Oxidation kinetics and hydrogen absorption of alloys of zirconium with niobium and chromium

The oxidation behaviour of Zr-Nb, Zr-Cr and Zr-Sn binaries, and of zircaloy-2 and an experimental Zr-0.5 Nb-1cr alloy has been studied in water at 360degC (saturation pressure) and steam at 550degC (1 atmosphere pressure). Kinetic measurements indicated two regions of oxidation - the initial parabolic or cubic oxidation region followed by linear oxidation, the change occurring at the transition point. Niobium addition (Zr-1Nb) was beneficial in aqueous medium (compared to Kroll Zr) while chromium addition (Zr-1Cr) was beneficial in steam. Zircaloy-2 has good resistance in aqueous medium, while the oxidation resistance is poor in steam at elevated temperatures. The corrosion rates of Zr-0.5 Nb-1Cr were comparable to zircaloy-2 in the aqueous medium and considerably lower in steam phase. The influence of alloying additions on the oxidation of zirconium has been briefly discussed. Metallographic examination of the cross-sections of the oxide revealed fairly uniform oxide covering on the alloy. The hydrogen absorption after oxidation in steam at 550degC was investigated for Zr-0.5 Nb-1Cr and the results compared with those for zircaloy-2. The hydrogen pick up by the experimental alloy was in the range 40-42% of the theoretical in the post-transition region as against 60-64% observed on zircaloy-2. (author)