Formation and corrosion of anodic oxide films on hafnium in nitric acid solutions

Oxide film formation on hafnium in nitric acid solutions (0.05-10.0M) was investigated by using polarization and impedance measurements. Under open circuit conditions, the oxide film was found to thicken linearly with logarithm of time in 1.0M HNO3 solution while the behaviour was complex in 10.0M HNO3 solution. The steady corrosion potential increases very slowly at lower acid concentrations (≤1.0M), but it increases very rapidly at higher concentrations. The kinetics of oxide formation under galvanostatic control was found to obey the exponential law due to Guntherschulze and Betz. The formation rate and the anodization field increase with increase of the acid concentration. The breakdown voltage was found to increase linearly with logarithm of the acid resistivity and was independent of the anodizing current density. The impedance of the anodized hafnium electrodes was purely capacitive and increases with increase of the film thickness. The hafnium oxide film was subject to corrosion in nitric acid solutions without current and the film thickness was found to decrease linearly with logarithm of time. The corrosion rate increases with increase of the acid concentration and the initial film thickness. (author). 10 figs., 1 tab., 30 re fs