STABILITY OF ANODICALLY PASSIVATED AL, AL-CU, AL-6061 AND AL-7075 IN NITRIC-ACID AND NITRIC-ACID SOLUTIONS CONTAINING CHLORIDE

The electrochemical behaviour of anodically passivated aluminium and a luminium alloys (Al-Cu, Al-6061 and Al-7075) has been investigated in nitric acid and nitric acid containing chloride solutions. The effect of chloride ion concentration on the stability of the anodic oxide fil m has been studied. Electrochemical impedance spectroscopy (EIS) and p olarization techniques are employed. The experimental impedance data h ave been fitted to the impedance characteristics of a parallel capacit or/resistor combination. The anodic oxide film formed on Al or Al-6061 approaches perfect dielectric behaviour, whereas that formed on Al-70 75 or AI-Cu deviate from ideal capacitor behaviour. Anodized Al-6061 e lectrodes are found to be the most stable either in nitric acid or nit ric acid containing chloride solutions. The anodic oxide film consists of two layers-a porous, less stable outer layer and a thin, more comp act stable inner one.