**1. Introduction**

376 Corrosion Resistance

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Protection of metals from ever progressing corrosion presents one of the topical issues of this century. The increasing industrialization of our life is accompanied with the evergrowing number of metals that corrode and become devalued. Corrosion is a chemical or electrochemical reaction process against certain material, usually metal and its environment which produce the deterioration of the material and its properties. The corrosion reaction produces a less desirable material from the original metal and resulted in the reduced function of a component or system, a significant problem encountered everyday.

Corrosion is a problem that impacts every industry. The serious consequences of the corrosion process have become a problem of worldwide significance. It is estimated that annual loss and damage due to corrosion in the United Kingdom costs about £5000 million; and approximately one tone of steel is lost through corrosion every 90 seconds. Further, it is estimated that 25% of this loss could be avoided by correct design, correct material selection and proper preventive processes (Barbara and Robert, 2006).

Even with the proper application of available countermeasures, the estimated cost by National Association of Corrosion Engineers (NACE) for replacing corroded piping systems in the United States alone stands well in excess of \$70 billion annually, which was 4.2% of the gross national product (GNP)-making corrosion one of the most potentially damaging losses to any commercial, private, or industrial property (Barbara and Robert, 2006).

Even though the term of corrosion is usually applied to metals; all materials including ceramics, plastics, rubber and wood deteriorate at the surface to some extent as a result of being exposed to certain combinations of liquids and/or gases. Few practical examples are the rusting of tools and automobiles over many years of use; the failure of pipelines delivering volatile components such as natural gases and environmentally harmful chemicals such as crude oil and hydrochloric acid; bridge failure, ship failure (due to pumps, fuel tanks, boiler and sensors) and aircraft crashes; for example, Aloha Airlines flight 737 jet landing gear failure in 1988 (Radia, 2004). Therefore, the importance of understanding corrosion is clear, especially in the analysis and the design systems that incorporate metal as a major component material which exposed to corrosive environments.

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 379

Although chromates, dichromates, molybdates, nitrate, nitrite and sulfate were found to be the effective inhibitor for the corrosion processes taking place at the electrode/electrolyte interface of aluminium and some of its alloys in acidic and basic solutions (El-Sobki *et al*., 1981; Kassab *et al*., 1987; Badawy *et al*., 1999), unluckily a major disadvantage is their toxicity and such as their use has come under severe criticism (Bethencourt *et al*., 1997; Song-mei *et* 

In recent days many alternative eco-friendly corrosion inhibitors have been developed, the range from rare earth elements (Neil and Garrard, 1994; Mishra and Balasubramaniam, 2007), and inorganic (Salem *et al*., 1978) to organic compounds (Onal and Aksut, 2000; Branzoi *et al*., 2002; Maayta and Al-Rawashdeh, 2004). Owing to the growing interest and attention of the world towards environmental problems and towards the protection of environment and the hazardous effects of the use of chemicals on ecological balance, the traditional approach on the choice of corrosion inhibitors has gradually changed. Researches

El-Etre and Abdallah (2000) study the natural honey as corrosion inhibitor for carbon steel in high saline water. It was found that natural honey exhibited a very good performance as inhibitor for steel corrosion in high saline water. The effect of fungi on the inhibition efficiency of natural honey is markedly decreased in high saline water. This is due to the high concentration of NaCl that retard the growth of fungi. This finding attracts the author to carry out further investigation on the effect of natural honey in seawater which contains

One of the aromatic groups that showed good inhibitive effect is vanillin. The inhibition effects of vanillin on the corrosion of steel in HCl and H2SO4 solutions were investigated by Emregul and Hayvali (2002) and Li *et al.* (2008), meanwhile El-Etre (2001) studied the effect of vanillin against acid corrosion of aluminium. They were explored that an aromatic aldehyde containing carbonyl, methoxy and hydroxyl groups arranged around the aromatic ring in vanillin contributed to the inhibition mechanism process. Lack of research on the effect of this inhibitor on the corrosion of aluminium alloy in seawater has motivated the author to explore this research area as contribution to the current interest on environmental-

The material employed was Al-Mg-Si alloy (AA6061). The chemical composition (weight %) of Al-Mg-Si is listed in Table 1 and the validity of composition was determined by EDS. Extruded shape of Al-Mg-Si alloy was selected in this study because of its well-proven medium strength structural alloy that satisfies the requirements of a number of

The samples were cut into 25 x 25 x 3 mm coupons and mechanically polished using #400, 500 and 600 silicone carbide emery papers (ASTM G 1) and lubricated using distilled water. The polished samples were cleaned with acetone (Merck, 99.8% purity) washed using

specifications and most applicable alloy used in marine applications.

distilled water, dried in air and stored in moisture-free desiccators prior to use.

are mainly focusing on non-toxic ''green" corrosion.

friendly and green corrosion inhibitors.

**3. Research methodology** 

**3.1 Materials** 

*al*., 2007).

NaCl.

Although much progress has been made in understanding the thermodynamics and kinetics of the corrosion process, the mechanisms of localized corrosion are not well understood, nor are those for imparting resistance or protection against aqueous or gases corrosion. With knowledge of the types of and a better understanding of the mechanism and causes of corrosion, it is possible to make measures to prevent them from occurring. For examples, we may change the nature of the environment by selecting a material that is relatively non reactive and/or protect the material from corrosion. Controlling corrosion in the infrastructure can prevent premature failure and lengthen useful service life, both of which save money and natural resources, promote public safety and protect the environment.

Due to the various industrial applications and economic importance of aluminium and its alloys, its protection against corrosion has attracted much attention (Aballe *et al*., 2001; Cheng *et al*., 2004; Hintze and Calle, 2006). Most aluminium alloys have good corrosion resistance towards natural atmospheres and other environments, because aluminium alloy surfaces are covered with a natural oxide film of thickness about 5 nm (Klickic *et al*., 2000). However, in the presence of aggressive ions, like chloride, the protective layer can be locally destroyed and corrosive attack takes place (Kliskic *et al*., 2000). Yet, if correctly protected, applications of aluminium alloy may be more reliable and have long service life.

One of the methods to protect metals or alloys against corrosion is addition of species to the solution in contact with the surface in order to inhibit the corrosion reaction and reduce the corrosion rate (Trabenelli *et al*., 2005) known as corrosion inhibitor. A number of corrosion inhibitors for aluminium alloys have been developed for this purpose such as lanthanide chloride, tolytriazole, bitter leaf, Schiff base compounds and polyacrylic acid (Benthencourt *et al*., 1997; Onal and Aksut, 2000; Avwiri and Igho, 2003; Yurt *et al*., 2006; Amin *et al*., 2009).

Owing to the growing interest and attention of the world towards environmental problems and towards the protection of environment and the hazardous effects of the use of chemicals on ecological balance, the traditional approach on the choice of corrosion inhibitors has gradually changed. Researches are mainly focusing on non-toxic ''green" corrosion inhibitors. Therefore, there is a great task to search for suitable natural source to be used as corrosion inhibitor as an alternative for the existing inhibitors.
