE% 40 40 LPR LPR 20 20 EIS EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm) 80 (c) (d) 60

IE% IE%

LPR 20 LPR 20 EIS EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm)

Figure 53. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C for EPPE

6.3.4 Adsorption isotherms for EPPE. The data collected from the EPPE experiments are used to plot four types of adsorption isotherms which are Langmuir, Temkin, Flory-Huggins and Frumkin. The best-fit adsorption isotherm will be chosen to represent EPPE.

119

250 350 (a) (b) 300 200

250

150 200

Ø C/ 100 C/ 150 100 50 y = 1.0258x + 13.681 y = 1.3667x + 15.488 50 R² = 0.9987 R² = 0.9985 0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

350 (c) (d) 300 450

250 400

200 Ø

Ø 350 C/ 150 C/ 300 100 y = 1.2767x + 30.377 250 y = 0.6527x + 278.07 50 R² = 0.993 R² = 0.6234 0 200 0 50 100 150 200 250 0 100 200 300 C (ppm) C (ppm)

Figure 54. Langmuir isotherms for adsorption of eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

120

0.75 0.95 (a) (b) 0.90 0.70 0.85

0.65

0.80

Ø Ø 0.75 0.60 0.70 0.55 y = 0.166x + 0.3167 0.65 y = 0.283x + 0.2757 R² = 0.9562 R² = 0.9412 0.60 0.50 1.3 1.7 2.1 2.5 1.3 1.8 2.3 2.8 Log C Log C

0.60 0.72 (c) (d) 0.50 0.67

0.62 0.40

0.57 Ø 0.30Ø 0.52 0.20 0.47 0.42 y = 0.2901x + 0.0207 0.10 y = 0.4453x - 0.564 R² = 0.9636 R² = 0.9494 0.37 0.00 1.3 1.8 2.3 2.8 1.3 1.8 2.3 Log C Log C

Figure 55. Temkin isotherms for adsorption of eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

121

-1.4 -1.6 (a) (b) -1.6 -1.8

-1.8

-2.0

/C)

/C) Ø

-2.0Ø log( log( -2.2 -2.2 y = 1.1927x - 1.0542 y = 4.3328x - 0.1987 -2.4 -2.4 R² = 0.9485 R² = 0.9327

-2.6 -2.6 -1.2 -1 -0.8 -0.6 -0.4 -0.55 -0.5 -0.45 -0.4 -0.35 -0.3 Log (1-Ø) Log (1-Ø)

-1.9

/C) /C) -2.5

-2.1

Ø log( -2.3log( -2.6 y = 0.4328x - 2.4666 y = 2.3875x - 1.242 -2.5 -2.6 R² = 0.3843 R² = 0.9132 -2.7 -2.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.35 -0.25 -0.15 -0.05 Log (1-Ø) Log (1-Ø)

Figure 56. F-H isotherms for adsorption of eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

122

3.5

(a) (b)

3.1 2.5

))

- -

2.7 /(1

/(1 2.1

Ø Ø

2.3 log log (C*

log log (C* y = 6.2582x - 2.4953 1.7 y = 7.526x - 2.618 1.9 R² = 0.9715 R² = 0.9649 1.3 1.5 0.5 0.55 0.6 0.65 0.7 0.75 0.6 0.7 0.8 0.9 1 Ø Ø

3.0 (c) (d)

2.6 2.5

))

)) Ø

Ø 2.0 -

2.2 -

/(1

/(1 Ø Ø 1.5 1.8

y = 5.1356x - 0.9058 1.0 log log (C* log log (C* y = 4.6208x + 0.1185 1.4 R² = 0.9857 0.5 R² = 0.9696

1.0 0.0 0.4 0.5 0.6 0.7 0.8 0 0.1 0.2 0.3 0.4 0.5 0.6 Ø Ø

Figure 57. Frumkin isotherms for adsorption of eggplant leave extract at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The Langmuir, Temkin, Florry-Huggins and Frumkin isotherms can be respectively seen in Figure 54, Figure 55, Figure 56 and Figure 57. Based on the four isotherms studied, it seems that even though Langmuir has the lowest residuals at 25°C, 40°C and 50°C, it becomes large for the 70°C case. This observation is consistent for the Flory-Huggins isotherm which is similar. However, the Temkin and Frumkin isotherm are able to fit the points with low residuals at all temperatures. Inspite of this, the Langmuir isotherm is chosen to represent the EPPE results.

It can be seen from Table 17 that the equilibrium adsorption constant (K) decreases with the increase of temperature which indicates the weakening of the adsorbed EPPE film. The higher the adsorption constant, the stronger is the bond

0 between the inhibitor molecules and the mild steel surface. The values of ∆퐺푎푑푠 are

123 positive for EPPE at all temperatures studied where it was found to increase with

0 temperature. Positive values of ∆퐺푎푑푠 are a result of values of K being lower than one. This indicates that the amount of EPPE inhibitor in the solution is more than the amount of EPPE molecules that is adsorbed on the surface of mild steel.

Table 17. EPPE thermodynamic data from Langmuir isotherm

∆퐺0 ( 풌푱 ) T (± 2°C) K 푎푑푠 풎풐풍

25 0.073 6.48

40 0.065 7.13

50 0.033 9.17

70 0.004 16.06

6.3.5 Activation energy of EPPE. The corrosion rate used in the Arrhenius plot is obtained from the cyclic sweep test. The purpose of this plot is to determine how the EPPE inhibitor affects the activation energy required for corrosion. The plot can be seen in Figure 58, and from it the activation energy is determined.

It is observed from Table 18 that increasing the concentration of EPPE in a

CO2-saturated 3.5wt% NaCl solution, results in an increase in the activation energy.

This is similar to the results of CPLE found in Table 12. The increase of Ea with EPPE concentration indicates the difficulty of the corrosion process occuring. High values of Ea are a good indicator that EPPE provides strong inhibitive action through raising the energy barrier of the corrosion process. The increase of the Ea can be attributed to the presence of the EPPE extract which increases the thickness of the double layer [115].

124

2.5

2.0

1.5

1.0

0.5

0.0 Blank y = -1511.2x + 6.2458

ln (CR) ln R² = 0.8159 -0.5 ELE-25ppm y = -3718.6x + 12.605 R² = 0.9796 ELE-50ppm y = -4746.6x + 15.555 -1.0 R² = 0.9885 ELE-100ppm y = -5719.3x + 18.367 -1.5 R² = 0.9709 ELE-150ppm y = -5998.2x + 19.02 -2.0 R² = 0.9372 ELE-200ppm y = -6704.9x + 21.084 R² = 0.9375 -2.5 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 1/T Figure 58. Arrhenius plot for eggplant leave extract

Table 18. Arrhenius parameters of EPPE 퐄퐚 Ea ( 풌푱 ) ln A 퐑 풎풐풍 Blank 6.246 1511.2 12.56

25ppm 12.605 3718.6 30.92

50ppm 15.555 4746.6 39.46

100ppm 18.367 5719.3 47.55

150ppm 19.020 5988.2 49.79

200ppm 21.084 6704.9 55.74

125

6.4 Commercial inhibitor A

6.4.1 Linear polarization resistance results for Inhibitor A. Inhibitor A was used to investigate the corrosion inhibition of mild steel in a

CO2-saturated 3.5wt% NaCl solution. Inhibitor A is a natural inhibitor which is used commercially in industry. The results are presented in this section at 25°C, 40°C, 50°C and 70°C using five different concentrations of 25ppm, 50ppm, 100ppm, 150ppm and 200ppm. The results from the LPR, EIS and CS tests are presented along with the adsorption isotherms and the activation parameters of the inhibition process.

6.4.1.1 Corrosion rate from LPR. It is observed in Figure 59(a), that when the concentration of Inhibitor A increases, the corrosion rate decreases for each temperature. When the concentration is fixed and the temperature is increased, the corrosion rate of mild steel increases. The only exceptions observed were at 50°C and 70°C where the corrosion rate slightly increases at 50ppm. This is likely due to desorption of the inhibitor molecules from the mild steel surface or due to experimental error since typically, the corrosion rate is expected to decrease with inhibitor concentration.

The general trend observed in Figure 59(b) is that the IE increases with the addition of Inhibitor A at all temperatures. The IE decreases for a fixed concentration as the temperature increases. The exception to this is seen for 25ppm of Inhibitor A where the 50°C point is higher than the 40°C. It is noticed that the increase of temperature from 40°C to 50°C results in little difference in the IE obtained.

126

3.0 (a) 25°C 40°C

50°C 2.0 70°C

1.0 Corrosion (mm/yr)rate

0.0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 59. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated temperatures from the LPR test

6.4.1.2 LPR parameters. The LPR test provides insights about the LPR value, corrosion potential and corrosion current density for each experiment using Inhibitor A. The LPR value is the long-term polarization resistance that is caused by the protective film formed over the mild steel. The protective film is formed through the adsorption of the Inhibitor A

127 molecules onto the mild steel surface. Increased values of LPR result in a lower current density that passes through the solution.

Table 19. Parameters obtained from the LPR test for Inhibitor A

T C (ppm) LPR (ohm.cm²) Icorr (mA/cm²) Potential (mV) IE% 0 243.5 0.0662 -710.2 - 25 817.4 0.0248 -679.8 84 50 1341.1 0.0151 -654.6 90 25°C 100 3531.3 0.0060 -646.2 96 150 3929.4 0.0054 -653.4 97 200 4378.2 0.0050 -655.3 97 0 178.3 0.1435 -719.8 - 25 230.3 0.1038 -719.2 44 50 267.9 0.0892 -716.2 52 40°C 100 814.8 0.0255 -682.9 84 150 1067.8 0.0195 -679.6 88 200 1221.5 0.0177 -677.9 90 0 132.67 0.2040 -722.9 - 25 166.7 0.1104 -724.5 23 50 186.9 0.1336 -725.1 31 50°C 100 571.8 0.0437 -698.6 78 150 703.4 0.0287 -689.1 82 200 874.3 0.0231 -688.2 85 0 128.1 0.2052 -739.0 - 25 136.4 0.1629 -746.9 6 50 138.3 0.1733 -746.4 7 70°C 100 260.3 0.0921 -747.8 51 150 250.4 0.0813 -748.1 49 200 408.2 0.0499 -741.3 69

A plot of LPR obtained for each concentration of Inhibitor A at 25°C, 40°C, 50°C and 70°C can be seen in plot (c) of Figure 59.This graph clearly illustrates that the LPR increases with Inhibitor A concentration for each temperature. However, the LPR decreases at elevated temperatures for a fixed concentration.

128

5000 25°C

40°C 4000 50°C

70°C

3000

LPR (ohm.cm²) LPR 2000

1000

0 0 50 100 150 200 Concentration (ppm) Figure 60. Effect of Inhibitor A concentration on the steady state LPR value

6.4.2 Electrochemical impedance spectroscopy results for Inhibitor A. The impedance test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of inhibitor A.

6.4.2.1 Corrosion rate from EIS. The corrosion rate of mild steel is observed to decrease with addition of Inhibitor A. This also results in the increase of the inhibition efficiency. It is noticed that at higher concentrations of Inhibitor A, the increase in the system temperature does not drastically affect the IE obtained. These results are represented graphically in Figure 61.

129

(a) 4.0 25°C

40°C

50°C 3.0 70°C

2.0 Corrosion (mm/yr)rate 1.0

0.0 0 50 100 150 200 Concentration (ppm)

(b) 100

60 IE%

40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 61. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated temperatures from the EIS test

6.4.2.2 EIS parameters and plots. This section consists of the impedance parameters and Nyquist plots from the EIS test. These depict different concentrations of Inhibitor A at 25°C, 40°C, 50°C and 70°C, respectively.

130

Table 20. Impedance parameters obtained using Inhibitor A

풎푭 Cdl ( ) T C (ppm) Rsol (ohms.cm²) Rct (ohms.cm²) 풄풎ퟐ IE%

0 16.53 221.20 367.60 - 25 16.93 788.50 66.44 72 50 16.72 1474.00 49.47 85 25°C 100 15.54 3448.00 38.77 94 150 15.57 3712.00 36.37 94 200 16.15 4356.00 39.08 95 0 14.53 138.60 231.60 - 25 12.56 199.30 174.80 30 50 11.97 228.80 146.00 39 40°C 100 12.62 770.10 55.30 82 150 12.94 1075.00 40.23 87 200 12.56 1206.00 50.91 89 0 11.29 101.20 302.80 - 25 11.64 139.60 195.30 28 50 11.11 158.20 176.50 36 50°C 100 10.39 475.70 74.21 79 150 11.04 665.20 63.60 85 200 10.86 800.10 56.12 87 0 8.80 84.10 306.80 - 25 7.88 95.05 277.80 12 50 7.69 99.20 281.60 15 70°C 100 7.69 224.50 145.40 63 150 7.78 212.60 135.30 60 200 7.90 373.40 84.59 77

The results from Table 20 confirms that the addition of Inhibitor A causes an increase in Rct and a decrease in Cdl. This signifies the decrease in formation of the anodic process controlling intermediates from the dissolution of mild steel. The Cdl values may have decreased due to the protective film formed by Inhibitor A on the mild steel surface. This trend occurs due to the adsorption of the inhibitor molecules onto the mild steel surface. These molecules are believed to have replaced the water molecules on the mild steel surface and thus decrease the diffusion rate of the reactant elements.

131

2000 600 (a) Blank (b) Blank A-25ppm 500 A-25ppm

A-50ppm A-50ppm

1500 A-100ppm A-100ppm 400 A-150ppm A-150ppm A-200ppm A-200ppm 1000 300

200

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 500 100

0 0 0 1000 2000 3000 4000 5000 0 200 400 600 800 1000 1200 1400 Z' (ohm.cm²) Z' (ohm.cm²) 400 Blank Blank (c) 160 (d) A-25ppm A-25ppm

A-500ppm A-50ppm

300 A-100ppm A-100ppm 120 A-150ppm A-150ppm A-200ppm 200 A-200ppm

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 100 40

0 0 0 500 1000 0 100 200 300 400 500 Z' (ohm.cm²) Z'' (ohm.cm²)

Figure 62. Nyquist plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C & (d) 70°C from the EIS test

The plots in Figure 62 illustrate that the addition of Inhibitor A results in the increase of the diameter of the semi-circle in the Nyquist plots. This indicates that the strength of the film, formed by Inhibitor A, is increasing. This trend is observed at all temperatures. This film is also thought to suppress the kinetics of the cathodic and anodic reactions [105].

The phase angle is plotted against the log of frequency as seen in Figure 63. These plots are useful since they can define a domain of pure capacitive behavior. At high frequencies, the phase angle decreases to almost zero and this is characteristic of a response to resistive behavior. At intermediate frequencies, the maximum phase angles obtained at various concentrations of Inhibitor A can be seen in Table 21.

132

90 90 Blank (a) Blank 80 25ppm 80 25ppm (b) 50ppm 50ppm 70 70 100ppm 100ppm

60 150ppm 60 150ppm

200ppm 50 50 200ppm 40 40

30 30

Phase Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency 90 90 Blank (c) Blank (d) 80 25ppm 80 25ppm 50ppm 50ppm 70 100ppm 70 100ppm

60 150ppm 60 150ppm

50 200ppm 50 200ppm 40 40

30 30

Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency

Figure 63. Theta-frequency plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

Table 21. Summary of maximum phase angles obtained for Inhibitor A Phase angle (°)

C (ppm) 25°C 40°C 50°C 70°C

0 47.26 45.36 43.19 44.05

25 65.34 51.97 48.95 46.63

50 71.61 54.99 50.67 46.02

100 75.61 68.24 64.60 55.67

150 75.20 69.70 66.34 55.67

200 75.96 70.39 67.92 61.06

133

Ideally, the phase angle should be 90° which is indicative of capacitive behavior. The difference between the maximum phase angle and 90° is thought to be due to deviations that arise from ideal capacitive behavior. The phase angle gradually approaches the ideal capacitive value due to the slowing down of dissolution rate with time. Thus, in the presence of the Inhibitor A, it is observed that the phase angle becomes closer to 90° through a faster attainment of steady state [77].

6.4.3 Cyclic sweep results for Inhibitor A. The cyclic sweep test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, using Inhibitor A.

6.4.3.1 Corrosion rate from CS. Figure 64(a) shows that the addition of Inhibitor A results in the decrease of the corrosion rate of mild steel at all temperatures. It was observed that the corrosion rate increased with temperature and this is similar to the corrosion plots obtained from the LPR and EIS tests with the only difference being the magnitude of the observed values. Figure 64(b), the increase in temperature results in reducing the IE. The general trend observed is that the IE increases with concentration and decreases at elevated temperatures. This is similar to the trends observed in the LPR and EIS tests.

6.4.3.2 CS parameters and plots. It is observed from Table 22 that there is a greater change in the cathodic Tafel slope (bc) than the anodic Tafel slope (ba). This is similar to observations of the other investigated inhibitor’s Tafel parameters. The corrosion current value was also observed to decrease with concentration.

134

7 (a) 25°C 6 40°C

50°C 5 70°C 4

2 Corrosion (mm/yr)rate

0 0 50 100 150 200 Concentration (ppm)

100 (b)

IE% 40 25°C 40°C 20 50°C 70°C 0 0 50 100 150 200 C oncentration (ppm)

Figure 64. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of Inhibitor A at elevated temperatures from the CS test

135

Table 22. Tafel parameters obtained from various Inhibitor A concentrations

T C (ppm) Ecorr ba (mV) bc (mV) Icorr IE% 0 -711.0 58.0 702.2 0.2514 - 25 -679.4 58.2 231.4 0.0668 73 50 -655.6 61.2 176.4 0.0590 77 25°C 100 -648.5 74.6 137.9 0.0442 82 150 -655.5 78.1 144.7 0.0214 91 200 -657.3 78.3 131.2 0.0196 92 0 -721.4 65.8 559.3 0.3841 - 25 -720.3 62.3 465.8 0.2961 23 50 -718.0 63.3 440.3 0.2664 31 40°C 100 -684.1 65.3 178.2 0.1411 63 150 -679.7 68.7 180.2 0.1207 69 200 -678.8 61.7 134.9 0.1180 69 0 -725.5 66.7 570.0 0.4811 - 25 -726.7 64.6 512.2 0.4389 9 50 -727.7 64.8 502.7 0.4098 15 50°C 100 -701.7 72.3 237.2 0.1901 60 150 -691.0 64.9 163.0 0.1887 61 200 -690.6 65.8 152.8 0.1865 61 0 -741.8 70.3 433.1 0.4852 - 25 -748.9 61.7 443.1 0.4562 6 50 -748.5 63.1 436.8 0.4408 9 70°C 100 -749.2 58.7 354.0 0.2432 50 150 -749.7 55.0 316.8 0.2263 53 200 -744.8 63.6 295.9 0.1880 61

It is observed from Figure 65, that there was a clear shift in both the anodic and cathodic Tafel slopes with the addition of Inhibitor A and this resulted in a decrease in the corrosion rate. The anodic shift is more noticeable than the cathodic shift from the blank case at 25°C, 40°C and 50°C. This indicates that the metal oxidation rate decreased. Thus, the adsorbed film decreases the metal dissolution at a higher rate than hydrogen reduction. However, at 70°C, the cathodic shift is observed to be slightly more prominent. The corrosion potential shift from the blank case is also seen to be greatly reduced. It is definitely less than 85 mV at all temperatures and based on these observations, Inhibitor A is classified as a mixed inhibitor [114].

136

-450 -500 (a) (b) -500 -550 -550

-600

-600 -650 -650 -700 -700 -750

Potential-750 (mV) Blank Blank Potential (mV) A-25ppm A-25ppm -800 A-50ppm -800 A-50ppm A-100ppm A-100ppm -850 A-150ppm -850 A-150ppm A-200ppm A-200ppm -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2) -500 -600 (c) (d) -550 -650

-600

-700 -650

-700 -750

-750 Blank Potential (mV) Blank Potential (mV)-800 A-25ppm A-25ppm -800 A-50ppm A-50ppm A-100ppm -850 A-100ppm -850 A-150ppm A-150ppm A-200ppm A-200ppm -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2)

Figure 65. CS plots for Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

6.4.3.3 Difference between test types. The inhibition efficiency calculated using the corrosion rates obtained from the LPR, EIS and CS tests are plotted together and compared at different temperatures. It can be seen Figure 66 that the inhibition efficiency of the LPR, EIS and CS methods follow the same trend at their respective temperatures.

137

100 100 (a) (b)

60 60

IE% IE% 40 40

LPR LPR 20 20 EIS EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm) 100 100 (c) (d) 80 80

60 60

IE% IE% 40 40

LPR 20 20 LPR EIS EIS CS CS 0 0 0 50 100 150 200 0 50 100 150 200 C oncentration (ppm) C oncentration (ppm)

Figure 66. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C for Inhibitor A

6.4.4 Adsorption isotherms for Inhibitor A. The data collected from the Inhibitor A experiments are used to plot four types of adsorption isotherms which are Langmuir, Temkin, Florry-Huggins and Frumkin. The best-fit adsorption isotherm will be chosen to represent Inhibitor A. The surface coverage used in these adsorption isotherms is determined using the inhibition efficiency obtained from the EIS test.

138

250 250 (a) (b) 200 200

150 150

C/ C/ 100 100 y = 1.0384x + 3.2533 y = 1.0402x + 17.34 50 R² = 0.9999 50 R² = 0.9996

0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

300 350 (c) (d) 250 300 250

200

200 Ø

150Ø C/ C/ 150 100 y = 0.9824x + 74.711 y = 1.0194x + 24.727 100 R² = 0.8252 R² = 0.9999 50 50

0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

Figure 67. Langmuir isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The three best-fit points are chosen for the Langmuir plots to obtain a trendline which has the lowest residual.

139

1.2 1.1 (a) (b) 1.0 1.0

0.8

Ø 0.9 Ø 0.6

0.8 y = 0.254x + 0.3927 0.4 y = 0.7369x - 0.7561 R² = 0.9084 R² = 0.9205 0.7 0.2 1.2 1.7 2.2 2.7 1.3 1.5 1.7 1.9 2.1 2.3 2.5 Log C Log C

1.1 1.0 (c) (d) 0.9 0.8

0.7 0.6

Ø Ø 0.5 0.4 y = 0.7541x - 0.8152 0.3 R² = 0.9273 0.2 y = 0.7764x - 1.0323 R² = 0.8997 0.1 0.0 1.3 1.5 1.7 1.9 2.1 2.3 2.5 1.3 1.5 1.7 1.9 2.1 2.3 2.5 Log C Log C

Figure 68. Temkin isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

140

-2.0 (a)(a) (b) -2.1

-2.1

-2.2

/C)

/C) Ø Ø -2.2

-2.3

log( log(

-2.3 -2.4 y = 2.7171x + 1.1767 y = 1.2989x - 1.111 R² = 0.905 R² = 0.9583 -2.5 -2.4 -1.3 -1.26 -1.22 -1.18 -1.00 -0.90 -0.80 -0.70 Log (1-Ø) Log (1-Ø)

-2.0 -2.1 (c) (d)

-2.1 -2.2

/C)

/C) Ø

-2.2 Ø -2.3

log( log(

-2.3 -2.4 y = 1.1156x - 1.3492 y = 0.4178x - 2.1312 R² = 0.9912 R² = 0.2383 -2.4 -2.5 -1 -0.9 -0.8 -0.7 -0.6 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 Log (1-Ø) Log (1-Ø)

Figure 69. F-H isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

141

4.0 (a) 3.4 (b)

3.5

2.9

))

Ø -

3.0 - /(1

/(1 2.4

Ø Ø

2.5 1.9

log log (C* log log (C* y = 7.2918x - 3.5353 y = 3.3465x + 0.0888 2.0 R² = 0.9502 1.4 R² = 0.9821

1.5 0.9 0.6 0.7 0.8 0.9 1.0 0.2 0.4 0.6 0.8 1.0 Ø Ø

3.0

)) ))

Ø Ø -

- 2.0 /(1

/(1 2.0

Ø Ø

1.0 log log (C* log log (C* 1.0 y = 3.2729x + 0.1535 y = 3.2746x + 0.2892 R² = 0.9827 R² = 0.9815

0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Ø Ø

Figure 70. Frumkin isotherms for adsorption of Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The Langmuir, Temkin, Florry-Huggins and Frumkin isotherms can be respectively seen in Figure 67, Figure 68, Figure 69 and Figure 70. The Langmuir isotherm is observed to have the best fit points at 25°C, 40°C and 50°C since its residuals are the lowest of the isotherms studied. At 70°C, the Langmuir residual becomes higher (0.82) when compared to the Temkin and Frumkin isotherms. However, the Langmuir isotherm is chosen to represent Inhibitor A.

It can be seen from Table 23 that the equilibrium adsorption constant (K) decreases with the increase of temperature which indicates the weakening of the adsorbed film. The higher the adsorption constant, the stronger is the bond between

0 the inhibitor molecules and the mild steel surface. The values of ∆퐺푎푑푠 are positive for Inhibitor A where it was found to increase with temperature. Positive values of 142

0 ∆퐺푎푑푠 are a result of values of K being lower than one. This indicates that the amount of Inhibitor A in the solution is more than the amount of Inhibitor A molecules that is adsorbed on the surface of mild steel.

Table 23. Thermodynamic data from Langmuir isotherm for Inhibitor A

∆퐺0 ( 풌푱 ) T (± 2°C) K 푎푑푠 풎풐풍

25 0.307 2.92

40 0.040 7.43

50 0.038 8.62

70 0.013 12.31

6.4.5 Activation energy of Inhibitor A. The corrosion rate used in the Arrhenius plot is obtained from the cyclic sweep test. The purpose of this plot is to determine how Inhibitor A affects the activation energy of the corrosion process. From the Arrhenius plot, the Arrhenius constant and activation energy can be determined.

It is observed from Table 24, that increasing the concentration of Inhibitor A resulted in an increase in the activation energy from the blank case for all concentrations. However, the activation energy obtained for each concentration did not follow a consistent trend. The increase in the activation energy makes it more difficult for the corrosion process to occur. High values of Ea are a good indicator that Inhibitor A provided strong inhibitive action through raising the energy barrier of the corrosion process.

143

2.2

1.7

1.2

0.7

0.2 Blank y = -1511.2x + 6.2458 R² = 0.8159 -0.3 ln (CR) ln A-25ppm y = -4274x + 14.484 R² = 0.7591 -0.8 A-50ppm y = -4486.4x + 15.067 R² = 0.7792 -1.3 A-100ppm y = -3788.5x + 12.304 R² = 0.8544 -1.8 y = -5239.2x + 16.618 A-150ppm R² = 0.8031 -2.3 y = -5028.8x + 15.88 A-200ppm R² = 0.7424 -2.8 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 1/T Figure 71. Arrhenius plot for Inhibitor A

Table 24. Arrhenius parameters of Inhibitor A 퐄퐚 Ea ( 풌푱 ) ln A 퐑 풎풐풍 Blank 6.246 1511.2 12.56

25ppm 14.484 4274.0 35.53

50ppm 15.067 4486.4 37.30

100ppm 12.304 3788.5 31.50

150ppm 16.618 5239.2 43.56

200ppm 15.880 5028.8 41.81

144

6.5 Commercial inhibitor B Inhibitor B was used to investigate the corrosion inhibition of mild steel in a

CO2-saturated 3.5wt% NaCl solution. Inhibitor B is a synergistic mix of corrosion inhibition active ingredients of ammonium salts with salted imidazoline which both contain nitrogen atoms. The results are presented in this section at 25°C, 40°C, 50°C and 70°C using three different concentrations of 100 ppm, 150 ppm and 200 ppm. The 5 sections presented will respectively show the LPR results, EIS results, CS results, adsorption isotherms and the activation parameters of the inhibition process.

6.5.1 Linear polarization resistance results for Inhibitor B.

6.5.1.1 Corrosion rate from LPR. It is observed that the corrosion rate of mild steel decreases drastically when Inhibitor B is used. This is similar to the results of other investigated inhibitors from the electrochemical tests. Moreover, it is observed to also be effective at elevated temperatures since its IE does diminish like the other inhibitors. From Figure 72, it is seen that the IE starts to plateau at concentrations above 150 ppm.

6.5.1.2 LPR parameters. The LPR test provides insights about the LPR value, corrosion potential and corrosion current density for each experiment using Inhibitor B. The LPR value is the long-term polarization resistance that is caused by the protective film formed over the mild steel. Increased values of LPR result in a lower current density that passes through the solution. This is similar to the trends of the other tested inhibitor LPR plots.

A plot of LPR obtained for each concentration of Inhibitor B at 25°C, 40°C, 50°C and 70°C can be seen in Figure 72. The plot shows that the LPR usually increases with Inhibitor B concentration. It is also seen that the magnitude of the LPR values decrease at elevated temperatures for a fixed concentration.

145

3 (a) 25°C 40°C

50°C

70°C 2

1 Corrosion (mm/ur)rate

0 0 50 100 150 200 Concentration (ppm) (b) 100

96 IE%

94 25°C 40°C 92 50°C 70°C 90 100 150 200 C oncentration (ppm)

Figure 72. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of Inhibitor B at elevated temperatures from the LPR test

146

Table 25. Parameters obtained from the LPR test for Inhibitor B

T C (ppm) LPR (ohm.cm²) Icorr (mA/cm²) Potential (mV) IE% 0 243.5 0.0662 -710.2 - 100 16368.0 0.0009 -586.6 98.51 25°C 150 30307.0 0.0005 -594.3 99.20 200 43112.0 0.0004 -588.7 99.44 0 178.3 0.1435 -719.8 - 100 10606.0 0.0017 -625.2 98.32 40°C 150 12365.0 0.0015 -616.3 98.56 200 12849.0 0.0014 -611.6 98.61 0 132.7 0.2040 -722.9 - 100 6099.7 0.0032 -628.3 97.82 50°C 150 7719.1 0.0026 -619.3 98.28 200 7131.8 0.0028 -615.9 98.14 0 128.1 0.2052 -739.0 - 100 1654.5 0.0121 -658.6 92.26 70°C 150 1784.1 0.0112 -656.6 92.82 200 1796.9 0.0107 -654.7 92.87

25°C

40°C 40000 50°C

70°C

30000

LPR (ohm.cm²) LPR 20000

10000

0 0 50 100 150 200 Concentration (ppm)

Figure 73. Effect of Inhibitor B concentration on the steady state LPR value

147

6.5.2 Electrochemical impedance spectroscopy results for Inhibitor B. The impedance test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, in the presence of inhibitor B. The test was conducted at 25°C, 40°C, 50°C and 70°C with each experiment lasting for 20 minutes.

6.5.2.1 Corrosion rate from EIS.

4 (a) 25°C 40°C

50°C 3 70°C

Corrosion (mm/ur)rate 1

0 0 50 100 150 200 Concentration (ppm) 101 (b) 100

97 IE%

96 25°C 95 40°C 94 50°C 70°C 93 100 150 200 C oncentration (ppm)

Figure 74. Corrosion rate (a) and inhibition efficiency (b) of mild steel in a CO2 saturated 3.5wt% NaCl solution in the presence of Inhibitor B at elevated temperatures from the EIS test

148

The corrosion rates of mild steel can be seen in plot (a) of Figure 74, where the introduction of Inhibitor B results in a huge decrease in the corrosion rate from the blank case. The corresponding IE can be seen in plot (b) where it is seen to be very high for all three concentrations of Inhibitor B. It is observed to not always increase with concentration of Inhibitor B and this could be due to experimental factors or to desorption of the inhibitor from the mild steel surface.

6.5.2.2 EIS parameters and plots. This section consists of the impedance parameters as well as the Nyquist and Theta-frequency plots which depict different concentrations of Inhibitor B at 25°C, 40°C, 50°C and 70°C respectively.

Table 26. Impedance parameters obtained using Inhibitor B

풎푭 Cdl ( ) T C (ppm) Rsol (ohms.cm²) Rct (ohms.cm²) 풄풎ퟐ IE% 0 16.53 221.2 367.60 - 100 21.42 46050.0 26.65 99.52 25°C 150 22.87 34960.0 22.06 99.37 200 22.16 43860.0 22.52 99.50 0 14.53 138.6 231.60 - 100 20.15 9936.0 30.26 98.61 40°C 150 20.54 11420.0 28.70 98.79 200 18.27 11600.0 29.82 98.81 0 11.29 101.2 302.80 - 100 15.39 6338.0 36.84 98.40 50°C 150 17.97 6948.0 28.69 98.54 200 16.79 6512.0 30.81 98.45 0 8.80 84.1 306.80 - 100 10.78 1439.0 46.50 94.16 70°C 150 11.37 1609.0 41.97 94.77 200 11.83 1603.0 42.18 94.75

The plots in Figure 75 illustrate that the addition of Inhibitor B results in the increase of the diameter of the semi-circle in the Nyquist plots. This indicates the strengthening of the film formed by Inhibitor B. This trend is observed at all temperatures. This film is also thought to suppress the kinetics of the cathodic and anodic reactions [105]. It should be noted that the size of the 200 ppm curves are slightly

149 smaller than the 150ppm curves at 50°C and 70°C. It is believed that the Inhibitor B molecules replace the water molecules on the mild steel surface and thus decrease the diffusion rate of the reactant elements.

18000 5000 (a) (b) 15000

4000

12000 3000 9000

2000 Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 6000 Blank Blank B-100ppm 3000 B-100ppm 1000 B-150ppm B-150ppm B-200ppm B-200ppm 0 0 0 10000 20000 30000 0 5000 10000 15000 Z' (ohm.cm²) Z' (ohm.cm²) 800 (c) 3000 700 (d)

2500 600

2000 500 400 1500

300 Z'' Z'' (ohm.cm²) 1000 Blank Z'' (ohm.cm²) Blank B-100ppm 200 500 B-100ppm B-150ppm 100 B-150ppm B-200ppm B-200ppm 0 0 0 2000 4000 6000 8000 0 500 1000 1500 2000 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 75. Nyquist plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C & (d) 70°C from the EIS test

The phase angle is plotted against the log of frequency as seen in Figure 76. These plots are useful since they can define a domain of pure capacitive behavior. At high frequencies, the phase angle decreases to almost zero and this is characteristic of a response to resistive behavior. At intermediate frequencies, the maximum phase angles obtained at various concentrations of Inhibitor B can be seen in Table 27.

150

90 90 Blank Blank (b) 80 100ppm (a) 80 100ppm 70 150ppm 70 150ppm

200ppm 200ppm

60 60 50 50 40 40

30 30 Phase Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency 90 90 Blank (c) Blank (d) 80 100ppm 80 100ppm 70 150ppm 70 150ppm

200ppm 200ppm

60 60 50 50 40 40

30 30 Phase Phase angle Phase Phase angle 20 20 10 10 0 0 -10 -10 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Log frequency Log frequency

Figure 76.Theta-frequency plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

Table 27. Summary of maximum phase angles obtained for Inhibitor B Phase angle (°)

C (ppm) 25°C 40°C 50°C 70°C

0 47.26 45.36 43.19 44.05

100 67.45 70.25 71.47 68.76

150 68.66 70.52 71.66 69.62

200 68.71 71.69 72.12 69.03

151

Ideally, the phase angle should be 90° which is indicative of capacitive behavior. The difference between the maximum phase angle and 90° is thought to be due to deviations that arise from ideal capacitive behavior. The phase angle gradually approaches the ideal capacitive value due to the slowing down of the dissolution rate with time. Thus, in the presence of Inhibitor B, it is observed that the phase angle becomes closer to 90° through a faster attainment of steady state [77].

6.5.3 Cyclic sweep results for Inhibitor B. The cyclic sweep test was used to investigate the corrosion of mild steel in a

CO2-saturated 3.5wt% NaCl solution, using Inhibitor B.

6.5.3.1 Corrosion rate from CS. It is seen from plot (a) in Figure 77 that the addition of Inhibitor B results in the decrease of the corrosion rate of mild steel for each temperature. This is similar to the corrosion plots obtained from the LPR and EIS tests and where it is observed to have the best performance of all the investigated inhibitors. From plot (b), it is seen that the IE increases with concentration. However, for 200ppm of Inhibitor B at 50°C, it is observed that the IE decreases. This could be due to desorption from the mild steel surface.

6.5.3.2 CS parameters and plots.

It is observed that there is a greater change in the cathodic Tafel slope (bc) than the anodic Tafel slope (ba). This is similar to observations of the other investigated inhibitor Tafel parameters. From Table 4, the corrosion current value is observed to decrease with concentration.

152

8 (a) 25°C 40°C 50°C

6 70°C

Corrosion (mm/ur)rate 2

0 0 50 100 150 200 Concentration (ppm)

(b)

92 IE%

25°C 87 40°C 50°C 70°C 82 100 150 200 C oncentration (ppm)

Figure 77. Plots obtained from the CS test using Inhibitor B where (a) Corrosion rate as a function of FLE concentration (b) Inhibition efficiency determined from the corrosion rate

153

Table 28. Tafel parameters obtained from various Inhibitor B concentrations at elevated temperatures

T C (ppm) Ecorr ba (mV) bc (mV) Icorr IE% 0 -711.0 58.0 702.2 0.2514 - 100 -566.8 46.8 127.9 0.0079 96.86 25°C 150 -596.9 67.1 128.4 0.0024 99.05 200 -590.4 68.8 119.5 0.0017 99.31 0 -721.4 65.8 559.3 0.3841 - 100 -628.0 62.8 125.9 0.0128 96.66 40°C 150 -619.8 59.2 133.0 0.0103 97.31 200 -615.9 60.8 119.3 0.0083 97.85 0 -725.5 66.7 570.0 0.4811 - 100 -631.2 60.9 119.2 0.0252 94.75 50°C 150 -622.3 71.8 124.8 0.0160 96.68 200 -619.1 63.2 114.7 0.0192 96.01 0 -741.8 70.3 433.1 0.4852 - 100 -661.8 66.3 151.8 0.0699 85.60 70°C 150 -659.1 65.3 137.9 0.0516 89.36 200 -658.0 68.8 134.0 0.0510 89.49

It is observed from Figure 78, that there was a clear shift in both the anodic and cathodic Tafel slopes with the addition of Inhibitor B. The anodic shift is more noticeable than the cathodic shift from the blank case at 25°C, 40°C and 50°C. This indicates that the metal oxidation rate has decreased. Thus, the adsorbed film decreases the metal dissolution at a higher rate than hydrogen reduction. It is noticed that the 100ppm curve in Figure 78 is higher than the 150ppm and 200ppm curves. This is likely due to an experimental error in the CS test.

The difference in the corrosion potential shift from the blank case to the various Tafel curves that correspond to the Inhibitor B concentrations is seen to be more than 85 mV at all temperatures. Thus, Inhibitor B can be classified as an anodic inhibitor.

154

-400 (a) (b) -500

-500

-600 -600

-700

Potential (mV) Potential (mV) Blank Blank -800 -800 B-100ppm B-100ppm B-150ppm B-150ppm B-200ppm B-200ppm -900 -900 0.000001 0.0001 0.01 1 0.000001 0.0001 0.01 1 Current (mA/cm2) Current (mA/cm2) -500 (c (d) -500 )

-600

-700

-700 Potential (mV) Potential (mV) Blank -800 Blank -800 B-100ppm B-100ppm B-150ppm B-150ppm B-200ppm B-200ppm -900 -900 0.000001 0.0001 0.01 1 0.000001 0.0001 0.01 1 Current (mA/cm^2) Current (mA/cm^2)

Figure 78. CS plots for Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

6.5.3.3 Difference between test types. It is observed from these comparison plots that as the concentration of Inhibitor B increases, the difference of IE between the tests becomes smaller. It is also noticed that there is a slight difference between the IE obtained from the cyclic sweep test and the LPR and EIS tests.

155

100 100 (a) (b)

98 IE% IE% 98

97 LPR 97 LPR EIS EIS CS CS 96 96 100 150 200 100 150 200 C oncentration (ppm) C oncentration (ppm) 100 100 (c) (d)

98 95

90 IE% IE% 96

94 LPR 85 LPR EIS EIS CS CS 92 80 100 150 200 100 150 200 C oncentration (ppm) C oncentration (ppm)

Figure 79. Difference between electrochemical tests at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C for Inhibitor B

6.5.4 Adsorption isotherms for Inhibitor B. The data collected from the Inhibitor B experiments are used to plot four types of adsorption isotherms which are Langmuir, Temkin, Florry-Huggins and Frumkin. The best-fit adsorption isotherm will be chosen to represent Inhibitor B. The surface coverage used in these adsorption isotherms is determined using the inhibition efficiency obtained from the EIS test.

156

250 250 (a) (b)

200 200

150 150

C/ C/ 100 100 y = 1.0053x + 0.0205 y = 1.01x + 0.3862 50 R² = 1 50 R² = 1

0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

250 250 (c) (d)

200 200

150 150

Ø C/ 100 C/ 100 y = 1.016x + 0.0317 y = 1.0487x + 1.2178 50 R² = 1 50 R² = 1

0 0 0 50 100 150 200 250 0 50 100 150 200 250 C (ppm) C (ppm)

Figure 80. Langmuir isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

157

0.9885 (a) (b) 0.9950

0.9875

0.9940

Ø Ø

0.9865 0.9930 y = 0.0022x + 0.9893 y = 0.0069x + 0.9725 R² = 0.825 R² = 0.8885

0.9920 0.9855 1.2 1.6 2.0 2.4 1.9 2.1 2.3 Log C Log C

0.9854 (c) (d) 0.9485

0.9848

0.9465

Ø 0.9842Ø 0.9445

0.9836 0.9425 y = 0.0209x + 0.9006 y = 0.0019x + 0.9799 R² = 0.8079 R² = 0.1511 0.9830 0.9405 1.9 2.0 2.1 2.2 2.3 2.4 1.9 2.0 2.1 2.2 2.3 2.4 Log C Log C

Figure 81. Temkin isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

158

-1.8 -1.9 (a) (b) -1.9 -2.0

-2.0

/C)

-2.1/C) Ø

Ø -2.1

log( log( -2.2 -2.2 y = -0.6242x - 3.5792 y = 3.8465x + 5.1361 -2.3 R² = 0.0694 -2.3 R² = 0.8919

-2.4 -2.4 -2.40 -2.30 -2.20 -2.10 -2.00 -1.90 -1.80 Log (1-Ø) Log (1-Ø) -1.8 (c) -2.0 (d) -1.9

-2.0 -2.1

/C) /C) Ø -2.1Ø

-2.2

log( log( -2.2 y = 2.7787x + 2.8278 y = 4.8732x + 3.9815 R² = 0.1422 -2.3 -2.3 R² = 0.8029

-2.4 -2.4 -1.83 -1.81 -1.79 -1.77 -1.75 -1.3 -1.28 -1.26 -1.24 -1.22 Log (1-Ø) Log (1-Ø)

Figure 82. F-H isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

159

4.30 3.60 (a) (b)

4.20

))

)) Ø

3.50 Ø 4.10

/(1

/(1 Ø Ø 4.00 3.40

3.90 log log (C* log log (C* y = 162.96x - 156.84 3.30 y = 29.17x - 24.175 R² = 0.9264 R² = 0.4497 3.80 3.20 3.70 0.940 0.942 0.944 0.946 0.948 0.950 0.9855 0.9865 0.9875 0.9885 Ø Ø

4.20 (c) 3.60 (d)

4.10

))

)) 3.50

Ø Ø -

4.00- /(1

/(1 3.40

Ø Ø 3.90 3.30

y = 106.59x - 100.93

log log (C* log (C* 3.80 R² = 0.2467 y = 47.036x - 41.078 3.20 R² = 0.8613

3.70 3.10 0.9830 0.9835 0.9840 0.9845 0.9850 0.94 0.942 0.944 0.946 0.948 0.95 Ø Ø

Figure 83. Frumkin isotherms for adsorption of Inhibitor B at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

The Langmuir, Temkin, Florry-Huggins and Frumkin isotherms can be respectively seen in Figure 80, Figure 81, Figure 82 and Figure 83. The Langmuir isotherm is observed to have the best fit points at 25°C, 40°C and 50°C since its residuals are the lowest of the isotherms studied. The data collected from the Inhibitor B experiments is used to plot the Langmuir isotherm which was found to give the best-fit representation.

160

Table 29. Thermodynamic data from Langmuir isotherm for Inhibitor B

∆퐺0 ( 풌푱 ) T (± 2°C) K 푎푑푠 풎풐풍

25 48.780 -9.64

40 2.589 -2.48

50 31.546 -9.27

70 0.821 0.56

It can be seen from Table 29 that the equilibrium adsorption constant (K) decreases with the increase of temperature (except in the case of 50°C) which indicates the weakening of the adsorbed film. The higher the adsorption constant, the stronger is the bond between the inhibitor molecules and the mild steel surface. The

0 values of ∆퐺푎푑푠 are negative for Inhibitor B where it was found to increase with 0 temperature. Negative values of ∆퐺푎푑푠 are a result of values of K being larger than one. This indicates that the amount of Inhibitor B in the solution is less than the amount of Inhibitor B molecules that is adsorbed on the surface of mild steel. It is 0 seen that at 70°C, the ∆퐺푎푑푠 becomes positive and this is due to K becoming less than one.

6.5.5 Activation energy of Inhibitor B. The corrosion rate used in the Arrhenius plot is obtained from the cyclic sweep test. The purpose of this plot is to determine how Inhibitor B affects the activation energy of the corrosion process. From the Arrhenius plot, the Arrhenius constant and activation energy can be determined.

It is seen from Table 30 that the activation energy increases with Inhibitor B concentration. This trend is similar to the trends seen for the other inhibitors. The increase in the activation energy indicates that it is more difficult for the corrosion process to occur. High values of Ea are a good indicator that Inhibitor B provided strong inhibitive action through raising the energy barrier of the corrosion process.

161

-1 ln (CR) ln

CO2-Blank y = -1511.2x + 6.2458 -2 R² = 0.8159

B-100ppm y = -5071.6x + 14.486 -3 R² = 0.9765

B-150ppm y = -6832.6x + 19.472 R² = 0.9845 -4 B-200ppm y = -7703.7x + 22.11 R² = 0.9773 -5 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 1/T

Figure 84. Arrhenius plot for Inhibitor B

Table 30. Arrhenius parameters for Inhibitor B 퐄퐚 Ea ( 풌푱 ) ln A 퐑 풎풐풍 Blank 6.246 1511.2 12.56

100ppm 14.486 5071.6 42.17

150ppm 19.472 6832.6 56.81

200ppm 22.110 7703.7 64.05

162

6.6 FTIR results The FTIR spectra for FLE, EPPE and CPLE are summarized in Figure 85. EPPE and FLE samples have identical FTIR spectra while CPLE have similar common peaks with the other two samples. The presence of a hydroxyl group is evident in all three samples. This is supported by the absorption peak at around 3400cm-1 which indicates an OH stretching mode that is expected within the range of 3500-3200 cm-1 [116].

The FLE FTIR spectrum, as seen in Figure 85(a), shows a strong absorption peak around 2917cm-1 displays the presence of aromatic C-H stretching. The peak at 1614cm-1 indicate the presence of C=O stretching (Carboxyl group). The peak at 1424 cm-1 can be assigned to the aromatic C=C. The observed peaks of 1270 cm-1 and 1077cm-1 indicate aryl OH and stretching mode of C-O respectively [116] [105].

For CPLE, the FTIR spectrum, as seen in Figure 85(b), shows the OH stretching absorption peak at 3394cm-1. The absorption at 2994cm-1 shows the presence of aromatic C-H stretching. The peaks at 1656cm-1 and 1543cm-1 indicate the presence of C=C stretching (alkene) and C-C stretching. The observed peaks of 1305cm-1 and 1122 cm-1 indicate the stretching mode of C-O. It is also noticed that the peak of 617cm-1 might represent C=C-H bonding.

The EPPE FTIR spectrum, as seen in Figure 85(c), shows the peaks of 1259cm-1 and 1074cm-1 that indicate aryl OH and the stretching mode of C-O respectively. The absorption at 2928cm-1 and 1633cm-1 respectively show the presence of aromatic C-H stretching and C=C stretching (alkene).

6.7 SEM results The images of the mild steel surface can be seen in Figure 86 for FLE, CPLE, EPPE and Inhibitor A. These micrographs were obtained using samples taken after the conclusion of the experiment. The sample chosen for each inhibitor was the 200ppm case. It is evident from the micrographs that inhibitor molecules which make up the protective film are present. They act as a barrier to prevent contact between the aggressive environment and the bare mild steel surface and are responsible for the decrease in the corrosion rate. The SEM images confirm the results obtained from the electrochemical techniques. 163

60 (a)

50 (b)

(c)

20 Transmittance (%)

ELE FLE CLE

0 4000 3600 3200 2800 2400 2000 1600 1200 800 400 Wavenumber (cm-1)

Figure 85. Combined FTIR spectra of FLE (a), CPLE (b) and EPPE (c)

Figure 86. SEM images of mild steel taken at 5um for the following: (a) Unexposed area (b) Film formed by Inhibitor A (c) Film formed by FLE (d) Film formed by CPLE and (e) Film formed by EPPE

164

6.8 Summary

6.8.1 Effect of temperature. The effect of temperature on the inhibition of mild steel is clearly illustrated in Figure 87 and Figure 88 where each respectively consist of four EIS plots and four CS plots. Each figure portrays FLE in plot (a), CPLE in plot (b), EPPE in plot (c) and Inhibitor A in plot (d). These plots are obtained using 200ppm of each inhibitor at 25°C, 40°C, 50°C and 70°C.

2000 1400 (a) FLE (25°C) (b) CPLE (25°C) FLE (40°C) 1200 CPLE (40°C)

1500 FLE (50°C) CPLE (50°C)

1000 FLE (70°C) CPLE (70°C) 800 1000

600

Z'' Z'' (ohm.cm²) Z'' (ohm.cm²) Z'' 500 400 200

0 0 0 2000 4000 6000 0 1000 2000 3000 Z' (ohm.cm²) Z' (ohm.cm²) 1400 2000 (c) EPPE (25°C) (d) A (25°C) 1200 EPPE (40°C) A (40°C) EPPE (50°C)

1500 A (50°C)

1000 EPPE (70°C) A (70°C) 800 1000

600

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 400 500 200

0 0 0 1000 2000 3000 0 2000 4000 6000 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 87. Nyquist plots depicting the effect of temperature for (a) FLE (b) CPLE (c) EPPE and (d) Inhibitor A

In Figure 87, it is observed that the inhibition at 25°C is represented by the largest semi-circle (for each inhibitor) while the smallest is seen at 70°C. When the temperature is raised, this results in a downward shift of the Nyquist plot. This 165 indicates a lower inhibition protection at higher temperatures. This observation is also confirmed by the CS test whose results are represented in Figure 88. It is seen that the plot shifts downwards and to the right and this indicates an increase in the corrosion current and thus corrosion rate. It is also clear that the corrosion potential decreases with temperature and this signifies a higher corrosion rate. Moreover, the trend of decreasing inhibition strength with temperature is also observed in the LPR tests for each inhibitor and can be seen in their respective sections.

-450 -500 -500 (a) -550 (b)

-550 -600

-600 -650 -650 -700 -700 -750 -750

-800 Potential (mV) -800 FLE (25°C) Potential (mV) CPLE (25°C) -850 FLE (40°C) -850 CPLE (40°C) CPLE (50°C) -900 FLE (50°C) -900 FLE (70°C) CPLE (70°C) -950 -950 0.00001 0.001Current (mA/cm0.12) 10 0.00001 0.001Current (mA/cm0.12) 10 -500 -450 (c) (d) -550 -500

-550

-600 -600 -650 -650 -700 -700

-750 -750 Potential (mV) Potential (mV) EPPE (25°C) -800 A (25°C) -800 EPPE (40°C) -850 A (40°C) EPPE (50°C) A (50°C) -850 -900 EPPE (70°C) A (70°C) -900 -950 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2)

Figure 88. CS plots depicting the effect of temperature for (a) FLE (b) CPLE (c) EPPE and (d) Inhibitor A

6.8.2 Comparison between inhibitors. The performance of FLE, CPLE, EPPE and Inhibitor A in protecting mild steel in a CO2-saturated 3.5wt% NaCl solution is clearly illustrated in Figure 89 and

166

Figure 90 where each respectively consist of four EIS plots and four CS plots. These plots are obtained using 200 ppm of each inhibitor at 25°C, 40°C, 50°C and 70°C.

In Figure 89(a), FLE provides better inhibition than Inhibitor A, EPPE and CPLE at 25°C. In plot (b), FLE is observed to show higher inhibition than CPLE, Inhibitor A and EPPE in that order. At 50°C, as seen in plot (c), it seems that CPLE provides the best inhibition followed by FLE, Inhibitor A and then EPPE. At 70°C, FLE was found to perform the best, followed by Inhibitor A, CPLE and EPPE as seen in plot (d). It is also noticed that the magnitude of impedance decreases drastically from 25°C to 70°C for each inhibitor.

2000 1200 (a) FLE (b) FLE

CPLE 1000 CPLE

1500 EPPE EPPE A 800 A 1000 600

400 Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 500 200 0 0 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 Z' (ohm.cm²) Z' (ohm.cm²) 500 200 (c) FLE FLE CPLE (d) CPLE 400

EPPE EPPE 150 A A 300 100

200

Z'' Z'' (ohm.cm²) Z'' Z'' (ohm.cm²) 100 50

0 0 0 500 1000 1500 0 200 400 600 Z' (ohm.cm²) Z' (ohm.cm²)

Figure 89. Nyquist plots for FLE, CPLE, EPPE and Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

In Figure 90, it is observed that for plot (a), the anodic shift of FLE is the highest, followed by Inhibitor A, CPLE and then EPPE. In plot (b), (c) and (d), FLE is observed to show higher inhibition than CPLE, Inhibitor A and EPPE in that order. It

167 is noticed that the rest potential of Inhibitor A is lower than EPPE and CPLE in plots (a) and (d) but its anodic shift is higher.

-450 -500 (a) (b) -500 -550

-550 -600

-600 -650 -650 -700 -700 -750

-750 Potential (mV) Potential (mV) FLE FLE -800 CPLE -800 CPLE EPPE -850 EPPE -850 A A -900 -900 0.00001 0.001 0.1 10 0.00001 0.001 0.1 10 Current (mA/cm2) Current (mA/cm2) -500 -500 (c) (d) -550 -550 -600

-600

-650 -650 -700 -700

-750 -750 Potential (mV) FLE Potential (mV) -800 -800 FLE CPLE CPLE -850 EPPE -850 EPPE A A -900 -900 0.00001 0.001 0.1 10 0.0001 0.001 0.01 0.1 1 10 Current (mA/cm2) Current (mA/cm2) Figure 90. CS plots depicting the difference between FLE, CPLE, EPPE and Inhibitor A at (a) 25°C (b) 40°C (c) 50°C and (d) 70°C

6.8.3 Summary of thermodynamic results.

6.8.3.1 Gibbs free energy of adsorption. The equilibrium adsorption constant (K) and standard Gibbs free energy of adsorption (∆퐺푎푑푠) are obtained from the Langmuir isotherm for each inhibitor and can be seen in Table 31 and Table 32. It was observed for all inhibitors that K decreased in value as the temperature was raised from 25°C to 70°C. The greater the adsorption equilibrium constant, the greater is the adsorption strength of the inhibitor molecules on the mild steel surface in a CO2-saturated 3.5wt% NaCl solution.

168

Based on the results, Inhibitor B was found to provide the greatest inhibition. Out of the natural inhibitors, it is seen that FLE produced the highest adsorption equilibrium constant for all temperatures. The order of best performing inhibitors after FLE, in terms of k, are as follows: at 25°C first is Inhibitor A, followed by CPLE and then EPPE; at 40°C first is CPLE, followed by EPPE and then Inhibitor A; and at 70°C first is Inhibitor A, followed by CPLE and then EPPE.

Table 31. Langmuir thermodynamic results for FLE, CPLE and EPPE FLE CPLE EPPE

0 0 0 ∆퐺푎푑푠 ∆퐺푎푑푠 ∆퐺푎푑푠 ( 풌푱 ) ( 풌푱 ) ( 풌푱 ) T (°C) K 풎풐풍 K 풎풐풍 K 풎풐풍

25 0.458 1.93 0.104 5.60 0.073 6.48

40 0.135 5.21 0.103 5.91 0.065 7.13

50 0.070 7.16 0.070 7.14 0.033 9.17

70 0.039 9.27 0.012 12.68 0.004 16.06

Table 32. Langmuir thermodynamic results for Inhibitor A and B Inhibitor A Inhibitor B

∆퐺0 ( 풌푱 ) ∆퐺0 ( 풌푱 ) T (°C) K 푎푑푠 풎풐풍 K 푎푑푠 풎풐풍

25 0.307 2.92 0.307 2.92

40 0.040 7.43 0.040 7.43

50 0.038 8.62 0.038 8.62

70 0.013 12.31 0.013 12.31

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6.8.3.2 Activation energy. It is seen from Table 33 that, typically, as the inhibitor concentration increases, the activation energy (Ea) increases. This increase indicates that the addition of an inhibitor results in raising the energy barrier of the corrosion reaction.

There are a few values of Ea that do not follow the observed trend such as those witnessed for FLE and Inhibitor A.

Table 33. Activation energy for all inhibitors FLE CPLE EPPE A B

Ea ( 풌푱 ) C (ppm) 풎풐풍

Blank 12.56

25 36.31 32.71 30.92 35.53 -

50 29.77 36.51 39.46 37.30 -

100 33.95 38.88 47.55 31.50 42.17

150 41.36 45.63 49.79 43.56 56.81

200 38.52 48.82 55.74 41.81 64.05

6.8.3.3 Enthalpy and Entropy. The values for enthalpy and entropy for each inhibitor were obtained from the Van’t Hoff (VH) plot using the following equation:

∆푺 ∆H 퐥퐧 푲 = 풂풅풔 − 풂풅풔 (ퟒퟕ) 풅 퐑 퐑퐓

where ∆푺풂풅풔is the entropy and ∆H풂풅풔is the enthalpy.

The values of Kd were obtained for each inhibitor and then plotted against the inverse of temperatures used such as 25°C, 40°C, 50°C and 70°C. ∆H푎푑푠 is obtained from the slope of the trendline and ∆푆푎푑푠is obtained from its y-intercept. These plots can be seen in Figure 91 and the results in Table 34.

170

-0.5 -1.5 (a) (b) -1.0 -2.0

-1.5 -2.5

-2.0 -3.0 ln Kd ln -2.5 -3.5 Kd ln -3.0 -4.0

-3.5 y = 5614.5x - 19.796 -4.5 y = 4941.9x - 18.417 R² = 0.9568 R² = 0.7823 -4.0 -5.0 0.0028 0.0030 1/T 0.0032 0.0034 0.0028 0.0030 1/T 0.0032 0.0034 -1.5 -2.2 (c) (d) -2.0

-3.2 -2.5

-3.0

-4.2 ln Kd ln -3.5 Kd ln -4.0 -5.2 y = 6705.4x - 24.619 -4.5 y = 4811.7x - 18.344 R² = 0.8405 R² = 0.9601 -6.2 -5.0 0.0028 0.0030 0.0032 0.0034 0.0028 0.0030 0.0032 0.0034 1/T 1/T 5.0 (e) 4.0

3.0

2.0 ln Kd ln 1.0

0.0 y = 7651.7x - 21.996 R² = 0.5136 -1.0 0.0028 0.0030 0.0032 0.0034 1/T Figure 91. Van’t Hoff plots for (a) FLE (b) CPLE (c) EPPE (d) Inhibitor A and (e) Inhibitor B

The negative values of ∆H풂풅풔 signify that the adsorption of inhibitor molecules on the surface of mild steel is an exothermic process. This is typically associated with physisorption while chemisorption is associated with an endothermic process. It is also evident that there is a negative change in entropy of the system which indicates that the disorder of the system has decreased. This also means that the disorder of the reactants is greater than the disorder of the products [117]. 171

Table 34. Summary of the enthalpy and entropy values for each inhibitor

∆H ( 풌푱 ) ∆푺 ( 풌푱 ) 풂풅풔 풎풐풍 풂풅풔 풎풐풍

FLE -46.68 -0.16

CPLE -41.09 -0.15

EPPE -55.75 -0.20

A -40.00 -0.15

B -63.62 -0.18

172

Chapter 7. Conclusion and Recommendations

The selected inhibitors exhibited excellent inhibition for mild steel immersed in a CO2-saturated and agitated 3.5wt% NaCl environment. The introduction of a tiny amount of inhibitor concentration, such as 25ppm, brought about a significant decrease in the corrosion rate. It was found that subsequent additions of each inhibitor resulted in further decreasing the corrosion of mild steel in the cases studied. The electrochemical tests offered conclusive proof that increasing the concentration of the inhibitor resulted in decreasing the corrosion rate. In the LPR test, it was determined that increasing the concentration of the inhibitor brought about an increase in the LPR values obtained. This trend was also observed in the EIS test where the addition of inhibitor resulted in the increase of the charge transfer resistance (Rct). The difference between the values obtained from the LPR and EIS tests, with regard to the polarization resistance, was minimal. It was also determined that the increase in temperature resulted in the values of LPR and Rct dropping for each concentration of inhibitor. However, even at elevated temperatures, the addition of inhibitor still resulted in increasing the LPR and Rct values obtained. However, the values obtained at elevated temperatures were lower than its counterparts at a lower temperature. This may be due to the weakening of the adsorbed film, formed by inhibitors, at elevated temperatures.

It was discovered that Inhibitor B performed the best at all the temperatures studied. It should be noted that Inhibitor B is a synthetic inhibitor which contains nitrogen and not a natural inhibitor like FLE, CPLE, EPPE and Inhibitor A which are environmentally friendly. Thus, for the natural inhibitors, it was discovered that FLE, CPLE and EPPE performed well when compared to Inhibitor A. The conclusion reached was based on all three electrochemical tests, and found the order of best- performing inhibitors for mild steel immersed in a CO2-saturated 3.5wt% NaCl solution are Inhibitor B > FLE > Inhibitor A > CPLE > EPPE.

The results, from the tests conducted, were fitted using four types of adsorption isotherms which are Langmuir, Temkin, Flory-Huggins and Frumkin. The best-fit adsorption model was chosen to represent the thermodynamic data for each inhibitor. It was observed for all inhibitors that the adsorption equilibrium constant 173

(K) decreased in value as the temperature was raised from 25°C to 70°C. The only exception is seen at 50°C for Inhibitor B and this may be due to experimental error or because of desorption from the mild steel surface. The greater the adsorption equilibrium constant, the greater is the adsorption of the inhibitor molecules on the mild steel surface. Inhibitor B displayed the largest values of K and this confirms that it is an effective inhibitor. Out of the natural inhibitors investigated, FLE was adsorbed the strongest followed by Inhibitor A, CPLE and then EPPE. The standard Gibbs free energy of adsorption for each green inhibitor was positive at the investigated temperatures, while for Inhibitor B it was negative except at 70°C. It was also determined from the Arrhenius plots that the activation energy typically increased with inhibitor concentration.

The experiments conducted are very sensitive to disturbances and requires precise work in a consistent manner. Changing parameters such as the CO2 flow rate or the CO2 bubbling rate in the solution can result in obtaining different results. This can also be affected by changing the level of the CO2 outlet tube (which is immersed in the test solution) and affects the pressure inside the system. It is recommended to investigate the inhibitors using higher/lower agitation or pressure for further analysis.

Inhibitor B is not made up of just one raw material but is instead a synergistic mix of a few active ingredients, which is the reason behind its super performance. Comparing the performance of Inhibitor B to inhibitors based on one natural extract is not fair. Thus, studies should be conducted in order to enhance the performance of the natural extract by mixing it with other extracts and/or adding some enhancers, such as surfactants and anti-foulants, in the formulation which would certainly help create a natural cocktail that could benefit the oil and gas industry. It might be worth using natural inhibitors such as FLE, CPLE and EPPE as synergizers or co-actives in commercial inhibitors. Mixing natural and synthetic inhibitors would certainly reduce the synthetic level of toxicity and biodegradation. This could be another area to look into since it would help industries meet environmental restrictions as well as performance criteria. Moreover, different ratios of the investigated inhibitors can be formulated to determine their inhibition effects and observe whether they synergize.

174

The experiments carried out in this thesis used only water as the test solution and this is considered as the worst case scenario. Production streams in the oil industry consist of oil and water and thus work should be done in adding small amounts of oil (hydrocarbon) to simulate more realistic conditions in order to confirm inhibitor performance. The amount of oil versus water (water cut) should be investigated since it would influence what formula and solvent package to use. The solvent package could be water only, hydrocarbon only, alcohol or a mixture. Moreover, it is also recommended to use different solvents, such as ethanol instead of double distilled water, in the extraction process from leaves. This should be done to determine the best solvent which can act as the carrier of the extracted inhibitor molecules.

175

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Vita

Elron Edgar Gomes was born on the 9th of June, 1991, in Sharjah, U.A.E. His formative years were spent at Saint Mary’s Catholic High School which he graduated from in 2007 achieving A-grades and above in the GCSE examinations. He then enrolled in the chemical engineering program at the American University of Sharjah (AUS) in Spring 2008 and graduated in Fall 2012 with a B.S in chemical engineering, minor in engineering management and a certificate for passing the NCEES Fundamentals of Engineering exam. He then enrolled in the chemical engineering graduate program at the American University of Sharjah with a fully funded assistantship from the AUS graduate assistantship program.

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