Mechanistic Studies of Zircaloy Nodular Corrosion

Abstract

The effects of alloy chemistry and thermal processing on corrosion performance of Zircaloy are shown to provide significant insights into the mechanisms governing nodular corrosion formation. Nucleation and growth behavior of nodular oxide in ex-reactor and in-reactor tests were studied phenomenologically, with the results suggesting that the behavior is similar in both environments. The sites of nodular oxide nucleation were found not to be at large precipitates or grain boundaries; rather, the sites were identified, though not unambiguously, to be free of precipitates. Zircaloy-2 and Zircaloy-4 show significant differences in response to heat treatment in the high alpha phase region; only Zircaloy-2 shows improvement in nodular corrosion resistance. Both alloys, however, become highly resistant to nodular corrosion after receiving heat treatment in the alpha-beta or beta phase fields at temperatures above 830°C. Both the high alpha annealing improvement of Zircaloy-2 and alpha-beta or beta annealing improvement of Zircaloy-2 and -4 are attributed to a redistribution of solute elements into the Zircaloy matrix. A nodular corrosion mechanism based on local solute depletion and its effects on the stoichiometric structure of the initial black oxide is proposed.