ABSTRACT
The corrosion inhibition of mild steel and zinc plates in 0.5M HCl in the presence and absence of 4-{[(Z)-(4-chlorophenyl)methylidene}amino)-N-(5-methy-l-furan-3-yl) benzenesulfonamide herein referred to as (MMBS) and 4-{[(Z)-(4-dimethyl amino) phenyl] methylidene} amino)-N-(5-methyl furan-3-yl) benzenesulfonamide herein refers to as (CMBS) was investigated at temperature ranges of 303 K to 343 K. Their inhibition performances were tested by weight loss and quantum chemical techniques and were characterised using Fourier Transform Infrared spectrophotometer (FT-IR). The FT-IR spectroscopy showed that the synthesised Schiff bases contained some functional groups which may have aided in the inhibition of the metal species under consideration. The results obtained from the weight loss method showed that inhibition efficiencies of the inhibitors increased as the concentrations of the Schiff-base molecules increased and with the highest inhibition efficiency observed at the optimum concentration of 1.0 g/L employed but decreased with rise in temperature. The inhibitor (MMBS) showed inhibition efficiency of 89.59 % while the inhibitor (CMBS) showed inhibition efficiency of 92.36 % for mild steel while for zinc plate, inhibition efficiencies were 38.19 % and 39.98 % for (MMBS) and (CMBS) respectively. Inhibition efficiencies were also found to decrease with increase in temperature. The inhibition efficiencies followed the order; (MMBS) < (CMBS). The Kinetic data obtained showed that the corrosion process followed first order kinetics. The following adsorption isotherms models including Langmuir and Freundlich employed in the interpretation of the inhibition data showed that Langmuir model provided the best fit to the experimental data (R2>0.96). Thermodynamic studies of the inhibition process showed that the ΔGads values for (MMBS) and (CMBS) 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 physisorption. The enthalpy values were positive for (MMBS) (121.650 – 148.904 Jmol-1K-1) on mild steel but negative for (MMBS) (-2.1213 to - 0.4059 Jmol-1K-1) on zinc showing that the reaction was endothermic for inhibition of mild steel but exothermic for inhibition of zinc. The quantum chemical parameters such as HOMO, LUMO, EHomo; Elijmo, and Energy gap (ΔE), aided in the elucidation of the probable points of interaction of these inhibitors with mild steel and zinc. 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 unoccupied molecular orbital (EHomo), energy of the lowest occupied molecular orbital (ELumo), dipole moment (µ), softness (σ), as well as the hardness (η) of the inhibitor. The quantum chemical parameter, as indicators to their comparative inhibition potentials, agreed with experimental data.
TABLE
OF CONTENTS.
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Table of Contents vi
List of Tables x
List of Figures xii
Abstract xvi
CHAPTER 1: INTRODUCTION 1
1.1 Background of the Study 2
1.2 Statement of the
Problem 4
1.3 Justification
for the Research 6
1.4 Aim
of the Research 8
1.5 Specific
Objective of the Study 8
1.6 Scope
of the Research 8
1.7 Limitation
and De-limitation of the study 9
CHAPTER 2: LITERATURE REVIEW 10
2.1 Definition
of Corrosion 10
2.2 Corrosion
Parameters 11
2.3 Classification
of Corrosion 16
2.4 Types
of Corrosion 16
2.5 Corrosion
Inhibitors 20
2.6 Types
and Mechanisms of Inhibitors 23
2.6.1
Passivating inhibitors. 23
2.7 Environmental
Conditions (Scavengers) 28
2.7.1 Organic
corrosion inhibitors 29
2.7.2. Green inhibitors 32
2.7.3. Amino acids as
corrosion inhibitor 34
2.8 Schiff
Base 35
2.8.1
Synthesis
of Schiff base 36
2.8.2. Schiff base as
corrosion inhibitor 36
2.8.3.
Coordination
chemistry of Schiff base 37
2.8.4 Mechanisms of
Schiff base inhibitors. 37
2.8.5 Review of current literature on the use of Schiff bases as
inhibitors for
corrosion 39
2.9 Literature Review of
Quantum Chemical Method Used in the
Study
of Corrosion 40
2.10 Methods
of Monitoring Corrosion 42
2.10.1 X-Ray diffraction (XDS) and X-ray photoelectron spectroscopy 42
2.10.2 Gasometric method 43
2.10.3 Gravimetric technique 44
2.10.4 Calculation of thermodynamic parameters 45
2.10.5 Activation energy (Ea) 46
2.10.6 Gibbs free energy of adsorption (ΔGads) 47
2.10.7 Fourier transform infra-red spectroscopy (FTIR) technique 47
2.10.8 Electrical resistance probe technique 48
2.10.9 Scanning electron microscopy technique 48
2.10.10 Thermometric technique 49
2.10.11. Electrochemical
measurements (polarisation) 49
2.10.12 Electrochemical
impedance spectroscopy (EIS) 50
2.11 Adsorption
Consideration 50
2.11.1 Langmuir adsorption isotherm 51
2.11.2 Temkin adsorption isotherm 52
2.11.3 Flory - Huggins
adsorption isotherm 52
2.11.4 Frumkin adsorption isotherm 53
2.11.5 Freundlich adsorption
isotherm 53
2.12 Computational
Chemistry and Corrosion Study 54
CHAPTER 3: MATERIALS
AND METHODS 56
3.1 Materials 56
3.1.1 Samples 56
3.1.2
Reagents
/ chemicals 56
3.1.3
Apparatus
/ equipment 56
3.2
Methods 56
3.2.1
Metal coupons 56
3.2.2 Preparation of Schiff base (MMBS). 57
3.2.3 Preparation of Schiff base (CMBS). 58
3.2.4 Preparation of 0.5 M HC1 from stock
solution 58
3.2.5 Preparation of acid solution 60
3.2.6 Preparation of washing solution (22 % NaOH and 22 g/l Zinc dust) 61
3.2.7 Preparation of inhibitor solutions 61
3.3 Corrosion
Studies 62
3.3.1. Weight
loss technique 61
3.3.2.
Quantum
chemical calculation 62
3.3.3. Fourier transform infrared
spectroscopy 63
CHAPTER 4: RESULTS AND
DISCUSSION 64
4.1 Fourier Transform Infrared
(FTIR) Spectroscopy 64
4.2:
Weight Loss Results 67
4.3 Surface Coverage and Inhibition Efficiency 78
4.4 Concentration
and Temperature Dependence on Inhibition Efficiency 93
4.5 Corrosion Rate Results. 94
4.6 Corrosion and Corrosion Inhibition Mechanism 99
4.7 Kinetic
Studies of Mild Steel and Zinc Corrosion in HCl Solution in
the
Presence of Inhibitor 100
4.8
Activation Energy and
Frequency Factors 123
4.9 Enthalpy and Entropy of Activation 129
4.10 Adsorption
Isotherm 135
4.10.1: The Langmuir
model 136
4.10.2 The Freundlich isotherm model. 142
4.11 Quantum Chemical Study 148
4.11.1
Global molecular reactivity 152
4.11.2 HOMO
energy (Ehomo) 152
4.11.3 LUMO energy (Elumo) 153
4.11.4.
Energy gap (ΔE) 153
4.11.5
Dipole moment (µ) 153
4.11.6
Softness (δ). 154
4.11.7
Hardness (η) 154
4.11.8
Electronegativity (χ) 154
4.11.9
Electrophilicity index (ω) 155
4.11.10 Number of transferred electrons
(ΔN) 155
CHAPTER 5: CONCLUSION AND
RECOMMENDATIONS 157
5.1 Conclusion 157
5.2 Recommendations 158
References
LIST
OF TABLES
4.1 Absorption frequencies of the various functional groups 66
4.2 Surface coverage and Inhibition
efficiency of MMBS on mild steel. 89
4.3 Surface coverage and inhibition
efficiency of CMBS on mild steel. 90
4.4 Surface coverage and inhibition
efficiency of MMBS on zinc. 91
4.5 Surface coverage and inhibition
efficiency of CMBS on zinc 92
4.6 Corrosion rates
of mild steel using MMBS at various concentrations and
temperatures 95
4.7 Corrosion rates of mild steel using CMBS at various concentrations
and temperatures. 96
4.8 Corrosion rates
of zinc using MMBS at various concentrations and
temperature 97
4.9 Corrosion rates of zinc using CMBS at various concentrations and
temperature 98
4.10 Rate constants
and half-life of steel for MMBS at 303 K 103
4.11 Rate constants
and half-life of steel for MMBS at 313 K 104
4.12 Rate constants
and half-life of steel for MMBS at 323 K 105
4.13 Rate constants
and half-life of steel for MMBS at 333 K 106
4.14 Rate constants
and half-life of steel for MMBS at 343 K 107
4.15 Rate constants
and half-life of steel for CMBS at 303 K 108
4.16 Rate constants
and half-life of steel for CMBS at 313 K 109
4.17 Rate constants
and half-life of steel for CMBS at 323 K 110
4.18 Rate constants
and half-life of steel for CMBS at 333 K 111
4.19 Rate constants
and half-life of steel for CMBS at 343 K 112
4.20 Rate constants and
half-life of zinc for MMBS at 303 K 113
4.21 Rate constants
and half-life of zinc for MMBS at 313 K 114
4.22 Rate constants and
half-lives of zinc for MMBS at 323 K 115
4.23 Rate constants
and half-life of zinc for MMBS at 333 K 116
4.24 Rate constants
and half-life of zinc for MMBS at 343 K 117
4.25 Rate constants
and half-life of zinc for CMBS at 303 K 118
4.26 Rate constants
and half-life of zinc for CMBS at 313 K 119
4.27 Rate constants
and half-life of zinc for CMBS at 323 K 120
4.28 Rate constants
and half-life of zinc for CMBS at 333 K 121
4.29 Rate constants
and half-life of zinc for CMBS at 343 K 122
4.30 Activation
energy and frequency factor values for MMBS on mild steel 125
4.31 Activation
energy and frequency factor values for CMBS on mild steel 126
4.32 Activation
energy and frequency factor values for MMBS on zinc 127
4.33 Activation
energy and frequency factor values for CMBS on zinc 128
4.34 Entropies
and enthalpies of activation values for MMBS on mild steel 130
4.35 Entropies
and enthalpies of activation values for CMBS on mild steel 131
4.36 Entropies
and enthalpies of activation values for MMBS on zinc 132
4.37 Entropies
and enthalpies of activation values for CMBS on zinc 133
4.38 Free energy and
adsorption parameters for MMBS on mild steel the plots 138
4.39 Free energy and
adsorption parameters for CMBS on mild steel the plots 139
4.40 Free energy and
adsorption parameters for MMBS on zinc the plots 140
4.41 Free energy and
adsorption parameters for CMBS on zinc the plots 141
4.42 Free energy and
adsorption parameters for MMBS on mild steel 143
4.43 Free energy and
adsorption parameters for CMBS on mild steel 144
4.44 Free energy and
adsorption parameters for MMBS on zinc 145
4.45 Free energy and
adsorption parameters for CMBS on zinc 146
4.46 Derived
quantum chemical parameters from semi-empirical (PM3)
method
for the Schiff bases MMBS and CMBS. 151
4.47 Comparative
study of calculated quantum parameters of inhibitors. 151
LIST OF FIGURES.
2.1 Evans type diagram showing corroding
system under anodic inhibition 25
2.2 Evans type diagram showing corroding
system under cathodic inhibition 26
2.3 Evans type diagram showing
corroding system under mixed inhibition 26
2.4 Type diagram – showing corrosion kinetics
for mixed inhibition 28
2.5 Structure of Schiff Base 36
3.1 Scheme 1-preparation of Schiff base CMBS 58
3.2 Scheme 2- preparation of Schiff base CMBS 59
4.1 The FTIR spectrum of MMBS 65
4.2 The FTIR spectrum of CMBS 67
4.3 Plot of weight loss (mild steel) vs time
for MMBS at 303K. 68
4.4 Plot of weight loss (mild steel) vs time
for MMBS at 313K. 68
4.5 Plot of weight loss (mild steel) vs time
for MMBS at 323K. 69
4.6 Plot of weight loss (mild steel) vs time
for MMBS at 333K. 69
4.7 Plot of weight loss (mild steel) vs time
for MMBS at 343K. 70
4.8 Plot of weight loss (mild steel) vs time
for CMBS at 303K. 70
4.9 Plot of weight loss (mild steel) vs time
for CMBS at 313K. 71
4.10 Plot of weight loss (mild steel) vs time
for CMBS at 323K. 71
4.11 Plot of weight loss (mild steel) vs time
for CMBS at 333K 72
4.12 Plot of weight loss (mild steel) vs time
for CMBS at 343K. 72
4.13 Plot of weight loss (zinc plate) vs time
for MMBS at 303K. 73
4.14 Plot of weight loss (zinc plate) vs time
for MMBS at 313K. 73
4.15 Plot of weight loss (zinc plate) vs time
for MMBS at 323K. 74
4.16 Plot of weight loss (zinc plate) vs time
for MMBS at 333K. 74
4.17 Plot of weight loss (zinc plate) vs time
for MMBS at 343K. 75
4.18 Plot of weight loss (zinc plate) vs time
for CMBS at 303K. 75
4.19 Plot of weight loss (zinc plate) vs time
for CMBS at 313K. 76
4.20 Plot of weight loss (zinc plate) vs time
for CMBS at 323K. 76
4.21 Plot of weight loss (zinc plate) vs time
for CMBS at 333K. 77
4.22 Plot of weight loss (zinc plate) vs time
for CMBS at 343K. 77
4.23 Plot of I.E. vs concentration of
inhibitors (MMBS) for mild steel at 303 K. 79
4.24 Plot of I.E. vs concentration of
inhibitors (MMBS) for mild steel at 313 K. 79
4.25 Plot of I.E. vs concentration of
inhibitors (MMBS) for mild steel at 323 K. 80
4.26 Plot of I.E. vs concentration of inhibitors
(MMBS) for mild steel at 333 K. 80
4.27 Plot of I.E. vs concentration of
inhibitors (MMBS) for mild steel at 343 K. 81
4.28 Plot of I.E. vs concentration of
inhibitors (CMBS) for mild steel at 303 K. 81
4.29 Plot of I.E. vs concentration of
inhibitors (CMBS) for mild steel at 313 K. 82
4.30 Plot of I.E. vs concentration of
inhibitors (CMBS) for mild steel at 323 K 82
4.31 Plot of I.E. vs concentration of
inhibitors (CMBS) for mild steel at 333 K. 83
4.32 Plot of I.E. vs concentration of
inhibitors (CMBS) for mild steel at 343 K. 83
4.33 Plot of I.E. vs concentration of
inhibitors (MMBS) for Zinc at 303 K. 84
4.34 Plot of I.E. vs concentration of
inhibitors (MMBS) for Zinc at 313 K. 84
4.35 Plot of I.E. vs concentration of
inhibitors (MMBS) for Zinc at 323 K 85
4.36 Plot of I.E. vs concentration of
inhibitors (MMBS) for Zinc at 333 K. 85
4.37 Plot of I.E. vs concentration of
inhibitors (MMBS) for Zinc at 343 K. 86
4.38 Plot of I.E. vs concentration of
inhibitors (CMBS) for Zinc at 303 K 86
4.39 Plot of I.E. vs concentration of
inhibitors (CMBS) for Zinc at 313 K 87
4.40 Plot of I.E. vs concentration of
inhibitors (CMBS) for Zinc at 323 K. 87
4.41 Plot of I.E. vs concentration of
inhibitors (CMBS) for Zinc at 333 K 88
4.42 Plot of I.E. vs concentration of
inhibitors (CMBS) for Zinc at 343 K. 898
4.43 A plot of –log
(weight loss of steel) vs. time for MMBS at 303 K 103
4.44 A plot of –log
(weight loss of steel) vs. time for MMBS at 313 K 104
4.45 A plot of –log
(weight loss of steel) vs. time for MMBS at 323 K 105
4.46 A plot of –log
(weight loss of steel) vs. time for MMBS at 333 K 106
4.47 A plot of –log
(weight loss of steel) vs. time for MMBS at 343 K 107
4.48 A plot of –log
(weight loss of steel) vs. time for CMBS at 303 K 108
4.49 A plot of –log
(weight loss of steel) vs. time for CMBS at 313 K 19
4.50 A plot of –log
(weight loss of steel) vs. time for CMBS at 323 K 110
4.51 A plot of –log
(weight loss of steel) vs. time for CMBS at 333 K 111
4.52 A plot of –log
(weight loss of steel) vs. time for CMBS at 343 K 112
4.53 A plot of –log
(weight loss of zinc) vs. time for MMBS at 303 K 113
4.54 A plot of –log
(weight loss of zinc) vs. time for MMBS at 313 K 114
4.55 A plot of –log
(weight loss of zinc) vs. time for MMBS at 323 K 115
4.56 A plot of –log
(weight loss of zinc) vs. time for MMBS at 333 K 116
4.57 A plot of –log
(weight loss of zinc) vs. time for MMBS at 343 K 117
4.58 A plot of –log
(weight loss of zinc) vs. time for MMBS at 303 K 118
4.59 A plot of –log
(weight loss of zinc) vs. time for CMBS at 313 K 119
4.60 A plot of –log
(weight loss of zinc) vs. time for CMBS at 323 K 120
4.61 A plot of –log
(weight loss of zinc) vs. time for CMBS at 333 K 121
4.62 A plot of –log
(weight loss of zinc) vs. time for CMBS at 343 K 122
4.63 An Arrhenius
plot of ln CR vs. 1/T for MMBS on mild steel 125
4.64 An Arrhenius
plot of ln CR vs. 1/T for CMBS on mild steel 126
4.65 An Arrhenius
plot of ln CR vs. 1/T for MMBS on zinc 127
4.66 An Arrhenius
plot of ln CR vs. 1/T for CMBS on zinc 128
4.67 Eyring
plot of ln (CR/T) vs. 1/T for MMBS on mild steel 130
4.68 Eyring
plot of ln (CR/T) vs. 1/T for CMBS on mild steel 131
4.69 Eyring
plot of ln (CR/T) vs. 1/T for MMBS on zinc 132
4.70 Eyring
plot of ln (CR/T) vs. 1/T for CMBS on zinc 133
4.71 Langmuir plots
for MMBS on mild steel at various temperatures. 138
4.72 Langmuir plots
for CMBS on mild steel at various temperatures. 139
4.73 Langmuir plots
for MMBS on zinc at various temperatures 140
4.74 Langmuir plots
for CMBS on zinc at various temperatures 141
4.75 Freundlich
isotherm plots for MMBS on mild steel 143
4.76 Freundlich
isotherm plots for CMBS on mild steel 144
4.77 Freundlich
isotherm plots for MMBS on zinc. 145
4.78 Freundlich
isotherm plots for MMBS on zinc 146
4.79 Optimised
geometry of MMBS Schiff base (inhibitor) 149
4.80 Homo orbitals of MMBS Schiff
base (inhibitor) 149
4.81 Lumo orbitals of MMBS
Schiff base (inhibitor) 149
4.82 Optimised
geometry of CMBS Schiff base (inhibitor) 150
4.83 Homo orbitals of CMBS
Schiff base (inhibitor) 150
4.84 Lumo orbitals of CMBS
Schiff base (inhibitor) 150
CHAPTER 1
INTRODUCTION
The surface
of iron
and steel,
existing in many forms ranging from exposed metal to oxidized
metal and to different degrees of states, finds many industrial
applications (Ballesteros et al., 2015; Barros et al., 2016; Souza et al., 2016). Some investigations have emphasized the importance for
the protection of the metal surfaces in various applications (Amitha, 2012). The spontaneous destruction
of metals, starting from
their surfaces, due to the
corrosive attack of environment brings the undesirable changes at the
surfaces and reduces
their lifetime, strength and changes the
desirable properties
of metal surfaces. In some cases, the corrosion
products formed are toxic. An accurate description of surface not only helps to identify the prevailing
form of corrosion but also the prescription of appropriate anti-corrosion measures.
Acid solutions,
(in particular HCl solutions) are widely used in
industries for many purposes, such as acid pickling, industrial acid cleaning,
acid descaling and oil well acidizing (Ochoa et
al., 2013; Danaee et al., 2013; Gerengi et al., 2014; Rocha et al., 2014). Due to the general
aggressive nature of acid solutions, the corrosive attack will be severed (Ochoa et al., 2013) and bring undesirable changes at the
surface of metals. Chemical inhibitors are often used to control the corrosive
attack and acid consumptions of environment (Ghasemi et al., 2013; Santana et al. 2015,).
Most of the well-known corrosion inhibitors are the organic compounds
containing N, S, O and P atoms
(Santos et al., 2017; Barreto et al., 2018). These organic compounds reduce the
metal dissolution by the adsorption onto the metal surfaces (Sivakumar et al., 2018). The adsorption and inhibition efficiency of these
compounds greatly depend on the
electron density around the hetero atoms,
the number
of adsorption
active canters in the molecule
and their surface charge density, molecular size, mode of adsorption and formation of metallic
complexes (Barreto et al., 2018; Abdallah et al., 2019).
However,
the choice of inhibitors is based on two considerations, first economic
consideration and seconds the presence of the electronegative atoms such as N,
O, P in the relatively long compounds. Thermodynamic model is an important tool
used for analyzing the corrosion inhibition mechanism of inhibitors and their
adsorption onto the metal surfaces (Fergachi et al., 2018; Santos et al., 2019). Based
on the type of adsorption isotherm that the inhibitor molecule follows for the
adsorption onto the metal surface, values of thermodynamic parameters such as
adsorption equilibrium constant (Kads)
and free energy change of adsorption(ΔGads)
can be calculated and used for predicting the nature of adsorption and
inhibition mechanisms.
1.1
BACKGROUND OF THE STUDY
Corrosion inhibitors are the compounds
that
are usually added in small
amount into an aggressive medium. The presence of these compounds in the medium, will reduce the
corrosivity of the medium significantly. Many authors have written papers on
this subject, and they are available in chemical literature on the varieties of inhibitors which have been developed
according to the metal/environment combinations. The inhibitors are either anodic or cathodic, oxidizing, or non-oxidizing
and organic or inorganic depending on the end use requirement. Majority of the effective inhibitors from this category are toxic or possess a hazard in their use. It would be advantageous
if environment friendly compounds would be used instead. For this
thesis, more environmentally friendly inhibitors will be used in this experiment.
Zinc
ions have long been considered as valuable corrosion inhibitors for carbon
steel in aerated water because of the protection afforded by a cathodic
polarization mechanism. Zinc increases the cathodic polarization and hence
inhibits corrosion of steel. Its action is attributed to precipitation of zinc
hydroxide on the cathodic areas because of locally high pH. Moreover, at the
cathodes, oxygen is reduced with the generation of hydroxyl ions. This zinc can
be added as any zinc containing salt, but most people use either zinc sulphate
or zinc chloride as the source. Zinc
chloride is readily available as a 50% aqueous solution for ease of handling
and safety as well as economy (Thomas,
1991).
The
adverse effect of corrosion
can be seen in daily
life. Corrosion causes accidents in industry, on highways, and in homes. It is wasteful financially, costing
industrialized nations 4-5% of their
gross domestic products (GDP)
annually. A little knowledge
of electrochemistry, material science and corrosion could save nations some
25% of this loss (Hansson, 2011).
Corrosion chemistry is the application of science and art to prevent or control corrosion damage in a safe and economical manner. To
perform this function properly, the corrosion
scientist
must rely on experimental research.
This is because the major aspects of corrosion chemistry are
largely empirical in nature.
Corrosion tests are conducted for several
reasons including:
1.
Establishing corrosion mechanisms.
2.
Defining corrosion resistance of materials
and how to develop new corrosion resistant alloys.
3. Estimating service life
of equipment.
4. Developing
corrosion protection processes.
5. Defining
the critical potential values
for
materials in various environments.
1.2 STATEMENT OF THE PROBLEM
Mild steel and zinc are the major items of
construction of the cisterns in the petrochemical industrial processes
involving storage of acids (often referred to as hold up tanks) before use.
This is a major operation in all industries utilizing these acids. The mild
steel option as a material of choice is because it is cheap and easily obtained
when compared to stainless steel (six times as expensive). Also, in the
transport of these acids from one point to another in the process plant, mild
steel pippings are used because of the cost advantage. These pippings are also
connected to fittings (valves, actuators and strainers) made of aluminium
alloys and some other metallic alloys in some instances. However, they are
prone to the damaging effects of the acid over time as they continuously
interact with the acids. The damaging effects become obvious when the load
carrying capacity of such facilities like shafts or shell thickness become
compromised by reduction in the effective diameter or thickness as a result of
metal loss from corrosive attack. The effective diameter or thickness is unable
to support the tensile, compressive, or radial load and so failure becomes
imminent and sometimes catastrophic.
Over the years, reports of product
spillage in industries have pervaded the tabloids.
Many of such incidents are the direct
effect of corrosion on facilities in service. In 2001, a United States (U.S)
refinery experienced an incident in which one worker was killed, eight injured
and significant offsite environmental impact on the surrounding water body
which led to loss of marine life (CSB, 2002). Subsequent ultrasonic test report
identified progressive corrosion of hydrochloric acid steel storage tank as the
major cause of the incidence. In specific terms it said approximately half of
the corrosion allowance was used up in the large sections of the entire exposed
area of the tank. This meant that the mechanical integrity of the tank was
compromised which led to failure of the tank. A finite elemental analysis of
the tank further revealed that the load carrying capacity of the tank had been
reduced drastically. Additional challenges associated with these failures
include:
(a) Economic
loss due to replacement of corroded equipment resulting in heavy expenditure
and man-hour loss.
(b) Facility
replacement cost,
(c) Litigation
for compensation by affected communities,
(d) Negative
publicity,
(e) Inferred
costs resulting from investigating/monitoring, overhaul and revamping and
(f) Untold
hardship on the populace because of withdrawal of such facilities from use.
It is thus essential that these metallic
structures are protected. Several means are available for preventing or
protecting metallic structures in service. These techniques include: materials
selection, coatings, cathodic/anodic protection and use of corrosion
inhibitors. The use of inhibitors has grown in popularity over the years. There
exist two major types of inhibitors. These are organic and inorganic/chemical
inhibitors.
The inorganic inhibitors contaminate the
environment after use and cause a lot of problems like disposal and destruction
of plant and animal life. Restrictions have been placed on the use of some of
these chemical inhibitors because of their toxic nature (CSPC, 1977). The
organic inhibitors are further sub-divided into synthetic or artificial organic
inhibitors and the green organic inhibitors. Some of the artificial organic
inhibitors are harmful to human existence. The focus of this study, therefore,
is on the utilization and partial substitution of chemical inhibitors with
green inhibitors because it is a sustainable means of arresting corrosion
problems.
Moreover, it is cheap and easily
available. It does not contaminate the environment and disposal is not an issue.
For this reason, litigations are not a common issue of concern. Also, the use
of green inhibitors could prove to be an employer of labour. Once suitable
plant extracts have been identified by appropriate testing in curtailing
corrosion in particular environments by researchers in the field, the
information could be communicated to growers who would in turn produce in
commercial quantities based on agreements with investors.
Due to its economic and ecological
implications and low-level awareness, the problems of metallic degradation
continue to reoccur worldwide. The world corrosion organization on April 24, 2009,
started a campaign of creating awareness for corrosion and the problems
associated with it. It was tagged international corrosion awareness day. The
purpose was to stimulate education and best practices in corrosion control for
socio-economic welfare of society, preservation of resources and protection of
the environment.
1.3
JUSTIFICATION FOR THE RESEARCH
Corrosion is the degradation of the metallic
properties of a metal. It progressively expends limited mineral resources and
the energy utilized in the mining and processing of metals together with that
employed in the production of machinery and infrastructures. Corrosion is known
to affect practically all facet of contemporary development or advancement. Therefore,
the deterrence of corrosion is of foremost commercial and ecological
significance. The world corrosion organization has posited that the annual cost
of corrosion globally is approximately 2.2 trillion US dollars. This represents
more than 3 % of the World’s Gross Domestic Product (GDP) (Koch et al., 2002). In Nigeria, the cost of
corrosion has not yet been surveyed; however, the Central Intelligence Agency
(CIA) world fact book on Nigeria however puts it at an estimated 3.2 billion
USD annually (CIA, 2006). This implies that further increases in corrosion
control measures are required in every area of human life and industry.
From the power sector where energy is
generated and the wastewater treatment plants that purify our water to the
pipelines and storage cisterns that transport our much-needed petroleum
products, corrosion control products are being used extensively. The addition
of an inhibitor to a system is one major technique of controlling corrosion. A
study conducted by the National Association of Corrosion Engineers (NACE) in
2002, showed that the total expenditure on corrosion inhibitors in the United
States increased by 83.3% from about $600 million in 1982 to almost $1.1 billion
in 1998 (NACE, 2002). This shows a fast-growing interest in the use of
inhibitors as a corrosion protection technique. A passive protective film is
formed on the metal when the inhibitor interacts with the metal. Inhibitors
that work this way are normally the types added to vehicle cooling units and
corrosion retarding extracts in protective coatings for metals. Conversely,
most of the inhibitors employed presently are harmful with attendant
undesirable effect on plant and animal life. Now in the industrialized
countries like the U.S, there is mounting demands by lawmakers for the
eradication of heavy metal mixtures and noxious inorganic and organic corrosion
inhibitors, thus making research efforts geared towards the enhancement and
development of efficient and eco-friendly inhibitors crucial. A look at
literature shows that there has been some progress made in the emergence of new
and efficient inhibitors currently (Shveta, and Ashish, 2020).
Metal-based
and anionic inhibitors like potassium, magnesium, lead, chromates, and those
comprising of a mixture of lethal anions (molybdates, benzoates, nitrites,
phosphates, and fluorides) are to a great extent adequate or suitable but not
eco-friendly. Accordingly, a greater part of inhibitors presently utilized in
the metal surface engineering and finishing, chemical, coatings and automobile
industry need substitution by eco-friendly materials. Unfortunately, too, there
are insufficient facts on eco-friendly corrosion inhibitors. Therefore, it is
the desire of this work to come up with corrosion inhibitor compositions
appropriate for use on hydrochloric acid solution on mild steel and zinc metals.
1.4
AIM OF THE RESEARCH
The aim of the present research pertains
to the generation of measurable and testable experimental data towards the
control of corrosion of mild steel and zinc in HCl through the development of environmentally
friendly synthesized Schiff bases as organic corrosion inhibitors.
1.5
SPECIFIC OBJECTIVES OF THE STUDY
The
objectives of the research are:
i.
To evaluate the effects of two Schiff bases on the corrosion of mild
steel and zinc in 0.5 M HCl through weight
loss measurement.
ii.
To determine metal-inhibitor interaction mechanism using some adsorption isotherms
namely, Langmuir, Freundlich, and Temkin.
iii.
To determine the mode of inhibition of the inhibitors using the kinetic parameters.
iv.
To investigate the surface morphology of the metals in the presence and absence
of the inhibitors and
v.
To carry out quantum chemical studies of the corrosion test.
1.6
SCOPE OF THE STUDY
The
scope of this study includes the following:
i.
Laboratory designed experimentation for
weight loss study of mild steel and zinc specimens in hydrochloric acid medium
in the presence of varying concentrations of green inhibitors (two Schiff bases)
at different temperatures.
ii.
Surface characterization of the inhibitors
using Fourier Transform Infra-red spectroscopy (FTIR).
iii.
Quantum chemical studies were also
performed.
1.7
LIMITATION AND DELIMITATION OF THE
STUDY
The experiment carried out shows
that the effectiveness of corrosion inhibitors depends upon factors such
as the electron density of the donor atom in the inhibitor molecules,
molecular geometry and the size of the inhibitor molecule and the solubility/dispersibility
of the inhibitor. The following factors can pose limitation to this as stated
below
·
Use non-corrosive metals, such as stainless
steel or aluminium.
·
State
of the metal surface stays which must be cleaned and dry.
·
Use drying agents.
·
Use a coating or barrier product such as grease,
oil, paint or carbon fibre coating.
·
Lay a layer of backfill, for example limestone,
with underground piping.
For
this reason, this study is not preoccupied with bioaccumulation study, storage
stability, durability of the Schiff base inhibitors and other non-electrochemical
methods. Of course, other non-electrochemical methods like chemical analysis
and X-ray fluorescence for determining corrosion products in process liquor; acoustic
emission with application in the aviation industry; ultrasonic techniques for thickness
measurements to detect metal losses caused by corrosion and hydrogen probes for
obtaining corrosion rate data have proved very useful in corrosion monitoring.
However, computer software studies together with weight or Weight loss
measurements are adequate to carry out the analytical work in this research and
results are valid with enough merit.
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