ABSTRACT

Corrosion of mild steel is a big problem in most of our industries causing huge losses of millions of dollars in maintenance. Synthetic corrosion inhibitors are carcinogenic and not biodegradable hence the urgent need for organic inhibitors.Shaded leaves and pods from African locust beans tree (Parkia biglobosa) constitute agricultural waste which dirty our environment. In investigating ways to tackle these problems, four extracts(from pods, leaves, stembark and root) of African locust beans tree (Parkia biglobosa) were used as corrosion inhibitors on mild steel immersed in 0.5M H2SO4 and 0.5M NaCl. The corrosion experiment is based on weight loss method and during the experiment, time, concentration of the inhibitors and temperature were varied; the results showed that inhibition efficiencies of all the extracts when compared to blanks(controls) increase with concentration of the inhibitors in both media at room temperature.In acidic medium,stembark extract has the highest inhibition efficiency of 86.02%, whereas in salt medium, root extract has maximum corrosion inhibition efficiency of 60.57%. Inhibition efficiencies in acidic and salt media decrease with increase in temperature. Corrosion rates at room temperature in absence and presence of extracts in both media decrease with increase in time. Activation energies of corrosion (Ea) increases with increase in inhibitors concentration in both media. Enthalpy and entropy change of corrosion values ( ie ΔHa and ΔSa) in both media are positive and negative respectively. Langmuir isotherm is found to fit the experimental datas obtained from acid and salt media better than Temkin isotherm.The overwhelmly negative values of Gibb‟s free energy of adsorption (ΔGads) in the media shows the spontaneous nature of the adsorption process and the formation of stable protective films. Scanning electron microscopy (SEM) analysis reveals that corrosion inhibition of mild steel actually took place at the optimum concentrations ( 0.5%v/v ) of the extracts. Mechanical properties investigated are more deteriorated in salt medium than in acidic medium, however, they are both significantly improved at 0.5%v/v stembark extract in acidic medium and 0.5%v/v root extract in salt medium.

TABLE OF CONTENTS

Title Page – – – – – – – – – – ii
Declaration – – – – – – – – – – iii
Certification – – – – – – – – – – iv
Acknowledgements – – – – – – – – – v
Dedication – – – – – – – – – – vi
Abstract – – – – – – – – – – vii
Table of content – – – – – – – – – viii
List of figures – – – – – – – – – – xii
List of tables – – – – – – – – – – xvi
List of plates – – – – – – – – – – xvii
List of appendices – – – – – – – – xviii
Abbreviations and Definitions – – – – – – xxiii
CHAPTER ONE
1.0 INTRODUCTION – – – – – – – – 1
1.1 Background – – – – – – – – – 1
1.2 State of the problem – – – – – – – – 2
1.3 The present research – – – – – – – – 2
1.4 Aims and objectives – – – – – – – – 3
1.5 Significance of the study – – – – – – – 4
1.6 Scope of the research – – – – – – – – 4
CHAPTER TWO
2.0 LITERATURE REVIEW – – – – – – – 6
2.1 Corrosion – – – – – – – – – 6
2.2 Electrochemical corrosion of metals- – – – – – 6
2.2.1 Standard electrode half-cell potentials of metals – – – – 7
2.3 Galvanic cells – – – – – – – – – 8
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2.3.1 Types of galvanic cells – – – – – – – 9
2.4 Corrosion rates – – – – – – – – 10
2.5 Polarization – – – – – – – – – 11
2.6 Passivity – – – – – – – – – 11
2.7 Forms of corrosion – – – – – – – – 12
2.7.1 Uniform or general attack corrosion – – – – – – 12
2.7.2 Galvanic or two metal corrosion – – – – – – 12
2.7.3 Pitting corrosion – – – – – – – – 13
2.7.4 Crevice corrosion – – – – – – – – 13
2.7.5 Intergranular corrosion – – – – – – – 14
2.7.6 Stress corrosion – – – – – – – – 15
2.7.7 Erosion corrosion – – – – – – – – 16
2.7.8 Selective leaching – – – – – – – – 16
2.7.9 Cavitation damage- – – – – – – – – 17
2.7.10 Fretting corrosion- – – – – – – – – 17
2.8 Methods of corrosion control – – – – – – 18
2.8.1 Material selection – – – – – – – – 18
2.8.2 Coatings – – – – – – – – – 19
2.8.3 Inhibitors – – – – – – – – – 20
2.8.4 Cathodic protection – – – – – – – – 22
2.8.5 Design – – – – – – – – – 22
2.9 Locust bean – – – – – – – – – 23
2.10 Mild steel – – – – – – – – – 25
2.10.1 Mechanical properties of carbon steel – – – – – 25
2.10.2 Application of mild steels – – – – – – – 26
2.11 Adsorption on solid surfaces – – – – – – – 27
2.11.1 Types of adsorption – – – – – – – – 27
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2.12 Chemical thermodynamics – – – – – – – 27
2.13 Review of related works – – – – – – – 28
CHAPTER THREE
3.0 MATERIALS AND METHODS – – – – – – 31
3.1 Materials – – – – – – – – – 31
3.2 Methods – – – – – – – – – 33
3.2.1 Elemental composition determination – – – – – 33
3.2.2 Sample preparation – – – – – – – – 34
3.2.3 Surface area determination of tensile sample – – – – 34
3.2.4 Density determination of the sample – – – – – 35
3.2.5 Collection, identification,preparation,physio-chemical analysis and
extraction of phytoconstituents of the plant parts – – – – 35
3.2.6 Quantitative analysis of the plant parts – – – – – 40
3.2.7 Preparation of 0.5M H2SO4 and 0.5M Nacl standard solutions – – 43
3.2.8 Stock solutions of the extracts – – – – – – 44
3.2.9 Preparation of test media – – – – – – – 44
3.2.10 Weight loss method- – – – – – – – 45
3.2.11 Effects of temperature on corrosion inhibition – – – – 45
3.2.12 Adsorption isotherms – – – – – – – 46
3.2.13 Scanning electron microscopy (SEM) – – – – – 48
3.2.14 Mechanical properties determination – – – – – 48
3.2.14.1 Tensile strength test – – – – – – – 48
3.2.14.2 Impact test – – – – – – – – – 48
3.2.14.3 Hardness test – – – – – – – – 49
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CHAPTER FOUR
4.0 RESULTS AND DISCUSSION – – – – – – 50
4.1 Results – – – – – – – – – 50
4.2 Discussion of results – – – – – – – 82
4.2.1. Corrosion rates at room temperature – – – – – 82
4.2.2. Effect of temperature – – – – – – – 82
4.2.3. Effect of concentration – – – – – – – 83
4.2.4. Effect of time – – – – – – – – 84
4.2.5 Adsorption consideration – – – – – – – 84
4.2.6. Scanning electron microscopy – – – – – – 85
4.2.7. Mechanical properties – – – – – – – 85
CHAPTER FIVE
5.0 CONCLUSIONS AND RECOMMENDATIONS – – – 87
5.1 Conclusions – – – – – – – – 87
5.2 Recommendations – – – – – – – – 88
References – – – – – – – – – 89

CHAPTER ONE

1.0 INTRODUCTION
1.1 Background
Mild Steel is one of the most common of all ferrous alloys, and one of the least expensive steels used. It is to be found in almost every product created from metal. It is weldable, very durable (although it rusts), it is relatively hard and is easily annealed. Having less than 2 % carbon, it will magnetize well and being relatively inexpensive, can be used in most projects requiring a lot of steel (properties of mild steel.). In addition, mild steel is used in the fabrication of reaction vessels, boilers, pipelines (as seen in chemical or oil and gas industries), automobile industry. However, under process called corrosion, carbon steel is chemically attacked and gets deteriorated.
Corrosion can be defined in many ways. Some definitions are very narrow and deal with a specific form of corrosion, while others are quite broad and cover many forms of deterioration. The word corrode is derived from the Latin corrodere which means “to gnaw to pieces.” The general definition of corrosion is to eat into or wear away gradually, as if by gnawing. For purposes here, corrosion can be defined as a chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties (ASM International,2000). Estimates of the annual cost of corrosion in the United States of America vary between $8billion and $126billion (Fontana, 2005). For Saudi Aramco‟s five domestic refineries, 36% of maintenance budget was due to corrosion. For gas sweetening plants, it was found that 25% of the maintenance budget was committed to corrosion control. For gas fractionation plants, 17% of the maintenance budget was due to corrosion. For production operations onshore, corrosion is responsible for 28% of maintenance costs. Offshore, corrosion accounts for 60% to70% of maintenance costs (Tems and Al Zahrani, 2006). Thus, corrosion needs to be
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controlled and prevented if the overall efficiencies of the systems and their durabilities are to be enhanced and cost of production reduced by using appropriate corrosion inhibitors.
1.2 Statement of the Problem
Corrosion is indeed a nuisance and it is easily seen in metallic infrastructure or facilities like oil and gas industries, petrochemical industry, bridges, railways. Corrosion causes failures of oil and gas pipe lines, stress corrosion collapse of bridges and the costs of replacement and maintenance is becoming intolerable. It can cause sudden failures in ships and submarines by gradually eating-up the steel bodies and propellers. Storage tankers for storing crude or refined oil can be aggressively weakened by corrosion if they are not adequately protected, and eventually causes oil spillage. Corrosion reduces the durability of galvanized roofing sheets.
Shaded leaves and emptied pods from African locust bean trees constitute agricultural waste which when disposed inappropriately can lead to environmental pollution.
In search for ways to control corrosion, myriad methods have been developed, one of which is the use of corrosion inhibitors. Synthetic (inorganic) inhibitors though proven most of the times to be effective in mitigating corrosion, are often carcinogenic and not biodegradable ( Olasehinde et al, 2013), hence when disposed off to the environment, they upset the ecosystem. Presently, green (organic) inhibitors are fast replacing inorganic onces because they prove to be equally effective in corrosion mitigation and not harmful to man and his environment.
1.3 The Present Research
Four different extracts were obtained from the pods, stembark, leaves and roots of locust bean plant. Then, their effects on mechanical properties and corrosion inhibitions of mild steel in 0.5M sulphuric acid and sodium chloride solutions were examined .
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Sulphuric acid of 0.5M concentration is preferred because it is the optimum concentration of most acids (Yawas, 2005).As for the salt solution, 0.5M is approximately the average concentration of salt (NaCl) in seawaters and oceans (Science daily, 2014).
1.4 Aim and Objectives
The aim of this study is to investigate the mechanical and corrosion behaviours of mild steel using locust beans tree extracts(ie pod, stembark, root and leaves) as corrosion inhibitors in 0.5M H2SO4 and NaCl solutions.
The following are the specific objectives: To carry out the physico-chemical and phyto-chemical analyses of the extracts(pods, leaves, bark and root). To carry out corrosion test on the extracts using gravimetric technique. Examine the effect of the four extracts on the corrosion rate of carbon steel in 0.5M H2SO4 and sodium chloride (NaCl) solutions. To compare the corrosion inhibition efficiencies of the extracts at different concentrations of the extracts and at different temperatures. To compare the photomicrographs of the carbon steel samples in contact with the sulphuric acid and sodium chloride solutions in the absence and presence of inhibitors. To investigate the mechanical properties (tensile strength, hardness and impact strength) of samples in contact with 0.5M sulphuric acid and sodium chloride solutions in the absence and presence of inhibitors. To determine the kinetic and thermodynamic parameters of the inhibitors and corrosion process.
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1.5 Significance of the study
(a) Development of non-toxic and environment friendly organic inhibitors capable of mitigating corrosion of mild steel in 0.5M sulphuric acid medium and 0.5M NaCl solutions.
(b) It has the potential of unlocking the possibilities of making agricultural wastes beneficial to man. For example, the pods of locust bean tree after the sugary pulp along with the seeds are removed, are thrown away as waste most of the time, except on rare occasions when it is used locally for cementing floors in some of our villages, and the shaded leaves also litter our surroundings.
(c) Acids, most especially sulphuric acid solution is used in our industries in processes like descaling, pickling, cleansing of our steel vessels. ( Saratha and Vasudha, 2010; Vijayalakshmi et al, 2011; James and Akaranta,2009). Carbon steel is normally aggressively attacked in acidic medium which would make most of our industries prone to corrosion failures if the corrosion is not mitigated or inhibited.
(d) Salty environment are also aggressive to steels, hence corrosion inhibitors would be required to reduce cost, due to corrosion .
1.6 Scope of the research
The scope of the research are as follows: Obtaining extracts from pods, leaves, stembark and roots of African locust bean tree. Carry out weight loss corrosion experiment, where the extracts would be used as inhibitors to the corrosion of mild steel immersed in 0.5MH2SO4 and 0.5M NaCl media. Study the surface morphologies of mild steel samples after corrosion experiment.
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Investigate the effects of inhibitors on the mechanical properties( tensile strength, impact strength and hardness) of mild steel.
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