Development of niobium-base corrosion resistant alloy for nitric acid equipment materials used in nuclear reprocessing plants

Niobium which is one of refractory metals is expected to be a promising corrosion resistant material applied for spent nuclear fuel reprocessing plants. For improving both environmental adaptability and mechanical properties of metallic Nb, the appropriate alloying elements of refractory metals were examined. Both the corrosion resistance and the solubility of candidate refractory metals in nitric acid solution were measured for evaluating the corrosion resistance index. Refractory metals of Hf, Ta and W are selected for candidate alloying elements of binary Nb alloys. Ingots of these alloys were made by electron beam melting. The mechanical strength of these alloys were examined by the measurements of the number of Vickers hardness as a function of the concentration of alloying elements. The corrosion behavior of these alloys were measured by immersion tests and electrochemical tests. As the results, the hardness of these alloys increased in the order of Nb-Ta, Nb-Hf and Nb-W. This tendency was explained with solution hardening factor due to the ion size difference between Nb and alloying elements. On the other hand, the corrosion resistance of these alloys was influenced by the solubility of their oxides which formed as passive films. Hf is the most superior alloying element in low concentration of nitric acid and W is in high concentration of one. The corrosion potential and the anodic current density at the passive potential of these refractory metals obtained by electrochemical tests were agreement with the results of the immersion tests. The corrosion resistance of niobium-base binary alloys is considered to be controlled by chemical stability of passive oxide films formed on surface. (author)