Anodization of hafnium in phosphate baths: radio tracer studies

The kinetics of anodic oxidation of hafnium in different phosphate baths: phosphoric acid and its three sodium salts of 0.1 M concentration have been carried out using galvanostatic technique at 8 mA.cm-2 current density and at room temperature (300 K). The conventional plots V-t, I/C-t, I/C-V are drawn. Thickness estimates are made from capacitance data. The above plots are found to be linear up to 140 ± 20 V. The breakdown voltage remained almost same. The parameters formation rate, current efficiency and differential field are calculated and are found to be decreasing regularly with the increase in the pH of the electrolyte. In order to establish the mechanism of oxide film formation on hafnium in Na3PO4 electrolyte, the uptake and distribution of phosphate ions during formation and thinning of the oxide film are studied using 32PO---4 as radio tracer and a specially designed radio active cell. For thinning experiments 0.7 μ diamond paste is used. The total mechanism is found to be via vacancy diffusion plus some exchange interstitial capture. The active-inactive transformation study clearly shows the migration of phosphate ions from one layer to another. Finally the effect of temperature and current density on the amount of incorporation of ions is also studied and linear relationships are established. (author)