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).