**3.1 Materials**

378 Corrosion Resistance

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,

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 can be controlled by suitable modifications of the environment which in turn stifle, retard or completely stop the anodic or cathodic reactions or both. This can be achieved by the use of inhibitors (Blustein *et al*., 2005; Emregul *et al*., 2005; Goa *et al*., 2008). Corrosion inhibitors are substances which when added in small concentrations to corrosive media decrease or prevent the reaction of the metal with the media. Inhibitors are added to many systems, *e.g.* cooling systems, refinery units, acids, pipelines, chemicals, oil and gas

A number of corrosion inhibitors have been developed to mitigate aluminium corrosion for the last two decades. A variety of inhibitors have been tested such as chromates, dichromates, molybdates, nitrate, nitrite and sulfate. Their high efficiency/cost ratio has made them standard corrosion inhibitors for a wide range of metals and alloys

production units, boiler and process waters etc. (Raja and Sethuraman, 2009).

applications of aluminium alloy may be more reliable and have long service life.

corrosion inhibitor as an alternative for the existing inhibitors.

**2. Literature review on corrosion resistance** 

(Benthencourt *et al*., 1997).

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 specifications and most applicable alloy used in marine applications.

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

Improvement of Corrosion Resistance of Aluminium Alloy by Natural Products 381

Windows-version 4.9.005 coupled to an Autolab potentiostat connected to a computer. The cell used consists of conventional three electrodes with a platinum wire counter electrode (CE), a working electrode (WE) and a saturated calomel electrode (SCE) as reference to

The WE was in the form of a square cut so that the flat surface was the only surface in the electrode. The exposed area to the test solution was 3.75 cm2. The WE was first immersed in the test solution and after establishing a steady state open circuit potential, the electrochemical measurements were performed. The cell was exposed to air and the measurement was conducted at room temperature (25.0 ± 0.1 ºC). Triplicate experiments were performed in each case of the same conditions to test the validity and reproducibility of the measurements. All procedures for electrochemical measurements were performed in accordance with the Standard Practice for Calculation of Corrosion Rates and Related

Many corrosion phenomena can be explained in terms of electrochemical reactions. Therefore, electrochemical techniques can be used to study these phenomena. Measurements of current-potential relations under carefully controlled conditions can yield information on corrosion rates, coatings and films, pitting tendencies and other important

The corresponding corrosion potential (*E*corr), corrosion current density (*i*corr), anodic Tafel slope (*b*a), cathodic Tafel slope (*b*c) and *CR* for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the *E*corr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic

Anodic and cathodic processes of aluminium corrosion in seawater are dissolution of

4Al 4Al3+ + 12e- (1)

Hence, Al3+ reacts with OH- to form aluminum hydroxide near the aluminium surface as

(2)

4Al + 3O2 + 6H2O 4Al(OH)3 (3)

Information from Electrochemical measurements (ASTM G 102).

aluminium and reduction of dissolved oxygen, respectively, as

3O2 + 6H2O + 12e- 12OH-

which all potentials are referred.

**4. Results and discussion** 

**4.1 Electrochemical measurements** 

hydrogen evolution reaction.

data.

below


Table 1. The chemical-composition of Al-Mg-Si alloy
