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Corrosion inhibition efficiency of Cassia fistula pod extract on mild in 0.5 M HCl solution was studied by gravimetric (weight loss) and quantum mechanical methods. Thermodynamic parameters such as activation energy, enthalpy, enthropy and Gibbs free energy of adsorption were determined. Results showed that the inhibition efficiency increased significantly by up to 82.9 % and 57.36 % for mild steel and aluminum respectively with increase in concentration of the inhibitor. However, the inhibition efficiency decreased slightly with increasing temperature in the range 303-343 K. This is supported by higher values of Kads (5.59-3.76) for mild steel and (1.10-0.73) for aluminium. It is observed that at lower temperature, there is higher value of Kads indicating that the inhibitor is more efficient at lower temperatures. The kinetic study shows that the inhibitory action follows a pseudo first order kinetics with the concentration of the extract. This was further supported by the thermodynamic parameters which reveal that the adsorption of both the individual seed extracts and their blends onto the metal surface was spontaneous, endothermic and followed physical adsorption mechanism. Cassia fistula was identified to have phytochemicals of phenol, saponins, tannins, alkaloids, terpenoids. These compounds are adsorbed by the surface of the metal, leading to corrosion inhibition. The experimental data fitted best into the Langmuir and Freundlich adsorption model for both mild steel and aluminium respectively at various temperatures studied with linearity coefficient (R2) of . The Gibbs free energy for Langmuir isotherm ranges from -15.230 to -14.453kJ/mol for mild steel and -10.558 to -10.238 kJ/mol for aluminium showing spontaneity and physisorption process. Thermodynamic adsorption consideration revealed that the positive values (31.890 - 35.63 kJ/mol) for mild steel and (19.44-21.018 kJ/mol)  aluminium of enthalpy of activation with the inhibitor concentration (0.2 – 1.0 g/L) shows that the process of adsorption of the inhibitor on the mild steel surface is endothermic and spontaneous (negative values of ΔG°ads) and supported by the mechanism of physisorption (ΔG°adsless negative than -20kJ/mol). The compounds were optimized using density functional theory (DFT) with Becke three Yang Parr (B3LYP) and 6-31G(d) basis set; Quantum chemical studies indicated that inhibition was due to adsorption of active molecules leading to formation of a protective layer on surface of mild steel. Quantum chemical parameters such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, HOMO–LUMO energy gap and electronic density were virtually identified. The calculated electronic properties and global reactivity description agree with experimental findings. The high dipole moment values (>4) and the electronic donating abilities (fraction of transferred electrons 0.8-1.08) of Cassia fistulafurther demonstrate good inhibition efficiency.


Title Page                                                                                                                    i

Declaration                                                                                                                 ii

Certification                                                                                                               iii

Dedication                                                                                                                 iv

Acknowledgements                                                                                                   v

Table of Contents                                                                                                       vi

List of Tables                                                                                                             x

List of Figures                                                                                                             xii

Abstract                                                                                                                      xv


CHAPTER 1: INTRODUCTION                                                                                      1

1.1       Background of Study                                                                                     1

1.2       Statement of the Problem                                                                               5

1.3       Justification of the Study                                                                               6

1.4       Aim of the Study                                                                                            6

1.5       Scope of the Study                                                                                         7


CHAPTER 2: LITERATURE REVIEW                          8

2.2        Corrosion Cells and Reactions                                                                        8

2.3        Factors Responsible for Corrosion                                                                  9

2.4        Types of Corrosion                                                                                         12

2.3.1.   Localized Corrosion                                                                                        12

2.3.2        Uniform surface corrosion                                                                              16

2.4          Corrosion in different media                                                                          18

2.4.1        Corrosion in alkaline solution                                                             18

2.4.2        Corrosion in acid medium                                                                               19

2.4.3        Corrosion in free organic liquid and gases                                                      19

2.4.4        Corrosion induced by bacteria                                                                        19

2.4.5        Corrosion in moist environment                                                                      19

2.5          Consequences of Corrosion                                                                            20

2.5.1        Economic effects                                                                                            20

2.5.2        Health effects                                                                                                 22

2.5.3        Safety effects                                                                                                  22

2.5.4        Technological effects                                                                                      23

2.5.5        Cultural effects                                                                                               23

2.6          Prevention and Control of Corrosion                                                             23

2.6.1        Applied coatings                                                                                             23

2.6.2        Anodization                                                                                                    23

2.6.3        Galvanization                                                                                                  24

2.6.4        Biofilm coatings                                                                                              25

2.7          Corrosion Inhibition and Inhibitors                                                                26

2.8          Types of Inhibitors                                                                                          26

2.8.1        Anodic (Passivating) inhibitors                                                                       29

2.8.2        Cathodic inhibitors                                                                                         29

2.8.3        Organic inhibitors                                                                                           32

2.8.4        Precipitating inhibitors                                                                                    34

2.8.5        Green corrosion inhibitors                                                                               35

2.9          Limitations and Advantages of Plant Extract as Corrosion Inhibitors           39

2.10      Brief History of Cassia fistula (a.k.a Golden shower tree )                            40

2.10.1    Medicinal uses                                                                         42

2.10.2    Other uses                                                          43

2.10.3    Places where cassia fistula can be found                     44

            2.10.4    Cassia fistula as a corrosion inhibitor                                    45


3.1       Materials                                                                                                         47

3.2        Methods                                                          48

            3.2.1        Determination of composition of metal coupons                     48

            3.2.2        Preparation of Cassia fistula seed extracts                                 49

            3.2.3        Preparation of 0.5 M HCl                                              49

            3.2.4        Phyto-chemical analysis                                             50

            3.2.5        Gravimetric techniques                                            53

            3.2.6        Corrosion data                                                              54

            3.2.7        Adsorption isotherm study                                                 55

            3.2.8        Thermodynamic studies                                                             57

            3.2.9        Corrosion adsorption kinetics                                            58

            3.2.10    Computational method                                                       59


CHAPTER 4: RESULTS AND DISCUSSION                                   62

4.1        Phytochemical Analyses                                      62

4.2        Determination of Composition of Metal Coupons            63

4.3        Analyses of FTIR Spectra                                         64

4.4        Effect of Concentration of Acetone Extract of Cassia Fistula Pod on the Corrosion Rate of Metals (Mild Steel and Aluminium) and Inhibition  Efficiency in 0.5 M HCl.                           65

4.5        Effect of Temperature                                  70

4.6        Kinetic Consideration                                            75

4.7        Thermodynamics                                                 78

            4.7.1        Arrhenius plots                                                             78

            4.7.2        Transition state plots                                           80

4.8        Adsorption Considerations                                        83

            4.8.1        Langmuir adsorption isotherm                                              83

            4.8.2        Freundlich adsorption isotherm                                     86

4.9        Quantum Studies                                           89


CHAPTER 5: CONCLUSION AND RECOMMENDATIONS                                     102

5.1       Conclusion                                                                    102

5.2       Recommendations                                                     103






3.1       List of materials used for the experiments                                                      47

3.2       List of equipment for the experiments                                                            48

4.1:      Phytochemical test of extract of Cassia fistula pods                                      62

4.2       Chemical composition of studied metal samples                                            63

4.3:      Surface coverage and inhibition efficiency of Cassia fistula pods

extracts on mild steel metal at varying time                                                   66

4.4:      Surface coverage and inhibition efficiency of Cassia fistula pods

extracts on aluminium metal at varying time                                                  67

4.5:      Surface coverage and inhibition efficiency of Cassia fistula pods extracts

on mild steel metal at varying temperature                                                     72

4.6:      Surface coverage and inhibition efficiency of Cassia fistula pods

extracts on aluminium at varying temperature                                                73

4.7:      Pseudo first order parameters for corrosion inhibition of mild steel by

extract of Cassia fistula pods                                                                          77

4.8:      Pseudo first order parameters for corrosion inhibition of aluminium by

extract of Cassia fistula pods                                                                          77

4.9:      Arrhenius parameters for the corrosion of mild steel and aluminium

in acid containing various concentrations of the studied inhibitor                 79

4.10:    Eyring-Transition state parameters for the corrosion of mild steel

            in acid containing various concentrations of the studied inhibitor                 82

4.11:    Langmuir isotherm parameters for the corrosion of mild steel and

            aluminium in   HCl medium containing various concentrations of the

            studied inhibitor                                                                                              85

4.12:    Freundlich isotherm parameters for the corrosion of mild steel and

aluminium in HCl medium containing various concentrations of the

studied inhibitor                                                                                              88

4.13     Electronic properties and global reactivity descriptors of Fistulic acid,

Catechin and Epicatechin                                                                               90

4.14:    Selected calculated Fukui functions and Mulliken atomic charges of

Fistulic acid                                                                                                     97

4.15     Selected calculated Fukui functions and Mulliken atomic charges of

Catechin                                                                                                          98

4.16     Selected calculated Fukui functions and Mulliken atomic charges of

Epicatechin                                                                                                     99









                                                  LIST OF FIGURES

2.1:      Schematic diagram of pitting corrosion                                                          13

2.2:      A schematic diagram of crevice corrosion                                                      14 

2.3:      A schematic diagram of intergranular corrosion                                             14

2.4:      A schematic diagram of filiform corrosion                                                     15

2.5:      A schematic diagram of uniform corrosion                                                    16

2.6:      A schematic diagram of dtress corrosion                                                        17

2.7:      A potentiostatic polarization diagram of a solution with

electrochemical behaviour of a metal in an anodic inhibitor                           28

2.8:      The mechanism of the anodic inhibitory effect                                              29

2.9:      Potentiostatic polarization diagram                                                                30

2.10:    Theoretical potentiostatic polarization diagram                                              33

2.11:    Illustration of the mechanism of actuation of the organic inhibitor:

acting through adsorption of the inhibitor on the metal surface. Where

the “Inh” represent the inhibitor molecules.                                                   33

2.12:    Pictures of the Cassia fistula tree, pod and pulp                                            41

3.1:      Pictures of separated pulp and pod                                                                  54

3.2:      Pictures of metal immersed in a solution of HCl acid and inhibitior               54

4.1:      FTIR Spectrum of the extract of Cassia fistula pods                                                 64

4.2:      Effect of concentration of Acetone Extract of Cassia fistula pods

            on the corrosion rate of mild steel in 0.5 M HCl                                            65

4.3:     Effect of concentration of acetone extract of Cassia fistula pods on the

           corrosion rate of aluminium in 0.5 M HCl                                                      66

4.4:     Effect of concentration of acetone extract of Cassia fistula pods on

inhibition efficiency on Mild Steel in 0.5M HCl at different contact times 69

4.5:     Effect of concentration of acetone extract of Cassia fistula pods on

inhibition efficiency on Aluminium in 0.5M HCl at different contact times 69

4.6:     Effect of concentration of acetone extract of Cassia fistula pods on

inhibition efficiency on Mild Steel in 0.5M HCl at different temperatures   71

 4.7:     Effect of concentration of acetone extract of Cassia fistula pods on

inhibition efficiency on aluminium in 0.5M HCl at different temperatures   71

4.8:     Pseudo first order plot for the corrosion of mild steel in 0.5M HCl in the

absence and presence of acetone extract of Cassia fistula pods                     75

4.9:     Pseudo first order plot for the corrosion of aluminium in 0.5M HCl

in the absence and presence of acetone extract of Cassia fistula pods           76

4.10:   Arrhenius plots for the corrosion of mild steel in 0.5M HCl containing

various concentrations of Cassia fistula pods extract                                     78

4.11:   Arrhenius plots for the corrosion of aluminium in 0.5M HCl containing

various concentrations of Cassia fistula pods extract                                     79

4.12:   Eyring Transition state plots for the corrosion of mild steel in 0.5

           M HCl containing various concentrations of Cassia fistula pods extract       81

 4.13:   Eyring Transition state plots for the corrosion of aluminium in 0.5

            M HCl containing various concentrations of Cassia fistula pods extract       81

 4.14:   Langmuir isotherm for the adsorption of the inhibitor on mild steel

            surface in 0.5M HCl solution at various temperatures                                    84

 4.15:   Langmuir isotherm for the adsorption of the inhibitor on aluminuim surface

            in 0.5 M HCl solution at various temperatures                                               84

 4.16:   Freundlich isotherm plots for the adsorption of the inhibitor on

            mild steel surface in 0.5 M HCl solution at various temperatures                  87

 4.17:   Freundlich isotherm plots for the adsorption of the inhibitor on

            aluminuim surface in 0.5 M HCl solution at various temperatures                 87

 4.18a: Optimized structure of fistulic acid                                                                92

 4.18b: HOMO map of fistulic acid                                                                            92

 4.18c: LUMO map of fistulic acid                                                                            92

 4.18d: Electrostatic potential map of fistulic acid                                                     93

 4.19a: Optimized structure of catechin                                                                      93

 4.19b: HOMO map of catechin                                                                                 94

 4.19c: LUMO map of catechin                                                                                  94

 4.19d: Electrostatic potential map of catechin                                                          95

 4.20a: Optimized structure of epicatechin                                                                 95

 4.20c: 4.20b: HOMO map of epicatechin                                                                   96

4.20c: LUMO map of epicatechin                                                                              96









            1.1              BACKGROUND OF STUDY

Metals are widely used in human activities due to their excellent mechanical and electrical properties (Ebenso et al., 2008). In order to preserve the desired state of these metals, their preventive maintenance is a priority. Corrosion is probably the most common undesired phenomenon that leads metals to become weaker (Mai et al., 2016; Zhu et al., 2020).

Corrosion can be defined as the destructive attack of a metal by a chemical or electrochemical reaction with its environment (Fontana, 1987; Eddy et al., 2014a).

Corrosion of metals remains a global scientific and industrial problem especially in the metallurgical, fertilizers, chemical, food processing and oil industries. Aviation, for instance, is a capital intensive industry in which the imperatives are flight safety, the protection of investment and uninterrupted operation of aircraft over a long design life. Food handling introduces aspects of public health, biological contributions to corrosion problems, and the mass production of food cans that are low-value corrosion-resistant artifacts constitute to corrosion hazards. Building construction, according to Uppal and Bhatia (2001), has different approaches to corrosion control from which solutions are selected to suit client requirements. In recent industrial history, many failures due to the use of metallic structures in contact with aqueous and non-aqueous media have been reported as a consequence of corrosion (Uppal and Bhatia, 2001).

In the petroleum and gas industries, more than half of the registered failures of pipelines are caused by corrosion and subsequent rupture of the pipe wall (Achebe et al., 2012). The Nigerian National Petroleum Cooperation (NNPC) reported 162 cases of failure due to corrosion between 2002 and 2004 (Adebiyiet al., 2003; Eddy, 2008). Oil pipeline failures in oil and gas industries in the Niger Delta area of Nigeria has been analyzed and showed corrosion as one of the major causes of failure (Achebe et al., 2012).

In natural and industrial environments, Engineering metals are unstable, all except gold are chemically unstable in air and air-saturated water at ambient temperatures and most are unstable in air-free water. In the long run, they inevitably revert to stable chemical species similar to the chemically combined forms from which they were extracted. Because of this, metals are only borrowed from nature for a short time (Talbot and Talbot, 1998).

Engineers have considered a number of factors in selecting materials for structures or piece of equipment. For any metal of choice, its amenability to fabrication requirements of a particular structure or equipment, their physical and chemical properties, corrosion resistance as well as real cost must be considered (Koch et al., 2002). The corrosion behaviour of metals is one of the important factors to be considered when choosing construction materials or engineering items. Materials selected should have the most economical life span which will depend on the intended use. A life of 20 to 100 years may be anticipated in buildings and other structures (Talbot and Talbot, 1998).

Among metals, mild steel is the most widely used in oil, food, energy, chemical and construction industries due to its different applications, most of which are based on its excellent mechanical properties like its high mechanical resistance, durability and toughness among others which makes it a highly available material and at a relatively low cost. Consequently, a solution to problems related to the degradation of mild steel by the corrosion should be a high-priority topic. This degradation can be reduced using corrosion inhibitors (Ladan et al., 2017).

Mild steel is an alloy of iron, containing iron, carbon, manganese, phosphorus and silicon (Kotz and Treichel, 1996). It is fairly ductile and malleable. It can be shaped by hammering and pressing while hot but cannot be hardened by heat treatment (Talbot and Talbot, 1998).

Mild steel is used in making boiler plates, tubes, rivet nuts and bolts. Pipelines are typically made of mild steel, and an oxide film forms on the iron surface under common operating conditions. This oxide film mostly presents a hematite (Fe2O3) structure. When mild steel corrodes, there is usually a loss of the metal to a solution in some form, in an oxidation-reduction reaction.

Since corrosion of metals and alloys pose great danger to the economy of many nations, it has become expedient for countries to invest huge sums of money in controlling the corrosion. The annual global cost of corrosion is $2.5 trillion according to a study by National Association of Corrosion Engineers (NACE) International (Koch et al., 2002). Applying corrosion prevention best practices could result in global saving of 15-35% of that cost, or $375-$875 billion (Koch et al., 2002).

Aluminium is the third most abundant element in the Earth’s crust. It is found in varying amounts in nature as aluminosilicates but can be obtained in pure form by electrolysis. Pure aluminum is weak and loses its strength rapidly above 3000C. To strengthen it, aluminium is therefore alloyed with small amounts of other elements. A more corrosion-resistant alloy of aluminium contains mainly manganese (Ita and Offiong, 1997). This type is used in construction of window frames, furniture, highway signs and cooking utensils.

Aluminium and its alloys are of economic importance because of their low cost, lightness and good corrosion resistance at moderate temperatures (Kotz and Treichel, 1996). Paradoxically, Uppal and Bhatia (2001) observed that aluminum theoretically tends to react with air and water by some of the most energetic chemical reactions known but provided that these media are neither excessively acidic nor alkaline and are free from contaminants, the initial reaction products form a vanishingly thin impervious barrier separating the metal from its environment. The protection afforded by this condition is so effective that aluminum and some of its alloys are standard materials for cooking utensils, food and beverage containers, architectural use, and other applications in which a normally bare metal surface is continuously exposed to air and water. Similar effects are responsible for the utility of some other metals exploited for their corrosion resistance, including zinc, cobalt, titanium, and nickel (Uppal and Bhatia, 2001).

Generally, increased corrosion-resistance can only be obtained at increased cost. However, the actual material-related costs incurred in a project will depend on the corrosivity of the environment concerned, the required design life, the physical requirements of the material, and the readily available stocks. The costs and problems associated with corrosive-resistant materials means that, in many cases, the use of corrosion inhibitors is a practical and economic alternative. In industry, use of corrosion inhibitors is therefore now broad based and extensive.

The use of inhibitors is one method of corrosion prevention among others such as cathodic protection, anodic protection and coating (Afolabi, 2007). The substances include phosphates, chromates, dichromates, silicates, bromates, arsenates, tungstates, molybdates, chlorides and their likes. These inorganic inhibitors exhibit toxic effects and are therefore not environmentally friendly (Afolabi,  2007).

Chromate based pigments have been used for many years as anti-corrosive pigments in wash primers and other alkyl and epoxy primers. In recent years, chromium and especially chromates (hexavalent chromium), have been found to cause irritation of the respiratory tract, produce ulcerations and perforations of the nasal septum, and  produce lung cancer in workers employed in chromium manufacturing plants in west Germany and the United States (Tchounwou et al., 2012). Due to its toxicity, chromates constitute a hazard and need to be replaced by more environmentally acceptable corrosion inhibitors. In this sense, a system containing tannins, a class of natural, non-toxic, biodegradable organic compounds has been proposed (Jiang et al., 2017). The use of common inhibitors is sometimes limited, since these are based on dangerous substances for human health, such as chromium-based inhibitors (Jiang et al., 2017). Recent approaches take advantage of organic compounds that can be obtained from expired pharmaceutical drugs, mushroom extracts, and even plant extracts (El-Hadded et al., 2019; Farahati et al., 2020; Espinoza-Vazquez et al., 2020). These extracts replace the toxic corrosion inhibitors.

Natural extracts have been widely used to protect metal materials from corrosion. The efficiency of these extracts as corrosion inhibitors is commonly evaluated using gravimetric method, computational/theoretical methods among others.

An extract is a solution composed by active constituents of a plant or its parts and a certain medium acting as solvent. These active ingredients (Phytochemicals) contain heteroatoms which are responsible for the inhibition of corrosion in metals. Phytochemicals are chemical compounds (alkaloids, saponins, tannins among others) formed during the plants normal metabolic processes (Okigbo et al, 2009).

This work provides a wide landscape of the use of Cassia fistula pod extract as corrosion inhibitors in mild steel and aluminium. Basic aspects of the extraction methods, characterization, and theoretical modeling and adsorption mechanisms are also discussed.

            1.2              STATEMENT OF THE PROBLEM

Mild steel and aluminium are valuable metals in the petroleum, fertilizer, metallurgical, food, household and other industries. Most times these metals come in contact with acids, bases and salts, thus exposing the metal to corrosion attack.

Corrosion has remained a worldwide industrial problem which has led to low durability of metals, contamination of industrial products and has resulted in low industrial productivity. Several kinds of corrosion inhibitors have been used to tackle this problem such as organic inhibitors, inorganic inhibitors, volatile inhibitors, green inhibitors among others. However, green inhibitors are more advantageous over the rest because, they are cheap, accessible, environmental friendly and after its application, its by-products are biodegradable. The other types of inhibitors are expensive, having by-products which are non-biodegradable and are toxic to the human health.

Hence, the present study focuses on tackling the problem of corrosion using a green inhibitor acetone extract of Cassia fistula pods.

            1.3              JUSTIFICATIONOF STUDY

Literature review has shown that little research has been done on Cassia fistula in the food industries, pharmaceutical industry and chemical industries. The young leaves, flower buds, pulp of the pod are edible. The root, bark, leaves and fruit pulp have laxative properties but in lesser extent. Powdered seeds are used in treatment of amoebiasis and bark extracts against inflammation. Water extract of the leaves has antifungal activity against human pathogens. The pods are used as anti-malaria,againstblood poisoning, anthrax, dysentery and diabetes.The phytoconstituents of these pods have not been fully documented and its corrosion inhibiting property, hence, it is necessary to study the Corrosion inhibiting property of Cassia fistula pods.

            1.4              AIM AND OBJECTIVES

The aim of this study is to inhibit corrosion of mild steel and aluminum using acetone extracts of Cassia fistula in HCl medium.

To achieve this aim, the following objectives have been outlined:

1.      To investigate the inhibiting effect of green extracts of on mild steel and aluminum metals

2.      To investigate the effect of temperature on the inhibition process.

3.      To study adsorption characteristics of the inhibitors by fitting adsorption data into different adsorption isotherms.

4.      To study the kinetic properties of the inhibition reaction

5.      To evaluate the relationship between the molecular structures of the plants extract and their inhibition efficiencies using Fourier transform infrared spectroscopic (FTIR) analysis.

6.      To evaluate the quantum mechanical consideration using a computational chemistry software.


            1.5              SCOPE

Corrosion inhibition of mild steel and aluminum in HCl will be independently investigated at different temperatures, different contact time and at different concentration. Acetone extracts of Cassia fistula will be used for corrosion inhibition tests.

The experimental aspect of the study was carried out using gravimetric method and surface characterization of the extract analyzed using Fourier Transform Infrared Spectroscopy (FTIR). Quantum Studies were performed.


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