Investigation on corrosion mechanism of Nb-added Zirconium alloy by analysis of electrical conductivity

Abstract

Zirconium alloys used as cladding materials of nuclear fuel are widely utilized by adding niobium as a major alloying element to improve corrosion resistance. At present, much research has been carried out experimentally to optimize the addition of niobium. And in recent years, as the integrity of high burn-up nuclear fuel is regarded as necessary, the research for further improving the corrosion resistance has been performed. However, the mechanism is still unclear. The mechanism to improve corrosion resistance has been explained in many aspects and it is generally understood that various macroscopic factors such as temperature, pressure, surface condition, heat treatment, and second phase particles interact complexly and consequently influence the lattice structure of $ZrO_2$ layer and charged particles’ diffusion. In this study, to understand clearly how niobium affects the ion transport in the lattice structure of oxide layer, we analyzed the electrical conductivity characteristics by impedance measurement, which is a typical method to understand ion transport. In particular, by manufacturing the monoclinic $ZrO_2$ bulk which was difficult to manufacture in the past but is similar to the structure of $ZrO_2$ layer, we were able to obtain evident electric conductivity characteristics in the monoclinic lattice structure of $ZrO_2$ bulk excluding the influence of various factors as described above. As a result, $ZrO_2$ bulk added with a small amount (1mol% $Nb_2O_5$) of niobium showed lower electrical conductivity than that of pure or tin-added $ZrO_2$. Besides, the $ZrO_2$ layer also showed the same tendency. Therefore, niobium is considered to lower the migration characteristics of oxygen ions in a microscopic viewpoint in the $ZrO_2$ layer. And the experimentally obtained Mott-Schottky plots provided the evidence of the reduced oxygen vacancy concentration for both $ZrO_2$ bulk and $ZrO_2$ layer.