Effect of precipitation of second-phase particles on corrosion resistance of zircaloy

Common zirconium alloys used in Japan as boiling water reactor (BWR) core materials are Zircaloy-2 (Zr-2) and Zircaloy-4 (Zr-4). In the past 20 yr of reactor operation, corrosion performances of Zr-2 and Zr-4 have been generally satisfactory. However, improvement of nodular corrosion resistance of Zircaloys are an important subject to assure core component reliabilities in the extended fuel burnup operation planned for the future. The authors experimentally investigated nodular corrosion resistance of Zr-2 and Zr-4, which were subjected to various heat treatments to clarify the effect of precipitation of second-phase particles on nodular corrosion resistance in high-temperature steam. Commercial Zr-2, Zr-4, and button arc-melted Zircaloy were used. Corrosion test specimens were cut from cold-worked and alpha-annealed plates. The specimens were water quenched from α + β phase and beta-phase temperatures (>8300C) and annealed at alpha-phase temperatures ranging from 500 to 8000C for 2 h. Average cooling rate in quenching was ∼200 0C/s. Specimens were exposed to 5000C steam in an autoclave for 50 to 60 h. The microstructure of heat-treated Zircaloy and the volume fraction of second-phase particles were evaluated using scanning and transmission electron microscopy