ABSTRACT
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.
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 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
CHAPTER
3: MATERIALS AND METHODS 47
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
References
LIST OF
TABLES
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
CHAPTER 1
INTRODUCTION
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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