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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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