Corrosion Study of Titanium-5% Tantalum Alloy in Hot Nitric Acid Condensate

In this study, a long-term corrosion tendency and metal salt effect in heating nitric acid solution on corrosion behavior of titanium-5% tantalum alloy (Ti-5Ta) in hot nitric acid condensate condition were mainly researched to discuss the aging behavior of reprocessing equipments such as evaporators made of titanium or its alloy. The hot pure nitric acid solution with continuous renewing such as the nitric acid condensate condition is severe corrosion environment for their materials because of the corrosion inhibition effect from titanium ions as corrosion products or oxidizing ions in nitric acid solution and is certainly formed in evaporator for spent nuclear fuel reprocessing. From the results of the long-term corrosion test for total 11,000 hrs, the corrosion of Ti-5Ta in the nitric acid condensate was accelerated with increase of the nitric acid concentration in the condensate (∼5.6 M). The corrosion rate was nearly constant during the immersion time and the test coupons suffered a uniform corrosion. Thus, from the viewpoints of nitric acid corrosion, the life-time of the reprocessing equipments made of titanium or its alloy will be roughly estimated based on the results of average corrosion rate in operation. It was also found that the kind and concentration of metal salt in the heating nitric acid solution gave a remarkable effect on the concentration of nitric acid vapor and the corrosion of Ti-5Ta in the hot nitric acid condensate. Most of the evaporators for reprocessing plants include metal ions in the heating nitric acid solution, so the metal salt effect is one of the corrosion factors to control the corrosion behavior of titanium alloy in condensate. The nitric acid concentration in the condensate increases by adding the metal salts in the heating nitric acid solution, in addition, the larger valence of metal ions was contributed to the increase of nitric acid concentration in the condensate. Consequently, the metal salts effect in the heating nitric acid solution accelerates the corrosion of Ti-5Ta in the nitric acid condensate. The corrosion of titanium or its alloy in nitric acid condensate condition should be carefully considered as one of severe corrosion environment in evaporators for reprocessing plant. This corrosion study would give useful information to estimate the lifetime of evaporators made of titanium alloy from the viewpoint of nitric acid corrosion.