Kinetics of zirconium passivation in phosphate solutions

Passivation of zirconium has been investigated by open circuit potential and impedance measurements in phosphate solutions of different pH values and concentrations. The results reveal that the thickening rate of the air-formed oxide film increases with increase of pH upto pH 6, then decreases again and also increases as the phosphate ion concentration increases at pH 2. The slope of the open circuit potential/pH relationship is shifted towards the cathodic direction with thickening of the air-formed film. This is attributed to the high insulating electrochemical properties of zirconium oxide film. Complex plane analysis shows that the reactivity of the zirconium surface is pH dependent. The reactivity has a maximum value at pH 6. Dissolution of the anodic oxide films formed at different pH values is also studied. The anodic oxide film formed at pH 6 dissolves at higher rate than the other films. This may be due to the high defects concentration within this oxides film which is formed at higher formation rate. (author). 3 tabs., 8 figs., 31 refs