Effect of metal ions in a heated nitric acid solution on the corrosion behavior of a titanium–5% tantalum alloy in the hot nitric acid condensate

For evaluating the application of titanium and its alloys as components of equipment for storing nitric acid condensate in spent nuclear fuel reprocessing plants, the corrosion behavior of titanium–5% tantalum alloy (Ti–5Ta) in a continuously renewed hot nitric acid condensate, and particularly the effect of metal ions in the heated nitric acid solution, was investigated. Corrosion experiments in an apparatus designed to renew the condensate at regular intervals showed that the corrosion rate of Ti–5Ta in the condensate increased linearly with the nitric acid concentration. The surface morphology of Ti–5Ta coupons after the corrosion experiments indicated uniform corrosion under any condition. The oxide film on the coupons had nearly constant thickness, and it was composed of mainly lower Ti oxides, such as TiO and Ti2O3, regardless of the nitric acid concentration in the condensate. The experimental results also showed that the addition of metal ions into the heated nitric acid solution increased the nitric acid concentration in the condensate, which resulted in a higher corrosion rate of Ti–5Ta. The corrosion rate increased noticeably as the valence of the metal ion increased and its ionic radius decreased. This effect of metal ions in the heated nitric acid solution on the corrosion rate of Ti–5Ta in the condensate was evaluated quantitatively based on the Gibbs free energy of hydration of the metal ions, and the calculated corrosion rates of Ti–5Ta in the condensate were found to be in good agreement with the experimental values.