Effect of Alloying Elements on Uniform Corrosion Resistance of Zirconium-based Alloys in 360°C Water and 400°C Steam

Abstract

The effects of alloying and impurity elements on the uniform corrosion properties of zirconium-based alloys were studied, Tin, iron, chromium, nickel, vanadium, niobium, and tantalum (as alloying elements), and oxygen, carbon, nitrogen, silicon, aluminum, and phosphorus (as impurities), were selected. These alloys consisted of two groups: Zircaloy-type alloys and zirconium-based alloys. Corrosion tests were performed in 633 K water and 673 K steam for a total of 7200 h. The effect on hydrogen pick-up was also studied. Zircaloy-type alloys showed typical corrosion behavior with a rate transition. In the pretransition region almost all alloys showed the same behavior, but the post-transition corrosion rate increased with tin content. Iron increased the post-transition corrosion rate in 673 K steam; the effect of chromium was negligible. A small addition of nickel was effective in reducing post-transition corrosion rate, but larger amounts did not markedly improve the resistance. The effect of impurities was also negligibly small within the variation of the contents in this study. The effects of alloying elements in the binary alloys were basically the same as that in Zircaloys, but niobium and tantalum had a remarkable effect in improving the corrosion resistance. The effects of tin, iron, and chromium on hydrogen pick-up fraction were small; nickel, however, had a significant effect. The composition of second-phase particles in Zircaloys varied with the contents of alloying elements, but seemed to have no relation to the uniform corrosion behavior of Zircaloys. The rate transition in corrosion behavior was assumed to be related to the relief of compressive stress in the growing oxide by the transformation from a monoclinic to a tetragonal or cubic structure. Furthermore, tin might have the effect of stabilizing monoclinic zirconium oxide. Second-phase particles, Zr(Cr.Fe)2, were oxidized to orthorhombic structure from hcp structure and seemed to have little effect on the morphology of oxide around them and consequently on corrosion behavior.