ABSTRACT
The
extract of Moringa oleifera leaf in
aqueous 5 M HCl and 1 M HCl was systematically investigated to ascertain its
inhibitory effect on the corrosion of mild steel and its mechanism of
inhibition by gasometric and gravimetric methods. The inhibition
efficiency of Moringa oleifera leaf on the corrosion of mild steel in the
acidic media increases simultaneously
with concentration and decreases with rise in temperature. The higher
activation energy observed in the presence of the extract compared to the blank
is indicative of physical adsorption mechanism. The nature of adsorption of the
extract on the mild steel surface was in conformity with Langmuir
isotherm.
TABLE OF CONTENTS
CHAPTER ONE - INTRODUCTION
1.1
Background of study
1.2
Corrosion
1.3
Cause of Corrosion
1.4
Basic process involved in corrosion
1.5
Forms of Corrosion
1.5.1
Uniform or general corrosion
1.5.2
Pitting corrosion
1.5.3
Erosion corrosion
1.5.4
Stress corrosion cracking
1.5.5
Galvanic or dimetallic corrosion
1.5.6
Hydrogen embrittlement
1.5.7
Crevice corrosion
1.6
Corrosion inhibitors
1.7 Mechanism of action of
corrosion inhibitors
1.8
Classification of inhibitors
1.8.1
Anodic inhibitors
1.8.2
Cathodic inhibitors
1.8.3 Mixed inhibitors
1.8.4
Green corrosion inhibitors
1.9
Moringa
oleifera
CHAPTER TWO - LITERATURE REVIEW
2.1
Inhibitory action of Phyllanthus amarus extract on the
corrosion of mild steel in acidic media
2.2
An interesting and efficient green
corrosion inhibitor for aluminium from extracts of Moringa oleifera in
acidic solution.
2.3
Moringa
Oleifera extract as green corrosion
Inhibitor for zinc in polluted Sodium chloride solutions.
2.4
Inhibitory Action of Phyllanthus Amarus Extracts on the
Corrosion of Mild Steel in Seawater
2.5
Spirulina
platensis - A novel green inhibitor for acid corrosion of mild steel.
2.6
Corrosion inhibitory action of some
plant extracts on the corrosion of mild steel in acidic media
2.7
A green approach: a corrosion
inhibition of mild steel by Adhatoda
vasica plant extract in 0.5 M H2SO4.
2.8 Influence
of Hibiscus esculenta leaves on the
corrosion of stainless steel in acid medium
2.9
Inhibition effect of Tea (Camellia sinensis) extract on the
corrosion of mild steel in dilute sulphuric acid.
2.10
Emilia
sonchifolia Extract as Green Corrosion Inhibitor for Mild Steel in Acid
Medium using Weight Loss Method.
2.11
Inhibition Effect of Reed Leaves Extract
on Steel in Hydrochloric Acid and Sulphuric Acid Solutions.
2.12
Green Approach to Corrosion Inhibition by
Black Pepper Extract in Hydrochloric Acid Solution
2.13
Fennel (Foeniculum vulgare) essential oil as green corrosion Inhibitor of
carbon steel in hydrochloric acid solution
2.14
Pericarp of the fruit of Garcinia mangostana as corrosion
inhibitor for mild steel in hydrochloric acid medium.
2.15
Synergistic and antagonistic effects of anions and Ipomoea invulcrata as green corrosion inhibitor for aluminium dissolution
in acidic medium.
2.16
Caffeic
acid as a green corrosion inhibitor for mild steel.
2.17
Plants as a source of green corrosion
inhibitors: the case of gum exudates from acacia species (A. drepanolobium and A. senegal)
2.18
Improvement Corrosion Resistance of Low
Carbon Steel by Using Natural Corrosion Inhibitor
2.19
Polyalthia
longifolia as a corrosion inhibitor
for mild steel in HCl Solution.
2.20
Corrosion Inhibition of Mild Steel by
Aloes Extract in HCl Solution Medium.
2.21
Inhibition of mild steel corrosion using Juniperus plants as green inhibitior.
CHAPTER THREE - MATERIALS AND
METHODS
3.1
Material preparation
3.2
Collection and drying of plant
materials
3.3
Extraction procedure
3.4
Phytochemical Analysis
3.5 Preparation
of solutions
3.6
Experimental methods
3.6.1
Weight loss method (gravimetric method)
3.4.2
Hydrogen evolution method (gasometric
measurement)
CHAPTER FOUR - RESULTS AND
DISCUSSION
4.1
RESULTS
4.2
DISCUSSION
4.2.1
Effect of time and concentration of
volume of hydrogen evolved
4.2.2
Effect of inhibitor's concentration on
inhibition efficiency
4.2.3
Effect of inhibitor's (plant extract)
concentration on corrosion rate.
4.2.4
Application of adsorption isotherm to
the study
4.2.5
Kinetic and thermodynamic consideration
CHAPTER FIVE - CONCLUSIONS
5.1
Conclusions
5.2
Suggestions for further studies
REFERENCES
Appendix 1: Raw
data's of hydrogen evolved (as taken from the burette) from mild steel in 5 M
HCl solution in the presence and absence of MOL extract at 300C.
Appendix 2: Raw
data's of hydrogen evolved (as taken from the burette) from mild steel in 5 M
HCl solution in the presence and absence of MOL extract at 600C.
Appendix 3: Raw
data's of weight loss measurement of mild steel in 1 M HCl solution in the
presence and absence of MOL extract at 300C for a period of 7days
(measurement made at an interval of 24hrs for 168 hours).
Appendix 4: Hydrogen
evolved (in mlcm-2) from mild steel in 5 M HCl solutions in the
presence and absence of MOL extract at 300C.
Appendix 5: Hydrogen
evolved (in mlcm-2) from mild steel in 5 M HCl solutions in the
presence and absence of MOL extract at 600C.
CHAPTER ONE - INTRODUCTION
1.1 Background of study
Corrosion
of materials has continued to receive interest in the technological world as
its effects on the structural integrity of materials has been a question for
some time. Metallic materials are still the most widely used group of materials
particularly in mechanical engineering and the transportation industry. In
addition, metals are commonly used in electronics and increasingly also in the
construction industry (Buchweishaija, 2009a).
However, the usefulness of metals and
alloys is constrained by one common problem known as corrosion. Hence, it has
been studied comprehensively since the industrial revolution in the late
eighteenth century (Sato, 2012). Corrosion is a naturally occurring phenomenon
defined as deterioration of metal surfaces caused by the reaction with the
surrounding environmental conditions (Buchweishaija, 2009a). Corrosion can
cause disastrous damage to metal and alloy structures causing economic consequences
in terms of repair, replacement, product losses, safety and environmental
pollution. Due to these harmful effects, corrosion is an undesirable phenomenon
that ought to be prevented.
Scientists are persistent in seeking
better and more efficient ways of combating the corrosion of metals. There are
several ways of preventing corrosion and the rates at which it can propagate
with a view of improving the lifetime of metallic and alloy materials
(Buchweishaija, 2009a). Hunag and Chen (2012) highlighted the measures in
preventing and control of corrosion as follows: use of resistant metal alloys,
cathodic and anodic protection, use of protective coatings (Stack, 2002) and addition
of corrosion inhibitors to the corrosion environment (Papavinasam, 2000).
Among the methods of corrosion control,
the use of inhibitors is very popular. It is one of the acceptable practices
used to reduce and/or prevent corrosion due to the ease of application. Mostly
heterocyclic compounds containing oxygen, sulphur and nitrogen as heteroatoms
serve as good inhibitors for corrosion (Kumar et al, 2009). To be effective, an
inhibitor must also transfer water from the metal surface, interact with anodic
and cathodic reaction sites to retard the oxidation and reduction corrosion
reaction, and prevent transportation of water and corrosion-active species on
the metal surface (Maqsood, 2011). Despite these promising findings about
possible corrosion inhibitors, most of these substances are not only expensive
but also toxic and non–biodegradable thus causing corrosion problems (Raja and
Sethuraman, 2008).
The
known hazardous effects of synthetic organic inhibitors, which have been in use
(Popova et al., 2007; Li, et. al., 2009) and the need to develop cheap,
non-toxic and ecofriendly processes have now made researchers to focus on the
use of natural product (Umoren et al.,
2008; Umoren & Ebenso, 2008; El-Etre, 2008). Plants have been recognized as
naturally occurring compounds, some with rather complex molecular structures
and having varying physical, chemical and biological properties (Buchweishaija,
2009a).
The
present work therefore, has been designed to evaluate the effect of the leaf
extracts of Moringa oleifera on the corrosion inhibition of
mild steel in 5M and 1M hydrochloric acid solution with a view to contributing
to the search for further beneficial uses of plant extract. Gravimetric and gasometric
methods were used for the investigation.
1.2 Corrosion
Corrosion
is nature’s method whereby metals and alloys return to their unrefined
naturally occurring forms as minerals and ores (Peter Maaß, 2011). Corrosion is
the deterioration of metals by chemical attack or interaction with its
environment (Acharya et. al., 2013). It can also be defined as the gradual
eating away or disintegration or deterioration of materials by chemical or
electrochemical reaction with its environment (Dara, 2007).
Corrosion
is a constant and continuous problem, often difficult
to eliminate completely. Prevention would be more practical and achievable than
complete elimination. Corrosion processes develop fast after disruption of the
protective barrier and are accompanied by a number of reactions that change the
composition and properties of both the metal surface and the local environment,
for example, formation of oxides, diffusion
of metal cations into the coating matrix, local pH changes, and electrochemical
potential (Rani and Basu, 2011).
1.3 Cause of Corrosion
In
nature, most metals are found in a chemically combined state known as an ore.
All the metals except gold, platinum and silver exist in nature in the form of
their oxides, carbonates, sulphides, sulphates, etc. These combined forms of
the metals represent their thermodynamically stable state (low energy state).
The metals are extracted from these ores after supplying a large amount of
energy.
Metals
in the uncombined condition have a higher energy and are in an unstable state.
It is their natural tendency to go back to the low energy state, that is,
combined state by recombining with the elements present in the environment.
This is the main reason for corrosion (Peter Maaß, 2011).
1.4 Basic process involved in
corrosion
The
basic process of metallic corrosion in aqueous solution consists of the anodic
dissolution of metals and the cathodic reduction of oxidants present in the
solution:
MM
→ M2+ (aq)+ 2e-M----------------- anodic
oxidation (1.1)
2Oxaq
+ 2e- M → 2Red.(e- redox)(aq)-------cathodic
oxidation (1.2)
In
the formulae, MM is the metal in the state of metallic bonding, M2+aq
is the hydrated metal ion in aqueous solution, e-M is the electron
in the metal, Oxaq is an oxidant, 2Red. is a reductant, and e-
redox is the redox electron in the reductant.
The
overall corrosion reaction is then written as follows:
MM
+ 2Ox(aq) → M2+(aq) +2Red (e-
redox)(aq)
---------------------------- (1.3)
These
reactions are charge-transfer processes that occur across the interface between
the metal and the aqueous solution, hence they are dependent on the interfacial
potential that essentially corresponds to what is called the electrode
potential of metals in electrochemistry terms. In physics terms, the electrode
potential represents the energy level of electrons, called the Fermi level, in
an electrode immersed in electrolyte.
For
normal metallic corrosion, in practice, the cathodic process is carried out by
the reduction of hydrogen ions and/or the reduction of oxygen molecules in
aqueous solution. These two cathodic reductions are electron transfer processes
that occur across the metal–solution interface, whereas anodic metal
dissolution is an ion transfer process across the interface (Sato, 2012).
1.5 Forms of Corrosion
Corrosion
can be classified into different categories based on the material, environment
or the morphology of the corrosion damage. According to the environment to
which materials are exposed, there are various forms of corrosion: uniform or
general, pitting, erosion, crevice, stress, galvanic and hydrogen embrittlement.
1.5.1 Uniform or general corrosion
General
corrosion occurs as a result of chemical or electrochemical reactions which
proceeds over the entire exposed surface at about the same rate. General
corrosion results in the metal becoming thinner and usually alters the
appearance of the surface. General corrosion could result in failure through
lowering the mechanical strength of components or by reducing wall thickness
until leaking results (Gray and Luan, 2002).
1.5.2 Pitting corrosion
This
is a localized attack, where some parts of the metal surface are free of
corrosion, but small localized areas are corrode quickly; this occurs when
solid corrosion product or neutralization salts are located on the metal
surface, causing deep holes which is known as pitting, these areas are the most
susceptible to the corrosion process (Marcus et al, 2008).
1.5.3 Erosion corrosion
An
increase in the rate of corrosion as a result of relative motion of the
environment is termed erosion corrosion. This type of corrosion provokes
uniform thinning of the metal surface, which is associated with the exposure to
a high velocity fluid, which causes the corrosion product to be stripped from
the metal surface, resulting in the exposure of the bare metal, which can be
corroded again, causing an accelerated attack. This type of corrosion is
further exacerbated when fluids contain solid particles that are harder than
the metal surface, which hit the metal constantly (Levy, 2002).
1.5.4 Stress corrosion cracking
This
type of corrosion promotes the formation of a fracture in the metal structure
due to mechanical stress and a chemically aggressive medium (Sieradzki and
Newman, 1987).
1.5.5 Galvanic or dimetallic
corrosion
Occurs
when there is a potential difference between dissimilar metals immersed in a
corrosive solution; the potential difference produces a flow of electrons
between the metals, where the less resistant metal is the anode (metal active),
and the most resistant is the cathode (noble metal). This attack can be
extremely destructive, dramatically accelerating the corrosion rate of the most
reactive metal, but the severity degree of galvanic corrosion depends not only
on the potential difference between the two metals, but also on the involved
surface area ratios, (Song et al, 2004) .
1.5.6 Hydrogen embrittlement
This
is associated with the hydrogen atoms that are produced on the metal surface in
an aqueous medium; a reduction reaction occurs when atomic hydrogen penetrates
the metal; the presence of defects allow the interaction between the hydrogen
atoms and the metal, forming molecular hydrogen, which being trapped by the
metal, provides enough pressure to form blisters, resulting in microcracks. This
type of failure occurs mainly in basic media, where there are compounds such as
sulfides and/or cyanides; this corrosion process is also present in plants with
catalytic refining processes. In this kind of corrosion process, some hydrogen
atoms diffuse through steel and become retained, where they recombine with each
other, forming a very strong internal pressure that exceeds the strength of
steel, forming blisters (González et
al,1997).
1.5.7 Crevice corrosion
This
is frequently observed in passivated metals and alloys when they are exposed to
environments that contain halide ions (especially chloride). crevice corrosion
could lead to initiation of cracks that propagate failure through leaking,
mechanical failure and freezing of joints(Gray and Luan, 2002).
1.6 Corrosion inhibitors
A
corrosion inhibitor is a chemical substance which when added in small
concentration to an environment, effectively decreases the corrosion rate
typically a metal or an alloy (Grafen et. al., 2002). In other words, Corrosion inhibitors are substances or mixtures that in low
concentration and in aggressive environment inhibit, prevent or minimize the
corrosion (Obot et. al., 2009). An
efficient inhibitor is compatible with the environment, economical for
application, and produces the desired effect when present in small concentrations.
A corrosion inhibitor can be added to a
fluid such as fuel or lubricant. In this case, the corrosion inhibitor travels
with the fluid, providing protection to the systems in which the fluid moves
through. Commonly, it forms a thin film which prevents reactions between
compounds in the fluid and systems such as pipes. This type of corrosion
inhibitor may be blended into the fluid continuously, or added periodically to
maintain a protective film. Corrosion inhibitors can also be sprayed or painted
on to create a thin layer which will provide protection from corrosion. Many
people do this on a regular basis when they oil locks and hinges to prevent
them from rusting and to keep them moving smoothly. The thin layer of oil acts
as a corrosion inhibitor to prevent oxidation, so that
rusting cannot occur. In order to work effectively, the surface needs to be
clean when the chemical is applied, as otherwise corrosive reactions can take
place underneath the corrosion inhibitor (McMahon and Wallace, 2014).
Once corrosion has already started, a
corrosion inhibitor may be used to slow the rate of damage, depending on the
corrosives involved and the situation. Some corrosion inhibitors will also
remove surface layers of corrosion to help restore a material to its original
finish before depositing a layer of protection. It is a good idea to regularly
inspect systems treated with corrosion inhibitors to confirm that the system is
still protected and to check for signs of corrosion and system failure (McMahon
and Wallace, 2014).
1.7 Mechanism of action of corrosion inhibitors
Generally
the mechanism of the inhibitor is one or more of three that are cited below:
•
The inhibitor is chemically or physically adsorbed on the surface of the metal
and forms a protective thin film with inhibitor effect or by combination
between inhibitor ions and metallic surface;
•
The inhibitor leads a formation of a film by oxide protection of the base
metal;
•
The inhibitor reacts with a potential corrosive component present in aqueous
media and the product is a complex. (Umoren and
Ekanem, 2010; Hong Ju et. al., 2008).
1.8 Classification of inhibitors
1.8.1 Anodic inhibitors
Anodic
inhibitors (also called passivation inhibitors) act by a reducing anodic
reaction, that is, blocks the anode reaction and causes a large shift of the
corrosion potential. This shift forces the metallic surface into the passivation region. In general, the
inhibitors react with the corrosion product, initially formed, resulting in a
cohesive and insoluble film on the metal surface. They are also sometimes
referred to as passivators. Chromates, nitrates, tungstate, molybdates are some
examples of anodic Inhibitors (Roberge, 1999).
1.8.2 Cathodic inhibitors
Cathodic inhibitors act by either slowing
the cathodic reaction itself or selectively precipitating on cathodic areas to
limit the diffusion of reducing species to the surface. These inhibitors have
metal ions able to produce a cathodic reaction due to alkalinity, thus
producing insoluble compounds that precipitate selectively on cathodic sites.
Deposit over the metal a compact and adherent film, restricting the diffusion
of reducible species in these areas. Thus, increasing the impedance of the
surface and the diffusion restriction of the reducible species, that is, the
oxygen diffusion and electrons conductive in these areas. These inhibitors
cause high cathodic inhibition (Talbot, 2000).
1.8.3
Mixed inhibitors
Mixed inhibitors work by reducing both the
cathodic and anodic reactions. They are typically film forming compounds that
cause the formation of precipitates on the surface blocking both anodic and
cathodic sites indirectly.
Hard water that is high in calcium and magnesium is less corrosive than
soft water because of the tendency of the salts in the hard water to
precipitate on the surface of the metal forming a protective film.
The most common inhibitors of this category are the silicates and the
phosphates. Sodium silicate, for example, is used in many domestic water
softeners to prevent the occurrence of rust water. In aerated hot water
systems, sodium silicate protects steel, copper and brass. However, protection
is not always reliable and depends heavily on pH. Phosphates also require oxygen
for effective inhibition. Silicates and phosphates do not afford the degree of
protection provided by chromates and nitrites; however, they are very useful in
situations where non-toxic additives are required (Roberge, 1999).
1.8.4 Green corrosion inhibitors
The
term “green inhibitor” or “eco-friendly inhibitor” refers to the substances
that are biocompatibility in nature, environmentally acceptable, readily
available and renewable source. Due to bio-degradability, ecofriendliness, low
cost and easy availability, the extracts of some common plants based chemicals
and their by-products have been tried as inhibitors for metals under different
environments (Ebenso et al, 2004). Green corrosion inhibitors are biodegradable
and do not contain heavy metals or other toxic compounds (Rani and Basu, 2011).
Green
corrosion inhibitors can be grouped into two categories, namely organic green
inhibitor and inorganic green inhibitors. Molecular structure of inhibitor is
the main factor determining its characteristics. Presence of hetero atom (S, N,
O) with free electron pairs, aromatic rings with delocalized π-electrons, high
molecular weight alkyl chains, substituent group in general improves inhibition
efficiency. It is noticed that organic compounds show higher inhibition
efficiency as compared to inorganic (Acharya et al, 2013).
1.9 Moringa oleifera
Moringa oleifera, also known as the horseradish
tree, is a pan-tropical species that is known by such regional names as
benzolive, drumstick tree, kelor, marango, saijhan, and sajna (Fahey, 2005). It
is the most widely cultivated species of a monogeneric family, the Moringaceae that is native to the
sub-Himalayan tracts of India, Pakistan, Bangladesh and Afghanistan where it is
used in folk medicine, it is now widely distributed all over the world (Lim,
2012). Today it has become naturalized in many locations in the tropics and is
widely cultivated in Africa, Ceylon, Thailand, Burma, Singapore, West Indies,
Sri Lanka, India, Mexico, Malabar, Malaysia and the Philippines (Fahey, 2005).
It is one of the newly discovered vegetable which is gaining wide acceptance in
Nigeria. It is widely grown and cultivated in the northern part of Nigeria
where it is locally called Zogeli (among the Hausa speaking people). Moringa oleifera can be grown in a variety of soil conditions preferring
well-drained sandy or loamy soil that is slightly alkaline (Anjorin et. al.,
2010).
It
is considered one of the world’s most useful trees, as almost every part of the
tree can be used for food, or has some other beneficial properties. The leaves,
especially young shoots, are eaten as greens, in salads, in vegetable curries,
and as pickles. The leaves can be eaten fresh, cooked, or stored as dried
powder for many months without refrigeration, and reportedly without loss of
nutritional value. The leaves are considered to offer great potential for those
who are nutritionally at risk and may be regarded as a protein and calcium
supplement. Moringa have a diverse
range of medicinal uses as an antioxidant, anticarcinogenic, anti-inflammatory,
antispasmodic, diuretic, antiulcer, antibacterial, antifungal and its antinociceptive properties, as well as its wound healing
ability has been demonstrated (Rajangam et al., 2001) . Phytochemical screening
reports have shown that the leaves contain phenolics, tannins, alkaloids,
saponins, flavonoids and steroids (Kasolo, 2010; Bamishaye, et. al., 2011)
which are very important in inhibition of corrosion of metals.
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