PHASE INSTABILITY AND CORROSION OF ALLOY 22 AS A HIGH-LEVEL NUCLEAR WASTE CONTAINER MATERIAL

摘要

Alloy 22 is currently proposed by the U.S. Department of Energy (DOE) for use as the outer container of the waste package for the disposal of high-level nuclear waste. DOE will need to address the extent to which phase instability resulting from fabrication processes (i.e., welding and postweld treatments) could limit the lifetime of the Alloy 22 waste packages. In this context, recent work involved the study of alloy specimens in mill-annealed condition after thermal exposure at 870 ℃ [1,598 ╚H] for periods of up to 30 minutes. The effect of metallurgical stability on localized corrosion susceptibility was evaluated using corrosion tests and analytical electron microscopy measurements. Results obtained from this study indicate that thermal exposure at 870 ℃ [1,598 ℉] for only 5 minutes resulted in the formation of topologically close-packed (TCP) phases at grain boundaries; however, no significant alloy depletion was detected in the matrix adjacent to the precipitates nor in the grain-boundary regions between precipitates. Nevertheless, precipitation of TCP phases was found to promote localized corrosion along grain boundaries and decrease the repassivation potential in chloride-containing solutions.