18O tracer study of the oxidation of zircaloy‐4 in steam

Abstract

AbstractThe corrosion resistance of Zircaloy‐4, used as fuel cladding in pressurized water reactors, in steam at 400°C is related to the annealing temperature used in fabrication. This change in corrosion behaviour appears to be related to the size and composition of intermetallic compound precipitates, which vary with annealing temperature.A SIMS study has been undertaken to clarify the mechanisms responsible for these variations in the initial stages of corrosion of Zircaloy‐4. A set of samples subjected to different annealing temperatures have been oxidized, initially in normal steam and subsequently in steam enriched in H2 18O. Corrosion mechanisms have been followed using depth profiling in a Cameca IMS 3F microscope together with three‐dimensional depth profiles and imaging of taper sections using a gallium microprobe (VG Scientific).Iron and chromium, present in intermetallics in the alloy, remain localized within the oxide scale. The size and frequency of these local concentrations of iron and chromium reflect the distribution of these elements in the alloy. There is, however, some diffusion of these elements into the surrounding oxide, the iron diffusion being greater than that for chromium. The 18O tracer studies indicate that grain boundary transport of oxygen dominates the initial oxide growth process for Zircaloy‐4.