ABSTRACT
The efficacy of blend of Afzelia africana (AA), Brachystegia eurycoma (BE) and Momordica charantia (MC) seed extracts was investigated as a cheap and ecologically friendly alternative mild steel corrosion inhibitor. The experimental aspect of the corrosion inhibition process was carried out in 1.0 M hydrochloric acid at temperatures ranging from 303K to 343K and concentration range of 0.1 to 0.5 g/L using Phytochemical Screening, gravimetric/weight loss, thermometric and hydrogen evolution techniques. The surface characterization was done using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopic (FTIR) techniques. The theoretical aspect was studied using the density functional theory calculations and modeling of the electronic structures of some of the most effective extract constituents, including physisorptive interactions with the mild steel surface. The inhibition efficiency was determined by comparison of the corrosion rates in the absence and presence of the seed extracts. The inhibition efficiency increases gradually in the order: blend > AA > BE > MC with the blend reaching a maximum value of 95.4% within the first 60 minutes of exposure at a concentration of 0.5 g/L and temperature of 303K. The inhibition efficiencies by both the hydrogen evolution and thermometric methods when compared with that obtained by weight loss method, followed the same trend. The adsorption parameters showed that the Freundlich isotherm was the best model for the individual seed extracts and the modified Langmuir adsorption isotherm for the blend. The kinetic study shows that the inhibitory action follows a first order kinetics with the concentration of both the individual seed extracts and their blends. This was further supported by the thermodynamic parameters which reveal that the adsorption of both the individual seed extracts and their blends onto the metal surface was spontaneous, endothermic and followed physical adsorption mechanism. The low values of adsorption equilibrium constant, Kads indicate low interaction between the adsorbed molecules and the metal surfaces which further confirmed that the extracts were physically adsorbed onto the metal surface. FTIR study of the blended seed extracts and the corrosion product of mild steel showed an interaction between the inhibitor and the metal surface. SEM analyses of the corrosion product also confirmed the formation of a protective layer on the surface of the metal. Quantum chemical studies indicated that inhibition was due to adsorption of active molecules leading to formation of a protective layer on surface of mild steel. Quantum chemical parameters such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, HOMO–LUMO energy gap and electronic density were virtually identified. Therefore, the inhibition of mild steel corrosion is proposed to occur through synergistic combination of the constituents of the inhibitor, which leads to the formation of inhibitor-metal complex and subsequent protection of the metal from further corrosion attack.
TABLE
OF CONTENTS
Cover page i
Title Page ii
Declaration iii
Certification iv
Dedication v
Acknowledgement vi
Table of Contents vii
List of Tables xiii
List of Figures xvi
List of Plates xxiv
Abstract xxv
1.0 Chapter
1: Introduction 1
1.1 Background
of Study 1
1.2 Statement of the problem 4
1.3 Aim
of the Study 5
1.4 Objectives
of the Study 5
1.5 Significance of the Study 6
1.6 Scope of the Study 6
1.7 Justification
of the Study 7
2.0 Chapter
2: Literature Review 8
2.1 Corrosion
8
2.2 Corrosion Chemistry 9
2.3 Classification
of Corrosion 11
2.3.1 Dry
Corrosion 11
2.3.2 Wet
Corrosion 11
2.4 Forms
of Corrosion 11
2.4.1 Uniform
Corrosion 12
2.4.2 Galvanic
or Two Metal Corrosion 12
2.4.3 Carbon
dioxide Corrosion 13
2.4.3.1 Pitting Corrosion 13
2.4.3.2 Mesa-Type Attack 14
2.4.3.3 Flow-Induced Localized Corrosion 14
2.4.4 Crevice
Corrosion 14
2.4.5 Intergranular
Corrosion 14
2.4.6 Selective
Leaching 15
2.4.7 Erosion
Corrosion 15
2.4.8 Stress
Corrosion 16
2.5 Factors
Affecting Corrosion 16
2.6 The
Problem of Corrosion 17
2.7 Corrosion Control 18
2.7.1 Materials selection and design 19
2.7.2 Surface Coatings 19
2.7.3 Modification of the Electrolyte 20
2.7.3.1
Removal of the Aggressive Species 20
2.7.3.2
The Addition of Corrosion Inhibitors 20
2.8 Types
of Corrosion Inhibitors 21
2.8.1 Organic
Inhibitors 21
2.8.1.1 Synthetic Organic Inhibitors 22
2.8.1.2 Natural
Organic Inhibitors 23
2.8.2 Inorganic
Inhibitors 24
2.8.2.1
Anodic Inhibitors 24
2.8.2.2
Cathodic Inhibitors 26
2.9 Corrosion
Monitoring Techniques 27
2.9.1 Weight
loss method 27
2.9.2 Gasometric/Hydrogen Evolution Method 28
2.9.3 Thermometric Method 30
2.10 Plant Description 31
2.10.1
Afzelia Africana (AA) 31
2.10.2
Brachystegia Eurycoma (BE) 32
2.10.3
Momordica Charantia (MC) 33
2.11 Methods
Used For Extraction Of Plant Materials 35
2.12 Phyto-Chemical
Analysis as a Tool in Corrosion Inhibition 36
2.13 Adsorption Isotherm 38
2.13.1 Langmuir
Adsorption Isotherm 40
2.13.2 Temkin
Adsorption Isotherm 42
2.13.3 Freundlich Adsorption Isotherm 43
2.13.4 Choice
of Appropriate Adsorption Isotherm Model 44
2.14 Determination
of Associated Thermodynamic and Kinetic Parameters 44
2.14.1 Thermodynamic
Considerations 44
2.14.2 Kinetic Considerations 46
2.14.2.1Effect
of Concentration on Reaction Rate 46
2.14.2.2Effect
of Temperature on Reaction Rate 48
2.15 Theoretical and Quantum Chemical Studies as
a Corrosion Monitoring
Techniques 49
2.16 Scanning Electron Microscopy (SEM) 54
2.17 Empirical
Review 56
2.17.1 Effects
of Concentration on Corrosion Inhibition 56
2.17.2 Effect
of Immersion Time on Corrosion Inhibition 61
2.17.3 Effect
of Temperature on Corrosion Inhibition 64
2.18 Adsorption
Isotherm Studies 68
2.19 Thermodynamics
and Kinetic Treatment 71
2.20 Quantum
Chemical Studies 76
3.0 Chapter 3: Methodology 79
3.1 Materials and Methods 79
3.1.1 Materials 79
3.2 Experimental
Procedure 80
3.2.1 Preparation
of Specimen 80
3.2.2 Surface Area and density of specimen 80
3.2.3 Preparation
of Blends of Afzelia Africana,
Brachystegia Eurycoma and
Momordica Charantia seed extracts 81
3.2.4 Preparation
of 1.0M HCl 82
3.3 Phyto-Chemical Analysis 83
3.3.1 Test
for Alkaloids 84
3.3.2 Test for Saponins 84
3.3.3 Test
for Tannins 84
3.3.4 Test
for Phenol 84
3.3.5 Test
for Flavonoids 84
3.3.6 Test
for Glycosides 84
3.3.7 Test
for Steroids 85
3.3.8 Test
for carbohydrates 85
3.3.9 Test
for Reducing Sugar 85
3.3.10 Test for Proteins 85
3.3.11 Test for Amino acids 86
3.4 Gravimetric Technique 86
3.5 Hydrogen
Evolution (Gasometric) Method 88
3.6 Thermometric
Method 88
3.7 Adsorption
Isotherm Study 89
3. 7.1 Langmuir Adsorption Isotherms 89
3.8 Thermodynamic and Kinetic Studies 90
3.8.1 Standard
Gibbs free energy change of Adsorption, ΔG°ads 90
3.8.2 Activation
Energy, Ea 90
3.8.3 Rate
Constant for the Corrosion Rate, k 91
3.8.4 Half-life, t1/2 91
3.9 Scanning Electron Microscopy (SEM) 91
3.10 Fourier Transform Infrared Spectroscopy (FTIR) 91
3.11 Theoretical and Quantum Chemical
Calculations 92
4.0 Chapter
4: Results and Discussion 93
4.1 Elemental
Composition of Mild Steel 93
4.2 Phytochemical
Screening 94
4.3 Weight
Loss Results 97
4.3.1 Effect of
Concentration of extracts on Corrosion Rate and Inhibition Efficiency 99
4.3.2 Effect of Immersion Time on Corrosion
Inhibition 104
4.3.3 Effect
of Temperature on Corrosion Rate and
Inhibition Efficiency 106
4.4 Gasometric Results 111
4.5 Thermometric Results 116
4.6 Adsorption
Mechanism 118
4.7 Thermodynamic and Kinetic parameters for
the corrosion inhibition of both the
individual seed extracts
and their Blends 128
4.7.1 Standard Gibbs free energy of
adsorption, ΔG°ads 129
4.7.2 Activation Energy (Ea) for the Corrosion Process 130
4.7.3 Activation
Parameters for the Corrosion Process 132
4.7.4 Rate
of Reaction and Rate Constant of the Reaction 134
4.7.5 Half
Life 147
4.8 Surface Studies by Scanning
Electron Microscopy 149
4.9 Fourier transform infrared spectroscopy (FTIR) analysis 151
4.10 Theoretical and Quantum Chemical Studies 155
4.10.1 Mulliken
charge distribution of Glucose, Arginine, Flavonol and Leucine 163
5.0 Chapter 5: Conclusion and
Recommendations 166
5.1
Conclusion 166
5.2
Recommendations 168
References
169
Appendixes 183
LIST
OF TABLES
3.1 List of
Materials Used for the Experiments 79
3.2 List of Equipment for the Experiments 80
4.1 Elemental
composition of mild steel employed in the study 93
4.2 Phytochemical Constituent of blends
of AA, BE and MC extract 94
4.3 Hydrogen
evolution data for mild steel corrosion in 1 M HCl solution in the absence and presence of
the blended seed extract 111
4.4 Thermometric data for mild steel in 1.0 M
HCl solution in the absence and presence
of blended seed extract (AA, BE and MC) 116
4.5 Isotherm parameters for the adsorption of
ethanolic extract of Afzelia africana seed
on the surface of mild steel in HCl 118
4.6 Isotherm parameters for the adsorption of
ethanolic extract of Brachystegia
eurycoma
seed on the surface of
mild steel in HCl 120
4.7 Isotherm parameters for the adsorption of
ethanolic extract of Momordica charantia
seed on the surface of
mild steel in HCl 122
4.8 Isotherm parameters for the adsorption of
ethanolic extracts of the blends of Afzelia africana,
Brachystegia eurycoma and Momordica charantia seeds on the surface of
mild steel in HCl 124
4.9 Calculated
values of thermodynamic and kinetic parameters for mild steel corrosion in
1 M
HCl in the absence and presence of Afzelia africana seed extract as inhibitor 128
4.10 Calculated
values of thermodynamic and kinetic parameters for mild steel corrosion in
1 M
HCl in the absence and presence of Brachystegia
eurycoma seed extract as
inhibitor 128
4.11
Calculated
values of thermodynamic and kinetic parameters for mild steel corrosion in
1 M HCl in the absence and presence of Momordica charantia seed extract as
inhibitor 128
4.12
Calculated
values of thermodynamic and kinetic parameters for mild steel corrosion in
1 M HCl in the absence and presence of blended seed extracts as inhibitor 129
4.13
Calculated values of the rate constant for
mild steel corrosion in 1 M HCl in the absence
and presence of Afzelia africana seed extract 141
4.14
Calculated values of the rate constant for
mild steel corrosion in 1 M HCl in the absence
and
presence of Brachystegia
eurycoma seed extract 141
4.15
Calculated values of the rate constant for
mild steel corrosion in 1 M HCl in the absence
and presence of Momordica
charantia seed extract 142
4.16 Calculated values of the rate constant for
mild steel corrosion in 1 M HCl
in
the absence and presence of blended
seed extracts 143
4.16
Calculated values of the half life for mild
steel corrosion in 1 M HCl in the absence
and presence of Afzelia africana seed extract 147
4.17
Calculated values of the half life for mild
steel corrosion in 1 M HCl in the absence
and presence of Brachystegia eurycoma seed extract 147
4.18
Calculated values of the half life for mild
steel corrosion in 1 M HCl in the absence
and presence of Momordica
charantia seed extract 148
4.19
Calculated values of the half life for mild
steel corrosion in 1 M HCl in the absence
and presence of blended
seed extracts 148
4.21 IR Absorption of Afzelia africana seed extract 151
4.22 IR Absorption of Brachystegia eurycoma seed extract 151
4.23 IR Absorption of Momordica charantia seed extract
151
4.24 Quantum
chemical parameters for most important components of the ethanol extract
of
Blend of AA, BE and MC 155
4.25 Mulliken
Charge Distribution on Flavonol, Leucine, Arginine and Glucose 165
LIST
OF FIGURES
2.1 Anodic Inorganic Inhibitors Effect and
their Mechanism of Action 25
2.2 Mechanism
of Actuation of the Cathodic Inhibitors 26
2.3 Gasometric
assembly for measurement of hydrogen gas evolved 29
2.4 Extraction method used for preparation
of plant extracts 36
2.5 Schematic
representation of an adsorption isotherm 39
4.1 Structures of the Phytochemicals Present
in the blended seed extracts 95
4.2 Variation
of weight loss with time for mild steel coupons in 1 M HCl solutions
containing
Afzelia africana at 303K 97
4.3 Variation
of weight loss with immersion time for mild steel coupons in 1 M HCl
solutions
containing Brachystegia eurycoma at
303K 97
4.4 Variation of weight loss with immersion
time for mild steel coupons in 1 M HCl
solutions
containing Momordica charantia at
303K. 97
4.5 Variation
of weight loss with immersion time for mild steel coupons in 1 M HCl
solution
containing the Blend at 303K 98
4.6 Variation
of corrosion rate with concentration of Afzelia
africana extract for mild steel
coupons in 1 M HCl solution at different time intervals at 303 K 99
4.7 Variation
of corrosion rate with concentration of Brachystegia
eurycoma extract for
mild
steel coupons in 1 M HCl solution at different time intervals at 303 K 99
4.8 Variation
of corrosion rate with concentration of Momordica
charantia extract for mild
steel
coupons in 1 M HCl solution at different time intervals at 303 K 100
4.9 Variation
of corrosion rate with the concentration of Blend extract for mild steel coupons in 1 M HCl solution at different
time intervals at 303 K 100
4.10 Comparisons
of corrosion rate of the individual extract with the blend at 60 min
at
303 K 101
4.11 Variation
of inhibition efficiency with concentration of Afzelia africana extract
for
mild steel coupons in 1 M HCl solution at different time intervals at 303
K 101
4.12 Variation
of inhibition efficiency with concentration of Brachystegia eurycoma extract
for mild steel in 1 M HCl solution at different time intervals at 303 K 102
4.13 Variation
of inhibition efficiency with concentration of Momordica charantia
extract
for mild steel in 1 M HCl solution at different time intervals at 303 K102
4.14 Variation
of inhibition efficiency with extract concentration of the blend for mild steel coupons
in 1 M HCl solution at different time intervals at 303 K 103
4.15 Comparisons
of inhibition efficiencies of the individual extracts with the blend
at
60 min at 303 K 103
4.16 Variation
of corrosion rate with different concentrations of Afzelia africana extract
showing the effect of temperature on
the corrosion inhibition process 106
4.17 Variation
of corrosion rate with different concentrations of Brachystegia eurycoma
extract showing the effect of
temperature on the corrosion inhibition process 106
4.18 Variation
of corrosion rate with different concentrations of Momordica charantia
extract showing the effect of
temperature on the corrosion inhibition process 107
4.19 Variation of corrosion rate with different
concentrations of the blend showing the effect
of temperature
on the corrosion inhibition process
107
4.20 Variation of inhibition efficiency with
different concentrations of Afzelia
africana
extract showing the effect of temperature on
the corrosion inhibition process 108
4.21 Variation of inhibition efficiency with
different concentrations of Brachystegia
eurycoma extract
showing the effect of temperature on the corrosion inhibition process 108
4.22 Variation
of inhibition efficiency with different concentrations of Brachystegia eurycoma
extract showing the effect of temperature on the corrosion inhibition
process 108
4.23 Variation
of inhibition efficiency with different concentrations of the blend
showing the effect of temperature on
the corrosion inhibition process
109
4.24 Variation
of volume of H2 gas evolved with time for mild steel corrosion in
1.0 M HCl in the absence and presence of the blended seed extract at 303 K 111
4.25 Variation
of volume of H2 gas evolved with time for mild steel corrosion in 1.0
M HCl in the absence and
presence of the blended seed
extract at 313 K 112
4.26 Variation
of volume of H2 gas evolved with time for mild steel corrosion in
1.0 M HCl
in the absence and presence of the blended seed extract at
323 K 112
4.27 Variation
of volume of H2 gas evolved with time for mild steel corrosion in
1.0 M HCl
in the absence and presence of the blended seed extract at
333 K 113
4.28 Variation
of volume of H2 gas evolved with time for mild steel corrosion in1.0
M HCl
in the absence and presence of the blended seed extract at
343 K 113
4.29 Variation of inhibition efficiency with the
concentration of the blended seed
extracts
for
mild steel corrosion in 1.0 M HCl solution at different temperatures 114
4.30 Variation of temperature with time in 1.0 M
HCl for mild steel corrosion in the absence
and presence of the blended seed extracts 116
4.31 Plot of RN against Log of inhibitor concentration 117
4.32 The
Langmuir isotherm for the adsorption of Afzelia africana extract on mild steel
surface
in 1.0 M HCl 118
4.33 The
Freundlich isotherm for the adsorption of Afzelia africana extract on mild steel
surface
in 1.0 M HCl 119
4.34 The
Temkin isotherm for the adsorption of Afzelia africana extract on mild steel surface in 1.0 M HCl 119
4.35 The
Langmuir isotherm for the adsorption of Brachystegia eurycoma extract on mild
steel surface in 1.0 M HCl 120
4.36 The
Freundlich isotherm for the adsorption of Brachystegia eurycoma extracts on mild
steel surface in 1.0 M HCl 121
4.37 The
Temkin isotherm for the adsorption of Brachystegia eurycoma extracts on mild
steel
surface in 1.0 M HCl 121
4.38 The
Langmuir isotherm for the adsorption of Momordica charantia extract on
mild
steel surface in 1.0 M HCl 122
4.39 The
Freundlich isotherm for the adsorption of Momordica charantia extract on
mild
steel surface in 1.0 M HCl 123
4.40 The
Temkin isotherm for the adsorption of Momordica charantia extract on
mild
steel surface in 1.0 M HCl 123
4.41 The
Langmuir isotherm for the adsorption of the blended
seed extracts on mild steel
surface
in 1.0 M HCl 124
4.42 The
Freundlich isotherm for the adsorption of the blended
seed extracts on mild steel
surface
in 1.0 M HCl 125
4.43 The
Temkin isotherm for the adsorption of the blended
seed extracts on mild steel
surface
in 1.0 M HCl 125
4.44 Plot
of log CR against 1/T for mild steel in 1 M HCl solution in the absence
and presence of various
concentrations of Afzelia africana extract
130
4.45 Plot
of log CR against 1/T for mild steel in 1 M HCl solution in the absence
and presence of various
concentrations of Brachystegia
eurycoma extract 130
4.46 Plot
of log CR against 1/T for mild steel in 1 M HCl solution in the absence
and presence of various
concentrations of Momordica
charantia extract
131
4.47 Plot
of log CR against 1/T for mild steel in 1 M HCl solution in the absence
and presence of various
concentrations of blended seed extracts 131
4.48 Plot
of log CR/T against 1/T for mild steel in 1 M HCl
solution in the absence and
presence
of various concentrations of Afzelia
africana extract 132
4.49 Plot
of log CR/T against 1/T for mild steel in 1 M HCl
solution in the absence and
presence
of various concentrations of Brachystegia
eurycoma extract 132
4.50 Plot
of log CR/T against 1/T for mild steel in 1 M HCl
solution in the absence and
presence
of various concentrations of Momordica
charantia extract 133
4.51 Plot
of log CR/T against 1/T for mild steel in 1 M HCl
solution in the absence and
presence
of various concentrations of blended seed extracts 133
4.52 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and
presence
of various concentrations of Afzelia
africana extracts at 303 K 134
4.53 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and
presence
of various concentrations of Afzelia
africana extracts at 313 K 134
4.54 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and
presence
of various concentrations of Afzelia
africana extracts at 323 K 135
4.55 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and presence
of
various concentrations of Afzelia
africana extracts at 333 K 135
4.56 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and presence
of
various concentrations of Afzelia
africana extracts at 343 K
135
4.57 Plot
of Log Wf vs Time for mild steel in 1 M HCl solution in the absence and presence
of various concentrations of Brachystegia eurycoma extracts at 303 K 136
4.58 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Brachystegia
eurycoma extracts at 313 K 136
4.59 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Brachystegia
eurycoma extracts at 323 K
136
4.60 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Brachystegia
eurycoma extracts at 333 K
137
4.61 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Brachystegia
eurycoma extracts at 303 K 137
4.62 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Momordica
charantia extracts at 303
K 137
4.63 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Momordica
charantia extracts at 313
K 138
4.64 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Momordica
charantia extracts at 323
K 138
4.65 Plot
of Log Wf vs Time
for mild steel in 1 M HCl solution in the absence and presence
of
various concentrations of Momordica
charantia extracts at 333
K 138
4.66 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of Momordica
charantia extracts at 343
K 139
4.67 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of blended seed extracts at 303 K 139
4.68 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of various
concentrations of blended seed extracts at 313 K 139
4.69 Plot
of Log Wf
vs Time for mild steel in 1 M HCl
solution in the absence and presence
of various concentrations of blended
seed extracts at 323 K 140
4.70 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of blended seed extracts at 333 K 140
4.71 Plot
of Log Wf
vs Time for mild steel in 1 M
HCl solution in the absence and presence
of
various concentrations of blended seed extracts at 343 K 140
4.72 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence of
various concentrations of Afzelia africana
extracts at 303K 144
4.73 Plot of 1/Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence
of various concentrations of Afzelia africana
extracts at 303K 144
4.74 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence of
various concentrations of Brachystegia eurycoma extracts at 303K
144
4.75 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence of
various concentrations of Brachystegia eurycoma extracts at 303K
145
4.76 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence andpresence of
various concentrations of Momordica charantia extracts at 303K
145
4.77 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence of
various concentrations of Momordica charantia extracts at 303K 145
4.78 Plot of Wf
vs Time for mild steel in 1M HCl
solution in the absence and presence of
various concentrations of blended
seed extracts at 303K 146
4.79 Plot of 1/Wf vs Time for mild steel in 1M HCl solution in the absence and presence
of various concentrations of blended
seed extracts at 303K 146
4.80 SEM micrograph of mild steel immersed in
hydrochloric acid without inhibitor at
200μm
magnification 149
4.81 SEM micrograph of mild steel immersed in
hydrochloric acid in the presence of
ethanol
extract of Blend of AA, BE and MC at 200μm magnification 149
4.82 FTIR spectra of the ethanol extract of
Blend of AA, BE and MC 152
4.83 FTIR spectra of the corrosion product of
mild steel in the presence of the blended extracts in 1 M HCl acid
152
4.84 Optimized
Structure of Glucose Leucine, Arginine and Flavonol 156
4.85 HOMO and
LUMO orbitals of Leucine 157
4.86 HOMO and
LUMO orbitals of Flavonol 157
4.87 HOMO and
LUMO orbitals of Glucose 157
4.88 HOMO and
LUMO orbitals of Arginine 158
4.89 ESP Optimized mapped density of (a) Leucine, (b)
Flavonol and (c) Glucose
(d) Arginine 162
LIST
OF PLATES
2.1 Afzelia africana (Akparata) seeds 32
2.2 Brachystegia
eurycoma (Achi) seeds 33
2.3 Brachystegia
eurycoma (Achi) processed seeds and fruits 33
2.4 Unripe Momordica charantia fruits and leaves 34
2.5 Ripe Momordica
charantia fruits and Seeds 35
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
The importance of mild steel in chemical
industry cannot be over-emphasized owing to its usefulness as structural
material in several applications including construction of tanks, gas
cylinders, pipelines, heat exchangers among others. These serve as basic tools
for the industrialization and development of a nation (Senthil et al., 2016; Peter
and Sharma 2017; Rathi et al., 2017).
Mild steel also known as plain-carbon steel is used in these applications due
to its easy availability, low cost and excellent physical properties, but its
use and lifespan is restricted in these applications due to its susceptibility
towards corrosion (Loto et al., 2012; Singh et
al., 2016). One of the most challenging and difficult tasks for industries
is the protection of metals from corrosion (Al-otaibi et al., 2012).
The increasing interest
in the manufacture of hydrochloric acid has created the need for obtaining
information on the corrosion resistance of mild steel to hydrochloric acid
attack. Hydrochloric acid is a very important chemical widely used
in many industrial activities such as industrial cleaning agent and acid
descaling, as well as oil well acid in oil recovery and pickling of mild steel
structures (Loto et
al., 2012; Chidiebere et al.,
2016).
Corrosion
as a highly destructive process negatively affects the performance of metallic
materials applied in many construction sites. Moreover, their consequences are
quite diverse and pose a great problem in the industries, construction and
other civil services, costing billions of dollars each year (Onukwube et al., 2016). Mild steel corrodes in
different concentrations of aggressive media resulting to a decrease in its
original weight. The corrosion of mild steel is attributed to the presence of
water, air and ions which accelerate the corrosion process.
In view of the above, efforts are being made to combat
this menace by adopting several options including electroplating, oiling,
cathodic and anodic protections, and the addition of inhibitors. Inhibitors are
often added during industrial processes so as to prevent metal dissolution
from inorganic and organic acids which are often employed (Quartarone et al.,
2008; Amitha-Rani and Bharathi, 2012). Most
effective inhibitors are organic compounds that contain heteroatoms like
nitrogen, sulfur, oxygen and phosphorus in a conjugated system (Ebenso et al., 2001; Umoren et al., 2010; Wang, 2012; Xhanari
et al., 2017; Lei et al., 2018). The inhibitors function
at the interface between the metal and aqueous corrosive media, and their
interaction with the metal surface through adsorption, results in a
modification in the mechanism of the electrochemical process. Polar functional
groups are regarded as the reaction center that promotes the adsorption process
(Roberge et al., 1999). Corrosion
inhibition helps to reduce the corrosion susceptibility of the metal surface (Umoren
et al., 2010, Chidiebere et al.; 2015). As a result, the service
life of the metal is prolonged. Most of these inhibitors previously in use are
either synthetic chemicals such as chromates and silicates or organic amines
with undesired toxic effects on the environment, animal and aquatic life, and
are expensive as well. This toxicity may manifest either during the synthesis
of the compound or during its application as these inhibitors may cause
reversible (temporary) or irreversible (permanent) damage to organ or system
viz, kidney, liver etc. hence disturbing the biochemical processes and enzyme
systems at some sites in the body (Mohammed-Dabo et al., 2011). The safety and environmental issues of corrosion
inhibition arisen in industries have always been a global concern.
Consequently, there exist the need to source
for environmentally friendly inhibitors with low toxicity and good efficiency
(Anees et al., 2016). In an attempt
to find corrosion inhibitors which are environmentally safe and readily
available, there has been a growing trend in the use of natural products such
as leaves or plant extracts as ecofriendly alternative in protecting metals and
alloys in acid cleaning process (Orubite et
al., 2004; Awe et al., 2015; Rathi
et al., 2017).
The exploration of natural products of plant
origin as inexpensive and eco-friendly sources of important inhibitors is an essential
field of research owing to the exploitation of abundant phytochemical resources
such as alkaloids, tannins, flavonoids,
amino acids, lignins, and carbohydrates. These could facilitate the development of
environmentally friendly alternatives as against the hazardous synthetic chemical
inhibitors. Plant extracts are used extensively in traditional medicine, where
the phytochemical constituents have been shown to be effective against
pathogenic (disease-causing) micro-organisms and form the basis for several
important pharmaceutical drug formulations (Okafor et al., 2005; Oyedeji et al.,
2005; Raju and Maridass, 2011). Attempts to extend the field of application of
these extracts to solving material corrosion problems in aqueous aggressive environments
and hence develop new, inexpensive, efficient, non toxic, readily available, biodegradable and environmentally friendly
corrosion-inhibiting additives from cheap and renewable sources are gaining increasing
interest (Kumar, 2008; Ekanem et al., 2010; Akalezi et al., 2012; Oguzie et al., 2012). The
extracts from the leaves, barks, seeds, fruits and roots comprise of
mixtures of organic compounds containing nitrogen, sulfur and oxygen atoms and
some have been reported to function as effective inhibitors of metal and alloy
corrosion in different aggressive environments. Such studies are justified by the
phytochemical constituents of the extracts, which have molecular and electronic
structures bearing close similarities with those of conventional corrosion
inhibitors and have been shown to also function through adsorption on the metal
interface (Awe et al., 2015).
Many scientific researchers have responded to this need and it has generated
increased research studies into the use of plant extracts (Loto, 2011).
Despite the increasing research
studies into the use of plant extracts as inhibitor for metals against
corrosion in different aggressive media, it is worthy to note that the use of
blends of extracts from Afzelia Africana
(Akparata) (AA), Brachystegia
eurycoma (Achi) (BE) and Momordica Charantia (MC) seeds as corrosion inhibitor has not been
reported in literature. An attempt at making a contribution to this growing
research area has necessitated the drive to examine the efficacy of the blends of the above mentioned plant extracts
(AA, BE and MC) in corrosion inhibition
of mild steel in acidic environment.
1.2 STATEMENT
OF THE PROBLEM
Corrosion is an increasingly serious
and costly problem that can lead to plant and equipment failures, leakages in
oil and gas pipelines as well as steel bridges, ship, and buildings. These
failures range from being an annoyance to being catastrophic. It could lead to
a direct failure of a component which could affect the entire system. Corrosion
cost is not only computed in terms of financial losses, but also in terms of material
repairs or replacement, man power and human injuries (Onuchukwu et al., 2004). Organic compounds
containing nitrogen, sulfur and oxygen have long been used as potential
corrosion inhibitors (Oguzie et al.,
2008). These compounds get adsorbed, form a protective layer or insoluble
complex on the metal surface and block the active corrosion sites. However,
most of these compounds are synthetic chemicals, expensive and very hazardous
to both human beings and the environments and need to be replaced with nontoxic
and eco-friendly compounds. Over the years, numerous classes of organic
compounds have been investigated as corrosion inhibitors. However, the trend in
green chemistry is geared towards the replacement of most of these inhibitors
with nontoxic, cheap and eco-friendly compounds. In recent years, a number of
eco-friendly corrosion inhibitors have been exploited as green alternative to
toxic and hazardous compounds (Samuel et
al., 2015). Hence, this study seeks to study the efficacy of blends of
extracts from Afzelia Africana (AA), Brachystegia eurycoma (BE) and
Momordica Charantia (MC) seeds on
the corrosion inhibition of mild steel in acidic medium.
1.3 AIM OF THE STUDY
The
aim of the study is to investigate
the efficacy of blends of extracts from Afzelia
Africana (AA), Brachystegia eurycoma (BE)
and Momordica Charantia (MC) seeds on
the corrosion Inhibition of Mild Steel in acidic medium.
1.4 OBJECTIVES OF THE STUDY
The
specific objectives of the study are;
1. To
determine the qualitative and quantitative phytochemical analysis of the blends
of extracts on the Afzelia africana (AA), Brachystegia eurycoma (BE) and
Momordica charantia (MC) seeds.
2. To
carry out corrosion test on mild steel using weight loss, hydrogen evolution
and thermometric techniques by varying the concentration of the individual seed
extract and blending each of the individual seed extracts together at equimolar
proportion.
3. To
investigate the effect of increase in concentration and temperature on
percentage inhibitive efficiency of the individual seed extracts and the blends
4. To
ascertain the effect of immersion time on corrosion inhibition of the
individual seed extracts and their blends
5. To
assess the kinetic and thermodynamic parameters of the inhibitors on the mild
steel surface in the aggressive medium.
6. To
evaluate the surface characteristics of mild steel in contact with hydrochloric
acid solution in the absence and presence of the inhibitor using scanning
electron microscope (SEM).
7. To
evaluate the relationship between the molecular structures of the plants
extract and their inhibition efficiencies using Fourier transform infrared spectroscopic (FTIR) analysis.
8. To
evaluate the adsorptive ability of the blended extracts by comparing the infrared spectroscopy (IR) of the
extract with the IR of the corrosion product of mild steel in the presence of
the blended extracts.
9. To
carry out quantum chemical studies on the effective component of the blends
of the seed extract.
1.5 SIGNIFICANCE
OF THE STUDY
Development of
effective and environmentally acceptable corrosion inhibitors as alternatives
to toxic and carcinogenic ones are being researched into, forming the interest of
the present research to serve as template for steel, oil and gas industries. Most
of the corrosion inhibitors are synthetic chemicals, expensive and very hazardous
to environment. Therefore, it is desirable to source for environmentally
friendly inhibitors. Owing to the increasing ecological
awareness as well as the strict environmental regulations, and consequently the
need to develop environmentally friendly processes, attention is currently
focused on the development of “green” alternatives to mitigating corrosion.
1.6 SCOPE OF THE STUDY
This Study was limited to investigation of
inhibitory effectiveness of blends of extracts from Afzelia africana (AA), Brachystegia
eurycoma (BE) and Momordica charantia (MC) seeds on mild
steel in 1M hydrochloric acid using weight loss, thermometric and hydrogen
evolution methods in the absence and presence of each inhibitor at various
temperatures. The adsorption and inhibition mechanism of the plant extract was
evaluated using quantum chemical calculation and Fourier transform infrared spectroscopic (FTIR) analysis. The surface
morphology of the sample was examined using scanning electron micrograph (SEM).
1.7 JUSTIFICATION OF THE STUDY
In
view of the aforementioned problem, this work is no doubt justifiable owing to
the increasing ecological awareness and the need to develop environmentally
friendly processes. Attention is currently focused on the development of
“green” alternatives to mitigating corrosion in which the application of blends
of extract from Afzelia africana (AA), Brachystegia eurycoma (BE) and Momordica charantia (MC) seeds happen
to fall into this category.
Several
studies have been carried out on the corrosion inhibition properties of the
individual seeds of the above plants, hence, it is most appropriate to utilize
the blends of extract from their seeds in the production of corrosion
inhibitors to assess their improved or reduced performance in inhibiting corrosion
of mild steel in HCl medium. This will also aid in the replacement of some of
the organic and inorganic chemical corrosion inhibitors which are known to be
hazardous and toxic to human and the environment.
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