ABSTRACT
The inhibition efficiencies of the two Schiff bases 2[2-diethylamino) ethyl methyl amino)-4-methy1-5-3 (3-methyl sulfanyl propy1 amino) methyldiene cyclohexdien-1-one (DEMS) and [1-(azepan-1-yl)2-2-[4-(2-tert-butyl sulfanyl ethyl piperazin-1-yl] ethanone (ATSP) synthesised from linoleic and benheric acids on mild steel and aluminised steel in 1 M HCl solutions were investigated. Their inhibition performance was tested by gasometric, weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and quantum chemical techniques and were characterised using FTIR spectroscopy. The FTIR spectra showed that the synthesised Schiff bases contained heterocyclic compounds with heteroatoms (N, S, and O) which had conjugated double bonds, indicating the presence of π- electrons. The results obtained from the weight loss method showed that inhibition efficiencies increase with increase in concentration of the Schiff base molecules and with the highest inhibition efficiency observed at the optimum concentration of 2.0 g/l. The inhibitor, DEMS showed inhibition efficiency of 81.14% while the inhibitor, ATSP showed inhibition efficiency of 71.21% for mild steel while for the aluminised steel, inhibition efficiencies were 88.64% (DEMS) and 86.90% (ATSP) respectively. Inhibition efficiencies were also found to decrease with increase in temperature. The following adsorption isotherms were employed in the study: Freundlich, Temkin and Langmuir isotherms with Langmuir showing best fit to the experimental data (R2=0.99). Thermodynamic studies of the inhibition process showed that the ΔGads values for DEMS and ATSP were both negative indicating the spontaneity of the reaction and the values were less than -20 kJmol,-1 indicating that the mechanism of adsorption is physiosorption. The negative entropy values for both inhibitors on mild steel and aluminised steel denote increase in disorderliness on the surface of the coupons as concentration of inhibitors increases. The enthalpy values were positive showing that the reaction was endothermic. The potentiodynamic polarisation results showed that the inhibitors were mixed type because the displacement obtained in the Ecorr value for both DEMS and ATSP were -15 mV and -63 mV respectively and these values were less than 85 mV. The highest inhibition efficiency values of the Nyquist plots obtained at 2.0 g/l from EIS for mild steel were 80.40% (DEMS) and 66.59% (ATSP) while 82.13% and 81.09% were obtained for aluminised steel for the inhibitor molecules, DEMS and ATSP respectively. These suggest the formation of protective layer by the inhibitor molecules. Molecular simulation studies were carried out using semi- empirical PM3 method to optimize the structure of the inhibitors. The results obtained from the chemical studies showed that there exists a relationship between the inhibitors’ efficiency and the energy gap (ΔE), energy of the highest occupied molecular orbital (E Homo), energy of the lowest occupied molecular orbital (E Lumo), dipole moment (μ), softness (σ) as well as the hardness (η) of the inhibitor molecules. Fukui function was used to determine the site where adsorption can occur on the inhibitor molecules. The inhibition efficiency trend of the Schiff base molecules follows DEMS > ATSP.
TABLE
OF CONTENTS
Title page
Certification i
Declaration ii
Dedication iii
Acknowledgements iv
Table of Contents vi
List of Tables xii
List of figures xv
Abstract xxiii
CHAPTER
1: INTRODUCTION
1.1 Background of the Study 1
1.2 Statement of Problem 2
1.3 Justification of the Study 3
1. 4 Aim and Objectives of the study 3
1.4.1 Specific Objectives 3
1.5 Scope of the Study 4
CHAPTER
2: LITERATURE REVIEW
2.1 Definition of Corrosion 5
2.2 Factors that Affect the Corrosion
of Metals 6
2.2.1 Water 6
2.2.2 Temperature 6
2.2.3 Condition and composition of
the metal surfaces 6
2.2.4 Air 7
2.2.5 Micro-organisms 8
2.3 Types of Corrosion 8
2.3.1 Uniform corrosion 8
2.3.2 Localised corrosion 8
2.3.2.1 Pitting corrosion 9
2.3.2.2 Crevice corrosion 9
2.3.2.3 Intergranular corrosion 9
2.3.2.4 Galvanic and thermo-galvanic corrosion 10
2.3.2.5 Selective leaching 10
2.3.2.6 Erosion corrosion 10
2.3.3 Stress or cracking corrosion 10
2.3.3.1 Fatigue corrosion 11
2.4 Inhibition
of Corrosion 11
2.4.1 Inhibitors 11
2.4.2 Uses/
application of inhibitors 12
2.4.3 Interaction
of the inhibitor with water molecules and metals
13
2.4.4 Classification of inhibitors 13
2.4.4.1 Passivation/anodic oxidizing
inhibitor 13
2.4.4.2 Adsorption/cathodic Inhibitor 13
2.4.4.3 Surface reaction product
inhibitor 14
2.4.4.4 Volatile corrosion inhibitor 14
2.4.4.5 Organic corrosion inhibitor 14
2.4.4.5.1 Green inhibitors 15
2.4.4.5.2 Amino acids as corrosion
inhibitor 17
2.4.4.5.3 Drugs as corrosion
inhibitor 19
2.4.4.5.4 Schiff bases as corrosion
inhibitor 19
2.5 Schiff Bases 19
2.5.1Synthesis of Schiff base 20
2.5.2 Co-ordination chemistry of
Schiff base 20
2.5.3 Mechanism of Schiff base
inhibitors 20
2.5.4 Review of current literatures
on uses of Schiff base as inhibitors for
corrosion 22
2.6 Literature Review of Quantum
Chemical Methods Used in the Study of
Corrosion 25
2.7 Methods of Used in Observing the
Progress of Corrosion 27
2.7.1Gasometric method 28
2.7.2 Gravimetric technique
parameters 29
2.7.3 Fourier Transform Infrared
Spectroscopy (FTIR) technique 30
2.7.4 Electrical resistance probe
technique 30
2.7.5 Scanning electronic technique 31
2.7.6 Thermometric technique 31
2.7.7 Electrochemical measurements
(Polarisation) 31
2.7.8 Electrochemical Impedance
Spectroscopy (EIS) 32
2.8 Calculation of Activation
parameters 32
2.8.1 Activation energy 32
2.8.2 Entropy and enthalpy 33
2.8.3 Gibbs Free energy (ΔG) 33
2.9 Adsorption Consideration 34
2.9.1 Mechanism of adsorption 34
2.9.2 Adsorption isotherm 35
2.9.2.1 Langmuir adsorption isotherm 35
2.9.2.2 Temkin adsorption isotherm 35
2.9.2.3 Flory Huggins adsorption
isotherm 36
2.9.2.4 Frumkin adsorption isotherm 36
2.9.2.5 Freundlich adsorption
isotherm 36
2.9.2.6 Thermodynamic parameters 37
2.10 Computational Chemistry and
Corrosion Study 37
CHAPTER
3: MATERIALS AND METHODS
3.1 Materials 39
3.1.1 Metal Coupons 39
3.1.2 Reagents/chemicals 39
3.2 Methods 40
3.2.1 Preparation of fatty acid hydrazine 40
3.2.2 Preparation of thiosemicarbazide 40
3.2.3 Preparation of Schiff base 40
3.2.4 Preparation of washing solution (22% NaOH and 22
g/l zinc dust) 41
3.2.5 Preparation of inhibitor solutions 42
3.3 Corrosion Studies 42
3.3.1 Weight loss technique 42
3.3.2 Gasometric technique 43
3.3.3 Electrochemical polarization measurement 43
3.3.4 Electrochemical Impedance
Spectroscopy (EIS) 44
3.3.5 Quantum chemical calculation 44
3.3.6 Fourier Transform Infrared
Spectroscopy 46
CHAPTER
4: RESULTS AND DISCUSSION
4.1 Fourier Transform Infrared
Spectroscopy 47
4.2 Gasometric technique 70
4.2.1 Effect of Immersion Time 70
4.3 Weight loss method 74
4.3.1 Effect of exposure time on the
rate of corrosion 74
4.3.2 Effect of concentration on
corrosion rate and inhibition efficiency 77
4.3.3 The effect of temperature on
corrosion rate and inhibition efficiency 87
4.3.3.1 Kinetics 90
4.4 Activation Parameters 100
4.4.1 Activation energy (Ea)
of the corrosion process 100
4.4.2
Enthalpy (∆Hact), entropy (∆Sact) and free energy
of activation (∆Gact) 103
4.5 Adsorption Isotherms 108
4.5.1 Temkin adsorption isotherm 108
4.5.2 Langmuir isotherm 112
4.5.3 Freundlich isotherm 117
4.5.4 Thermodynamic parameters 120
4.6 Potentiodynamic Polarisation
Measurements 123
4.7 Electrochemical Impedance
Spectroscopy 129
4.8 Quantum Chemical Study 133
4.8.1 Global molecular reactivity 136
4.8.1.1 Homo energy (EHomo) 136
4.8.1.2 Lumo energy (Lumo) 136
4.8.1.3 Energy gap (∆E) 137
4.8.1.4 Dipole moment (µ) 137
4.8.1.5 Softness (δ) 137
4.8.1.6 Hardness (η) 138
4.8.1.7 Electronegativity (X) 138
4.8.1.8 Electrophilicity Index (ω) 138
4.8.1.9 Number of transferred electrons (∆N) 139
4.8.2 Mulliken charges 139
CHAPTER
5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 143
5.2 Recommendations 145
REFERENCES 146
APPENDICES 156
LIST
OF TABLES
4.1: Peaks, intensities and assigned
functional groups for
FTIR absorption of methyl ester of
linoleic acid 48
4.2: Peaks, intensities and assigned functional groups
for
FTIR
absorption of synthesised fatty acid hydrazine of linoleic acid 51
4.3: Peaks, intensities and assigned functional groups
for FTIR
absorption
of the synthesised Thiosemicarbazide of linoleic acid 54
4.4: Peaks, intensities and assigned
functional groups for
FTIR absorption of the synthesised Schiff base (DEMS) 57
4.5: Peaks, intensities and assigned
functional groups for
FTIR absorption of methyl ester benheric
acid 60
4.6: Peaks, intensities and assigned
functional groups for
FTIR absorption of the synthesised fatty acid hydrazine of benheric acid 63
4.7: Peaks,
intensities and assigned functional groups for
FTIR absorption
of the synthesised Thiosemicarbazide of benheric acid 66
4.8: Peaks, intensities and assigned
functional groups for
FTIR absorption of the synthesised Schiff base (ATSP) 69
4.9: Inhibition efficiencies and degree of surface coverage of the
various
concentrations of DEMS inhibitor on
mild steel and aluminised
steel in 1 M HCl 73
4.10: Inhibition efficiencies and degree of surface coverage of
the various
concentrations of ATSP inhibitor on
mild steel and aluminised steel
in 1 M HCl 73
4.11: Calculated values for
corrosion rate (CR), surface coverage (θ) and
inhibition
efficiencies (%) of mild steel in 1M HCl solution in the
presence
and absence of different concentrations of inhibitor (DEMS)
at temperatures 303-343 K 79
4.12: Calculated values for
corrosion rate (CR), surface coverage (θ) and
inhibition
efficiency (%) of mild steel in 1M HCl solution in the presence
and
absence of different concentrations of inhibitor (ATSP)
at temperatures 303-343 K 81
4.13: Calculated values for corrosion rate (CR), surface coverage (θ)
and
inhibition efficiencies (I%) of aluminised
steel in 1M HCl solution in
he absence and presence of different
concentrations of inhibitor (DEMS)
at temperatures 303-343 K 83
4.14: Calculated values for corrosion rate (CR), surface coverage
(θ)
and inhibition efficiencies (I %) of
aluminised steel in 1M
HCl solution in the absence and
presence of different
concentrations of the inhibitor
(ATSP) at temperatures 303- 343 K 85
4.15: Activation energy (Ea)
for the adsorption of DEMS on the surface
of mild steel 101
4.16: Activation energy (Ea)
for the adsorption of ATSP on the surface
of mild steel 101
4.17: Activation energy (Ea)
for the adsorption of DEMS on the surface
of aluminised
steel 102
4.18: Activation energy (Ea)
for the adsorption of ATSP on the surface
of aluminised
steel 102
4.19: Thermodynamic parameters (∆Hads,
∆Sads and ∆Gads) for the adsorption
of DEMS on the
surface of mild steel 105
4.20: Thermodynamic parameters (∆Hads,
∆Sads and ∆Gads) for the adsorption
of ATSP on the
surface of mild steel 106
4.21: Thermodynamic parameters (∆Hads,
∆Sads and ∆Gads) for the adsorption
of DEMS on the
surface of aluminised steel 106
4:22: Thermodynamic parameters (∆Hads,
∆Sads and ∆Gads) for the adsorption
of ATSP on the
surface of aluminised steel. 107
4.23: Temkin adsorption parameters for the
adsorption of DEMS on the
surface of mild
steel 111
4.24: Temkin adsorption parameters for the
adsorption of ATSP on the
surface of mild
steel 111
4.25: Temkin adsorption parameters for the
adsorption of DEMS on the
surface of aluminised
steel 111
4.26: Temkin adsorption parameters for the
adsorption of ATSP
on the surface of aluminised
steel 112
4.27: Langmuir adsorption parameters for
the adsorption of DEMS on the
surface of mild
steel 115
4.28: Langmuir adsorption parameters for
the adsorption of ATSP on the
surface of mild
steel 115
4.29: Langmuir adsorption parameters for
the adsorption of DEMS on the
surface of aluminised
steel 116
4.30: Langmuir adsorption parameters for
the adsorption of ATSP on the
surface of aluminised
steel 116
4.31: Frumkin adsorption parameters for
the adsorption of DEMS on the
surface of mild
steel 119
4.32: Frumkin adsorption parameters for
the adsorption of ATSP on the
surface of mild
steel 119
4.33: Frumkin adsorption parameters for
the adsorption of DEMS on the
surface of aluminised
steel 120
4.34: Frumkin adsorption parameters for
the adsorption of ATSP on the
surface of aluminised
steel 120
4.35: Enthalpy of adsorption and entropy
of adsorption of the inhibitors
mild steel and
aluminised steel 124
4.36: Electrochemical kinetic parameters
obtained from potentiodynamic
polarisation
curves of mild steel electrode in 1 M HCl in different
concentrations of
DEMS and ATSP 128
4.37: Electrochemical kinetic
parameters obtained from potentiodynamic
polarisation
curves of aluminium metal electrode in 1M HCl in different
concentrations of
DEMS and ATSP. 128
4.38: Electrochemical Impedance Parameters
for the corrosion of mild
steel in 1M HCl
containing different concentration of DEMS and ATSP
at 303 K 132
4.39: Electrochemical Impedance Parameters
for the corrosion of aluminised
steel in 1M HCl
containing different concentrations of DEMS and ATSP
at 303 K 132
4.40: Derived quantum chemical parameters
from semi – empirical (PM3)
method for the
Schiff bases DEMS and ATSP 135
LIST OF URES
4.1: The FTIR spectrum of methyl ester of
linoleic acid 47
4.2: Structure of methyl ester of linoleic
acid
49
4.3: The FTIR spectrum of the synthesised fatty
acid hydrazine of linoleic acid 50
4.4: Structure of fatty acid hydrazine of
linoleic acid
52
4.5:
The FTIR spectrum of the synthesised Thiosemicarbazide of linoleic acid 53
4.6: Structure of Thiosemicarbazide of
linoleic acid 55
4.7:
The FTIR spectrum of the synthesised Schiff base of linoleic acid (DEMS) 56
4.8: Structure of the synthesised Schiff
base of linoleic acid (DEMS) 58
4.9:
The FTIR spectrum of methyl ester of benheric acid 59
4.10: Structure of methyl ester of benheric
acid
61
4.11: The FTIR spectrum of the synthesised fatty
acid hydrazine of benheric acid 62
4.12: Structure of fatty acid hydrazine of
benheric acid
64
4.13: The FTIR spectrum of the Synthesised
Thiosemicarbazide of benheric acid 65
4.14: Structure of Thiosemicarbazide of
benheric acid
67
4.15: The FTIR spectrum of the synthesised
Schiff base of benheric acid (ATSP) 68
4.16: Structure of the synthesised Schiff
base of benheric acid (ATSP) 70
4.17: Variation of volume of hydrogen gas evolved with time for
the corrosion
of mild steel in various
concentrations of DEMS in 1 M HCl solution 71
4.18: Variation of volume of hydrogen gas evolved with time for
the corrosion
of aluminised steel in various
concentrations of DEMS in 1 M HCl solution
71
4.19: Variation of Volume of Hydrogen Gas Evolved with Time for
the Corrosion
of Mild Steel in Various
Concentrations of ATSP in 1 M HCl Solution 72
4.20:
Variation of volume of hydrogen gas evolved with time for the corrosion
of
aluminised steel in various concentrations of ATSP in 1 M HCl solution 72
4.21: Variation of weight loss
(g) of mild steel versus time (h) in the absence
and
presence of different concentrations of DEMS at 303 K 74
.
4.22: Variation of weight loss (g) of mild
steel versus time(h) in the absence
and presence of
different concentrations of ATSP at 303 K. 75
4.23: Variation of weight loss (g) of aluminised
steel versus time (h) in
the absence and
presence of different concentrations of DEMS at 303 K. 75
4.24: Variation of weight loss (g) of aluminised
steel versus time (h) in the
absence and presence
of different concentrations of ATSP at 303 K. 76
4.25: Variation of weight loss (g) of mild
steel versus time (h) in the absence
and presence of
different concentrations of DEMS at 303 K. 77
4.26: Variation of weight loss (g) of mild steel
versus time (h) in the absence
and presence of
different concentrations of ATSP at 313 K. 77
4.27: Variation of inhibition
efficiency versus different concentrations of DEMS
on aluminised steel at
temperature ranges of 303-343 K 78
4.28: Variation of inhibition
efficiency versus different concentrations of ATSP
on aluminised steel at
temperature ranges of 303-343 K. 78
4.29: Plots of log
CR versus 1/T for the corrosion of mild steel in 1M HCl
containing
various concentrations of DEMS 87
4.30: Plots of log
CR versus 1/T for the corrosion of mild steel in 1M HCl
containing
various concentrations of ATSP 88
4.31: Plots of log
CR versus 1/T for the corrosion of aluminised steel in 1M
HCl
containing various concentrations of DEMS 88
4.32:
Plots of log CR versus 1/T for the corrosion of
aluminised steel in 1M HCl
containing various concentrations of ATSP.
89
4.33: Plots of -log weight loss of mild steel
against time at temperature
303 K in the
presence of various concentrations of DEMS in 1M HCl. 90
4.34: Plots of -log weight loss of mild steel
against time at temperature
313 K in the
presence of various concentrations of DEMS in 1M HCl 90
4.35: Plots of -log weight loss of mild steel
against time at temperature
323 K in the
presence of various concentrations of DEMS in 1M HCl 91
4.36: Plots of -log weight loss of mild steel
against time at temperature
333 K in the
presence of various concentrations of DEMS in 1M HCl 91
4.37: Plots of -log weight loss of mild steel
against time at temperature
343 K in the
presence of various concentrations of DEMS in 1M HCl 92
4.38: Plots of -log weight loss of mild steel
against time at temperature
303 K in the
presence of various concentrations of ATSP in 1M HCl 92
4.39: Plots of -log weight loss of mild steel
against time at temperature
313 K in the
presence of various concentrations of ATSP in 1M HCl 93
`
4.40: Plots of -log weight loss of mild steel
against time at temperature
323 K in the
presence of various concentrations of ATSP in 1M HCl 93
4.41: Plots of -log weight loss of mild steel
against time at temperature
333 K in the
presence of various concentrations of ATSP in 1M HCl 94
4.42: Plots of -log weight loss of mild steel
against time at temperature 343 K in
the presence of
various concentrations of ATSP in 1M HCl 94
4.43: Plots of -log weight loss of aluminised
steel against time at temperature
303 K in the
presence of various concentrations of DEMS in 1M HCl 95
4.44: Plots of -log weight loss of aluminised
steel against time at temperature
313 K in the
presence of various concentrations of DEMS in 1M HCl 95
4.45: Plots of -log weight loss of aluminised
steel against time at temperature
323 K in the
presence of various concentrations of DEMS in 1M HCl 96
4.46: Plots of -log weight loss of aluminised
steel against time at temperature
333 K in the
presence of various concentrations of DEMS in 1M HCl 96
4.47: Plots of -log weight loss of aluminised
steel against time at temperature
343 K in the
presence of various concentrations of DEMS in 1M HCl 97
4.48: Plots of -log weight loss of aluminised
steel against time at temperature
303 K in the
presence of various concentrations of ATSP in 1M HCl 97
4.49: Plots of -log weight loss of aluminised
steel against time at temperature
313 K in the
presence of various concentrations of ATSP in 1M HCl 98
4.50: Plots of -log weight loss of aluminised
steel against time at temperature
323 K in the
presence of various concentrations of ATSP in 1M HCl 98
4.51: Plots of -log weight loss of aluminised
steel against time at temperature
333 K in the
presence of various concentrations of ATSP in 1M HCl 99
4.52: Plots of -log weight loss of aluminised
steel against time at temperature
343 K in the
presence of various concentrations of ATSP in 1M HCl 99
4.53: Plots of log
(CR/T) versus 1/T for the corrosion of mild steel in 1M HCl
containing various concentrations of DEMS 103
4.54: Plots of log
(CR/T) versus 1/T for the corrosion of mild steel in 1M HCl
containing
various concentrations of ATSP 104
4.55: Plots of log (CR/T) versus 1/T for the
corrosion of aluminised steel
in 1M HCl
containing various concentrations of DEMS 104
4.56: Plots of log (CR/T) versus 1/T for the
corrosion of aluminised steel
in 1M HCl
containing various concentrations of ATSP 105
4.57: Temkin
adsorption isotherm for the adsorption of DEMS on the surface
of
mild steel at various temperatures 109
4.58: Temkin adsorption
isotherm for the adsorption of ATSP on the surface of
mild
steel at various temperatures 109
4.59: Temkin
adsorption isotherm for the adsorption of DEMS on the surface of
aluminised steel at various temperatures 110
4.60: Temkin adsorption
isotherm for the adsorption of ATSP on the surface of
aluminised steel at various temperatures 110
4.61: Langmuir
adsorption isotherm for the adsorption of DEMS on the surface
of mild steel at
various temperatures 113
4.62:
Langmuir adsorption isotherm for the adsorption of ATSP on the surface of
mild steel at various temperatures 113
4.63: Langmuir adsorption isotherm for the
adsorption of DEMS on the surface of
aluminised
steel at various temperatures 114
4.64: Langmuir adsorption isotherm for the
adsorption of ATSP on the
surface
of aluminised steel at various temperatures 114
4.65: Freundlich
adsorption isotherm for the adsorption of DEMS on the surface
of mild steel at various temperatures 117
4.66: Freundlich
adsorption isotherm for the adsorption of ATSP on the surface
of mild steel at various temperatures 118
4.67: Freundlich
adsorption isotherm for the adsorption of DEMS on the
surface
of aluminised steel at various temperatures 118
4.68: Freundlich
adsorption isotherm for the adsorption of ATSP on the
surface
of aluminised steel at various temperatures 119
.
4.69: Plot of log Kads
versus 1/T for the corrosion process of mild steel in 1M HCl
containing
the inhibitor DEMS 122
4.70:
Plot of log Kads versus 1/T for the
corrosion process of mild steel in 1M HCl
containing
the inhibitor ATSP 122
4.71: Plot of log Kads
versus 1/T for the corrosion process of aluminised steel
in 1M
HCl containing the inhibitor DEMS 123
4.72: Plot of log Kads
versus 1/T for the corrosion process of aluminised steel
in 1M
HCl containing the inhibitor DEMS 123
4.73: Polarization curves of mild steel in
1 M HCl with different concentrations
of DEMS at 303 K 125
4.74: Polarization curves of mild steel in 1 M
HCl with different concentrations
of ATSP at 303 K 126
4.75: Polarization curves of aluminised steel
in 1 M HCl with different
concentrations DEMS at 303 K 126
4.76: Polarization curves of aluminised steel
in 1 M HCl with different
concentrations of ATSP at 303 K 127
4.77: Nyquist plots for mild steel in 1M HCl
in the absence and presence
of different
concentrations of DEMS 129
4.78: Nyquist plots for mild steel in 1M HCl
in the absence and presence
of different
concentrations of ATSP 130
4.79: Nyquist plots for aluminised steel in 1M
HCl in the absence and presence
of different
concentrations of DEMS 130
4.80: Nyquist plots for aluminised steel in 1M
HCl in the absence and presence
of different
concentrations of ATSP 131
4.81a: Optimised geometry of DEMS Schiff base
(inhibitor) 133
4.81b: Molecular orbital of DEMS Schiff base
showing LUMO and HOMO 133
4.82a: Optimised geometry of ATSP Schiff base
(inhibitor) 134
4.82b: Molecular orbital of ATSP Schiff base
showing LUMO and HOMO 135
4.83: Mulliken charge network showing the
sites for electrophilic and nucleophilic
attacks for the Schiff base molecule (DEMS). 140
4.84: Mulliken charge network showing the sites for electrophilic and
nucleophilic
attacks for the Schiff base molecule- ATSP. 141
CHAPTER
1
INTRODUCTION
1. 1 BACKGROUND OF THE STUDY
Metals
are common substances that are widely used in everyday life. They are utilised
both at home and in the industries. Their usage ranges from extraction
industries, manufacturing industries and production industries and as well as
production of machines used in various forms of industrial purposes, but their susceptibility to
rusting in humid air and their very high dissolution rate in acidic or alkaline
medium are the major obstacles for their uses on larger scale.
Steel, which is an alloy made from the
combination of two or more metallic elements and/or non-metallic element, is
commonly made from the fusing of iron and carbon together, and with varying
amount of other elements (such as vanadium, chromium, tungsten, aluminium
etc.). The presence of these alloying elements determines the properties such
as hardness, ductility and tensile strength possess by a particular steel metal
(Umoren et al., 2008b).
Steel when compared with pure iron
has more resistance to rusting and has a better weld ability. Most times, other
metals are incorporated to iron/ carbon mixture in order to affect the
properties of the steel produced. Metals such as manganese and nickel help to
improve the tensile strength of steel, while chromium helps to improve the
hardness and elevate melting point. When compared to aluminum, steel is more
malleable and it is used in the manufacture of different types of tools,
utensils, automobiles, weapons and building materials. The expanded industrial
applications of this metal based on its properties has been recognized by
researchers (Eddy and Odoemelam, 2008b; Dadgarinezhad and Baghaei, 2011; Oguzie
et al., 2006a). Despite the economic
and industrial importance of metals, the problems linked with corrosion still
remain a cause of concern to all as corrosion tends to reduce life span of
metals if not properly protected.
Over the years, corrosion of metals
has led to huge losses of artificial and natural resources annually. In the gas
and oil industries, most of the reported cases of pipelines failure are
attributed to corrosion. For example, about 162 cases of oil spill resulted
from the corrosion of pipelines between 2002 and 2004 (Ajayi, 2003; Eddy and
Odoemelam, 2008b) and in 2016 NACE international impact estimated the global
cost of corrosion to be US2.5 trillion www.impact.nace.org.
These deterioration processes took place because of exposure of these metals to
aggressive acidic and basic environments by industries and individuals that
utilize them.
Corrosion of metal is a major
industrial problem that has attracted much attention and researches (Abiola et al., 2007; Arora et al., 2007; Kumar, 2008; Umoren and Ebenso 2007, 2008). Corrosion
is one of the common ways by which metals degrade. Most metals corrode when in
contact with moist air, water, solutions such as acids, bases, salt, aggressive
polishes as well as gaseous materials such as acid vapours, formaldehyde gases,
ammonia gases and sulphur containing gases.
1.2 STATEMENT OF THE PROBLEM
The effort to limit the problem of
corrosion does not only lie on the type of inhibitor used to control it but
also on the effect of the inhibitor to the environment. Inorganic and synthetic
inhibitors which are mostly lead, chromium and cadmium based compounds have
been considered to be environmentally unfriendly and in some cases attack
either cathodic or anodic corrosion sites. They are considered to be unsafe for
usage. Schiff bases, which are environmentally friendly organic inhibitors
double up as mixed- type inhibitors, which make them more preferred to
inorganic inhibitors. This forms the basis of this research, which involves the
use of Schiff bases, in the control of corrosion.
1.3
JUSTIFICATION OF THE STUDY
The damage caused to industrial installations by
corrosion is unimaginable and this attaches huge cost for remediation. Brown
(1999) estimated the cost of damages resulting from corrosion to be $170b. This
showed an astronomical increase from the estimate $17.9b spent on remediation
of damage resulting from corrosion in 1998 (Wanlin et al., 2005).
In 2016, NACE international impact
estimated the global cost of corrosion to be US $2.5 trillion (Shell Imperial
Aim Centre). As the global quest for industrialization through technology and
metals utilization increases, the cost of management of damages due to
corrosion also increases. Corrosion is a problem threatening metal
installations including cars, engines, bridges, buildings and in factories such
as oil and gas, fertilizer industry, textile industry, etc.
1.4
AIM AND OBJECTIVES OF THE STUDY
The aim of the study is to
investigate the inhibition potentials of the Schiff bases: 2[2-diethylamino) ethyl methyl
amino)-4-methy1-5-3 (3-methyl sulfanyl propy1 amino) methyldiene
cyclohexdien-1-one (DEMS) and [1-(azepan-1-yl)2-2-[4-(2-tert-butyl sulfanyl
ethyl piperazin-1-yl] ethanone (ATSP) synthesized from linoleic acid and benheric acid on mild steel
and aluminised steel in 1M HCl solutions.
1.4.1
Specific Objectives
The specific objectives of the study
include:
(i)
To synthesise the two
Schiff bases to be used for the study from linoleic and benheric acids.
(ii)
To study the inhibition
potentials of the two synthesised Schiff bases on mild steel and aluminised
steel
(iii)
To characterise the
inhibition properties/mechanism of the two Schiff bases on mild steel and
aluminised steel in HCl solution.
(iv)
To investigate the
correlation between the inhibitive effect and molecular structure of the
inhibitors (Schiff bases).
1.5
SCOPE OF THE STUDY
Organic compounds are known to
contain nitrogen, oxygen or sulphur in their conjugated systems, which serve as
sites for adsorption of their molecules on the surfaces of metals creating
barrier to substances which may cause corrosion (Wang et al., 2007; El Ashry et al.,
2006; Ebenso, 2003a). Schiff bases which are organic inhibitors are also known
to contain functional groups with high electron densities which enable them to
adsorb onto metal surfaces. These molecules possess hetero atoms in their
functional groups (-C=O, -N=O, -NR2, SH) as well as π-electrons in
their double bonds which enable them to donate lone pair of electrons to metals
with vacant sites. These functional groups contribute to the effectiveness of
the Schiff base inhibitors. Corrosion inhibition studies of the two Schiff
bases (DEMS and ATSP) on mild steel and aluminised steel in acidic media (HCl)
is a work that has not yet been reported.
The inhibition efficiencies of the
two Schiff bases were carried out using gasometric technique, weight loss
method, potentiodynamic polarisation measurement, electrochemical impedance
spectroscopy and quantum studies. The characterization of the Schiff bases was
done using FTIR spectral analysis. The thermodynamic properties of the Schiff
bases were also studied and the adsorption properties of the Schiff bases on
the mild steel and the aluminised steel coupons were determined from various
adsorption isotherms models.
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