1. INTRODUCTION:

Aluminium plays an important role in several industrial applications because of its excellent physical and mechanical properties. It shows outstanding corrosion resistance. For this reason, it is used in lithium batteries, where ionic liquid could be employed as electrolytes. Aluminium shows corrosion resistance in neutral aqueous solutions particularly when halides are not present¹. This resistance is due to the formation of a thin, adherent and protective passive film, which naturally or electrochemically formed on the surface^2-4. In acidic and alkaline environment, the solubility of this protective layer increases and aluminium shows uniform corrosion^5-7. The work on the kinetics of corrosion of aluminium particularly in alkaline medium is very scanty. Present investigation deals with corrosion susceptibility of aluminium in alkaline medium. The effect of alkali concentration and temperature was investigated. Further the kinetics of corrosion was studied and activation parameters were reported.

2. EXPERIMENTAL:

Aluminium sheets (Jindal Aluminium Limited, India) were purchased from local market and specimen of 5cm x 5cm were prepared for weight-loss measurements. All the specimen surface were polished with emery papers and washed with water.

The specimen were then immersed in 1.0 M sodium hydroxide solution for 3 minutes to dissolve any oxides that had previously formed in air, then washed with distilled water and alcohol and finally dried. All the specimen were weighed on the analytical balance to an accuracy of 5%. The weight of specimens were noted and then dipped into test solution of potassium hydroxide at 20.0°C. At regular time intervals (up to 60 minutes), the specimen were removed from the test solution and washed with distilled water and alcohol and then dried. Finally the weights of specimen recorded. The differences in weight were noted at each interval and the corrosion rates and specific reaction rates were evaluated.

3. RESULT AND DISCUSSION:

The corrosion of aluminium was studied in various concentrations of potassium hydroxide ranging from 0.5 M to 3.0 M at 20.0°C for an exposure period of one hour. The study was carried out by weight-loss method. It is observed that aluminium get dissolved in different concentrations of potassium hydroxide, it can be seen from the decrease in original weight of the aluminium coupons. The corrosion is due to the presence of air (oxygen), which initiates the corrosion process at the surface of aluminium coupons. The plot of corrosion rate versus concentration of potassium hydroxide show that corrosion rate increases with increase in concentration of potassium hydroxide and exposure time (Fig.1).

Figure 1: Kinetics of corrosion of Aluminium in Potassium Hydroxide: Effect of Concentration.

The corrosion study was also extended to various temperatures ranging from 20.0°C to 50.0°C (Fig. 2). It is observed that increase in weight-loss do take place as the temperature increases from 20.0°C to 50.0°C, which indicates that corrosion of aluminium in potassium hydroxide increases at higher temperature. It is accordingly to the principles of chemical kinetics. It may possible that as temperature increases the protective layer of aluminium oxide on aluminium coupon get destroy very quickly, possibly because of adsorption of OH^‾ ions.

Figure 2: Kinetics of corrosion of Aluminium in Potassium Hydroxide: Effect of Temperature.

The specific reaction rates and half-life time of corrosion of aluminium in potassium hydroxide were calculated using first order rate expression. The specific reaction rate increases with increase in temperature and the corresponding half-life decreases with increase in temperature. This confirms the first order kinetics for the corrosion of aluminium in potassium hydroxide (Table1). Further the activation parameters such as energy of activation, Ea^*, the enthalpy of activation, ∆H^* and entropy of activation, ∆S^* are found to be 28.24 KJ mol^-1, 25.74 KJ mol^-1and - 106.98 J mole⁻¹K⁻¹respectively (Table 2).

Table 1: Kinetics of corrosion of Aluminium in Potassium Hydroxide: Effect of Temperature.

Sr.	Temp.	Temp.	Specific	Half Life	Average E_a
No.	°C	K	Reaction Rate k/10^-3min	t_1/2/10²	(KJ/mole)
1	20	293	1.34	5.172
2	30	303	1.79	3.872	28.2449
3	40	313	2.79	2.484	KJ/mol
4	50	323	3.91	1.772

Table 2: Kinetics of corrosion of Aluminium in Potassium Hydroxide: Activation parameters.

Sr. No.	Activation Parameters
1	Energy of activation, E_a^* (KJ mole⁻¹)	28.24
2	Entropy of activation, ∆S^*(J mole⁻¹K⁻¹)	- 106.98
3	Enthalpy of activation, ∆H^* (KJ mole⁻¹)	25.74

ACKNOWLEDGEMENT:

The authors owe their sincere thanks to Director, Institute of Science, Nagpur, India and Principal, Manoharbhai Patel Institute of Engineering and Technology, Gondia, India for their constant support and encouragement to complete this research work.

REFERENCES:

1. Moore K.L. et al, Corros. Sci., 50, 2008, pp 3233.

2. Alwitt R.S. et al, J. Electrochem. Soc., 129, 1982, pp 711.

3. Simizu K. and Kobayashi K., J. Electrochem. Soc., 132, 1985, pp 1384.

4. Juttner K. et al, Corros. Sci., 29, 1989, pp 279.

5. Metikos-Hurovic M. et al, J. App. Electrochem., 24, 1994.

6. Metikos-Hurovic M. et al, J. App. Electrochem., 32, 2002.

7. Doche M.L. et al, Corros. Sci., 41, 1999, pp 805.

Received on 09.03.2011 Modified on 25.03.2011

Asian J. Research Chem. 4(6): June, 2011; Page 963-964