ABSTRACT

The potentials of African Star Apple (Chrysophyllum albidium; (CA) and Cock’s Comb (Heliotropium indicum;(HI)as corrosion inhibitorsfor medium carbon low alloy steel in 1.0M H2SO4 and HCl acidic media were evaluated and compared with a known conventional standard inhibitor (Shell Ensis SX; (SE)).The plant extracts were first characterized to determine their phytochemical constituents; saponins, tannins, alkaloids and flavonoids. These constituents were found to be present. Proximate analysis and physicochemical parameters such as pH, colour, viscosity and solubility of the extracts were also evaluated. The rheological and biodegradability of plant extracts were studied, Chrysophyllum indicum showed a shear-thickening behaviour while Heliotropium indicum gave a shear-thinning behaviour, rapid biodegradation occurs in the formulated plant extracts during Organization for Economic, Cooperation and Development (OECD) marine Biochemical Oxygen Demand (BOD) tests. Ultraviolent-visible (UV-Vis) and Fourier Transform Infra-Red (FTIR) spectrometry were used to characterize the bioactive compounds present in the leave extracts. Corrosion inhibition efficiencies and ability of the plant extracts were studied by weight loss and Linear Resistance Polarization (LPR) techniques. Solutions of concentrations ranging from 100 to 500part(s) per million(ppm) of formulated and standard inhibitors were prepared. Results showedthat inhibitionefficiencies (IE %) increasedwith increase in concentration of inhibitors for gravimetric-based mass loss and electrochemical methods. For gravimetric based mass loss studies, rate of corrosion decreased progressively with increase in inhibitor concentration and exposure time. Potentiodynamic polarization curves revealed that the studied inhibitors function as mixed–type inhibitors withinhibition efficiency values of 98 % (CA) and 96% (HI and SE) respectively at concentration of 500ppm of inhibitors. Kinectic and thermodynamic studies showedthat inhibitionmechanism occurs due to the adsorption of the inhibitor molecules onto the steel surface is an endothermic process; both physical and chemical takes place on metal surface and best described by the Langmuir isotherm. Scanning Electron Microscope (SEM) analysis after corrosion test showed the appearance of smooth surfaces for inhibited solutions, which may be due to the absorption of inhibitor molecules onto the metal surface that serves as a protective layer. Mechanical properties decreased drastically for uninhibited samples but increased with application of inhibitors.

TABLE OF CONTENTS

Title Page ii
Declaration Page iii
Certification Page iv
Dedication Page v
Acknowledgements vi
Abstract viii
Table of contents ix
List of Tables xiii
List of Figures xiv
List of Plates xviii
List of Appendices xix
CHAPTER ONE
1.0 INTORDUCTION 1
1.1 Background of Study 1
1.2 Statement of the Research Problem 3
1.3 The Present Research 4
1.4 Aim and Objectives of the Research 5
1.5 Justification for the Research 5
1.6 Scope of Study 6
1.7 Contribution to Knowledge 7
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CHAPTER TWO
2.0 LITERATURE REVIEW 8
2.1 Corrosion 8
2.1.1 The nature of metallic corrosion 8
2.1.2 Parameters influencing corrosion rate 12
2.1.3 Effects of corrosion 15
2.2 Forms of Corrosion 15
2.2.1 General corrosion 17
2.2.2 Localized corrosion 18
2.2.3 Environmentally Assisted Cracking 20
2.3 Material Selection to Ensure Corrosion Resistance 25
2.4 Impact of Corrosive Environment 25
2.5 Important of Corrosion Testing and Information 26
2.6 Corrosion Testing and Control Methods 27
2.6.1 Cathodic protection 28
2.6.2 Anodic protection 29
2.6.3 Sacrificial Anodes 29
2.6.4 Surface treatments 30
2.6.5 Corrosion inhibitors 31
2.7 Natural Products as Corrosion Inhibitors for Metals 35
2.8 Recent Advances on Natural Products as Corrosion Inhibitors for Metals 37
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CHAPTER THREE
3.0 MATERIALS AND METHODS 41
3.1 Materials 41
3.2 Methods 42
3.2.1 Design of Experiment 42
3.2.2 Chemical Analysis of As-received Low Alloy steel 43
3.2.3 Microstructural Examination of As-received Low Alloy Steel 43
3.2.4 Plant collection and Identification 43
3.2.5 Biochemical Extraction 44
3.2.6 Phytochemical Screening 45
3.2.7 physico-chemical Analysis 48
3.2.8 ProximateAnalysis 51
3.2.9 UV and FTIR Analysis of Plant Extracts 53
3.2.10 Preparation of Low Alloy steel for Corrosion Coupon 53
3.2.11 Preparation of Test Solution 54
3.2.12 Corrosion Tests 54
3.2.13 SEM Analysis 58
3.2.14 Determination of Mechanical Properties 57
CHAPTER FOUR
4.0 RESULTS AND DISSCUSION 59
4.1 Chemical analysis of As-received Low Alloy steel 59
4.2 Mechanical Properties and Microstructural Examination 60
4.3 Biochemical Extraction 61
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4.4 Phytochemical Screening 62
4.5 Physicochemical Properties 63
4.6 Biodegradability Test 66
4.7 Proximate Analysis 68
4.8 FTIR Analysis 69
4.9 Corrosion Tests 72
4.9.1 Gravimetric Based Mass Loss Method 72
4.9.2 Absorption Isotherm and Thermodynamic Consideration 93
4.9.3 Linear Polarization Resistance Method 98
4.10 SEM Analysis 105
4.11 Mechanical Properties Test 107
CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION 110
5.1 Conclusion 110
5.2 Recommendation 111
REFERENCES 112
APPENDICES 123
xiii

CHAPTER ONE

1.0 INTRODUCTION
1.1 Background of Study
Steel of different grades are extensively used in numerous applications, such as oil and gas pipelines, offshore structures, and industrial equipment (Lebriniet al., 2011). Also, they find extensive use for storage tanks and farm machinery system in whichthey are expected to provide specific desirable combinations of better properties such as strength, toughness, formability, weldability and atmospheric corrosion resistance than carbon steel. Stainless steels may be the best choice but are limited by cost constraints. Cost considerations may lead to pruning down to other acceptable material types with built-in safety net with regards to having plans for maintenance (repair or replacement).
Corrosion causes gradual decay and deterioration in metals when exposed to the action of fluids in industrial processes.Among the effective methods of combating corrosion, the use of inhibitors is one of the best and widely used methods for steels in the oil and gas industry and has proven to be very successful (Ebenso, 2011).
The safety and environmental issues in the use of corrosion inhibitors have always been of global concern. Safety is an important consideration in selecting any material in order to meet stringent requirements for engineering application among otherproperties such as mechanical, relative weight, reliability, cost and availability. Notwithstanding the importance of theenvironmental impact of corrosion as a problem which has been a motivation over the years for researchers to
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focus on the use of corrosion inhibitors from the eco-friendliness, biodegradability and renewability point of view.
More recently, the practice of corrosion inhibition is greatly influenced by legislations developed such as the US Environmental Protection Agency (USEPA, 1996) and Organization for Economic Cooperation and Development (0ECD, 1984; 2004)as a result of chronic hazardous effects of some compounds, especially those that contain hexavalent chromium ions (Revie and Uhlig, 2008; Sangeetha et al., 2011). The injection of film-forming corrosion inhibitors is a common practice to protect carbon steel against internal and external corrosion, plant extracts possess potentialsas corrosion inhibitors, thereby the trend in replacing some common hazardous inhibitors with inhibiting action of bio-extracts to form a protective barrier between the steel surface and the corrosive species in the fluids, irrespective of the corrosive environment(Sangeetha et al., 2011; Amithaet al., 2012).
Most of the inorganic inhibitors are synthetic in nature which pose chemical treat to the environment thereby limiting their usages. Moreover, bio-extract by simple procedure with presence of constituents like tannins, flavonoid, terpenoid, alkanoids and saponins are obtainable from plants extracts, a cost effective, eco-friendly, and biodegradable renewable sources of materials (Yin Jin, 2004; Omotoyinboet al., 2013). The use of plant extract as inhibitors is gaining research trend due to their mode of inhibiting action and economically friendly(Amithaet al., 2012).
Organic compounds containing nitrogen, oxygen, phosphorous and sulphur are the widely used inhibitors (Yaro et al., 2010). Corrosion inhibition by organic compounds takes effect generally by mechanism of absorption of molecules and ions at metal surface. Inhibitors retard corrosion
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either by physisortion (electrostatic) or chemisortion (charge transfer) depending on their chemical structure and nature. However, the chemical composition and structure of various plant extracts used as inhibitors is another significant factor influencing surface-active principles when exposed under the action of fluids in various service conditions(Sangeetha et al., 2011).
1.2 Statement of the Research Problem
Studies have that showninhibitors play a vital role in combating metallic corrosion, especially in acidic media but those that are non-toxic or have low toxicity are the widely used ones (Nnannaet al., 2012). One of the main draw backs with most of synthetic type organic inhibitors is their severe environmental hazards such as toxicity, expensive to synthesize, non-biodegradation and bioaccumulation. Hence, these deficiencies have prompted the search for their replacement.
Recently, researches in corrosion control and prevention are concentrated withthe development of “green corrosion inhibitors”. These are compounds with good inhibition efficiency and low risk of environmental pollution. Green corrosion inhibitors are biodegradable and do not contain heavy metals or toxic compounds.Green approaches to corrosion mitigation entail the use of naturally occurring compounds in a sustainable way, while protecting the environment (Amithaet al., 2012). Therefore, there is the need to put more efforts.
Most countries in West Africa especially Nigeria are endowed with forests and luxuriant pastures, among which grow many plant origin with potential bioactive properties for green corrosion inhibitors. Plants are available in nature and considered as the most natural of all the other natural things and, therefore attracted the attention of scientific community due to environmental benefits.
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Over the years, numerous classes of organic compounds have been investigated as corrosion inhibitors.Chrysophyllum albidium and Heliotropium indicum are one of the most popular plants, their natural occurrence have been reported in diverse zones in Nigeria but has not been maximally utilized.Although, Chrysophyllum albidium (African star apple) is a forest tree widely known for its fleshy pulp of the fruits while the popularity of Heliotropium indicum as a vegetable is due to the distinct, pleasant aroma it releases when used in cooking soups in northern Nigeria. They are known to possess several antioxidative capacities and bioactive compounds (Akaneme, 2008; Jyoti and Sharma, 2014).
Chrysophyllum albidiumand Heliotropium indicumplants are of interest, because of theiravailability,more importantly relatively less expensive andenvironmentally friendlyamidst other plants of biodegradable and renewable sources. Most of the compounds extracted from these plants are been used in traditional medicinal applications such as aromatic spices, medical plants and herbs (malaria therapy and pharmaceutical drug development). However, the use of their extracts as corrosion inhibitors to best of my knowledge has not been tested.
1.3 The Present Research
In spite of great number of bio-extracts assessment as corrosion inhibitors carried out and reported in the literature, researchers have continued to identify more bio-extract as inhibitors for specific applications (Yaro et al., 2010). However, no work so far carried out onChrysophyllum albidiumandHeliotropium indicum leaves extract using electrochemical studies for low alloy steel inhibition, and coupled with the fact it readily available within the proximity of this research work.Even then, there is need for further research to be done in this direction, notwithstandingthe development of effective corrosion inhibitor which is the trust for this study.
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The present research focused on the use ofChrysophyllum albidiumandHeliotropium indicum leaves extract as organic inhibitors for medium carbon low alloy steel in 1M HCl and 1M H2SO4environments.
1.4 Aim and Objectives of the Research
The aim of this research is to assess the suitability of utilizing extracts of Chrysophyllum albidiumandHeliotropium indicum leavesas corrosion inhibitors of medium carbon low alloy steel in 1M HCl and 1M H2SO4environments.
While the specific objectives of the research are to:
(i) Assess the biodegradability limits of formulated and standard Shell Ensis SX inhibitors using OECD Marine BOD model.
(ii) Carry out corrosion tests on medium carbonlow alloy steel using gravimetric basedmass loss and linear resistance polarization techniques in the presence and absence of the formulated and standard inhibitors.
(iii) Carry out mechanical tests and microstructural examination on the low alloy steel before and after corrosion test.
1.5 Justification for the Research
The problem of corrosion has been a subject of concern for several years and this happens to be one of the major problems facing many industries in the world today; ranging from the petroleum industries to mining industries and even to our everyday life. Several countries and industries all over the world have spent and allocated huge sums of money into researches aimed at combating this menace.
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Also, environmental protection has become a major consideration in the petrochemical industry which has led to reduction in the use of inorganic and/or synthetic inhibitor.Asides, the use of agro-based organic corrosion inhibitors to protect structures in services had been reported to have little or no adverse effects on the environment, as well as reduce the rate of corrosion in practice such as the oil well acidizing and offshore drilling operations.
1.6 Scope of the Study
The scope of the work encompasses the followings:
(i) Elemental analysis of as-received steel sample by Emission Optical Spectrometer
(ii) Extraction of compounds from ground Chrysophyllum albidiumandHeliotropium indicum leaves using the Soxhlet extractor.
(iii)Corrosion inhibition tests by gravimetric based mass loss and linear polarization resistance methods.
(iv) Chemical analysis of the formulated inhibitor by Fourier Transform Infrared (FTIR) and Ultraviolent (UV) Spectroscopy.
(v) Biodegradability tests of Chrysophyllum albidium, Heliotropium indicum and Shell Ensis SX inhibitors by OECD Marine BOD Model for 28 days.
(vi) Scanning Electron Microscope (SEM) analysis of medium carbon low alloy before and after corrosion test.
(vii) Mechanical tests such as hardness, impact and tensile strength of the steel sample before and after the corrosion test.
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1.7 Contribution to knowledge
This work has a number of significant contributions to the field of corrosion science and engineering in corrosion control and prevention. Among these contributions have implications for researchers, practitioners and educators.
(1) The primary goal of this research is the development of organic corrosion inhibitors for low alloy steel in acidic environment from the extracts of Chrysophyllum albidium and Heliotropium indicum leaves. Organic inhibitors were successfully formulated and tested.
(2) Formulated inhibitors from extracts of Chrysophyllum albidium and Heliotropium indicum leaves has greater tendency of degradation far more than that of Shell Ensis SX inhibitor formulation thereby does not show clearly duration over which it will persist in the environment based on the results obtained from the OECD marine BOD model studies in this research.
(3) Extracts ofChrysophyllum albidium and Heliotropium indicum leaves will serve as effective and eco-friendly inhibitors for medium carbon low alloy in acidic media with inhibition efficiencies percent upto 98.2 and 96.0 respective, thereby complete favourably with the value of 96.2 obtained for Shell Ensis SX standard inhibitor.
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