Dissolution rates, electrochemical and passivation properties of alpha ZrO solid solutions in HF

Abstract

Studies were conducted into the mechanism of dissolution in hydrofluoric acid of alpha solid solution specimens of oxygen (up to 7·01 per cent by weight) in zirconium. They show that the electrochemical and corrosion properties of the metal are significantly influenced by the presence of oxygen. The rate equations were first order with respect to the unionized HF concentration. The activation energies increased slightly with the oxygen content of the alloys (from 4·1 kcal/mole for 1·52 per cent (by weight) oxygen to 5·2 kcal/mole for 6·97 per cent (by weight) oxygen). Essentially the same values were found in HFHCl acid mixtures, however, the rate constants increased by as much as 50 per cent in the presence of 2·0 N HCl. ZrO x specimens dissolving in hydrofluoric acid exhibited a strong positive difference effect, the K-values increasing slightly with oxygen concentration (in the metal) and more rapidly with the concentration of HCl in the acid mixture. The dissolution potentials of the alloys in hydrofluoric acid became more noble with increasing concentrations of oxygen. Additions of HCl made the potentials still more noble. These studies confirm that the rate controlling step in the dissolution process is the diffusion of unionized HF to the metal surface, where it reacts with the Zr to evolve hydrogen. ZrO 2 accumulates on the surface causing a decrease in dissolution rate, an increase in activation energy, in dissolution potential and in the K-value of the difference effect. Dissolved oxygen in the metal does not alter the dissolution mechanism, i.e., the free or unbound zirconium of the alloy matrix reacts as if there were no oxygen present. However, the latter also exhibits in the form of ZrO 2 some shielding passivation ability.