Depletion of Fe and Cr within precipitates during Zircaloy-4 oxidation

Analyses of the oxide layers which were formed in a static autoclave of 400 degreesC steam after the heat-treatment to coarsen the precipitates were performed with the optical microscope, scanning electrical microscope (SEM), transmission electron microscope, electron probe micro-analyzer (EPMA), synchrotron X-ray diffractometer (XRD), and secondary ion mass spectroscope (SIMS), paying attention to the oxidation of Fe and Cr within Zr(Fe, Cr)(2) precipitates in Zircaloy-4. The oxide analyses were concentrated on the oxidized samples in the regime of pre- and post-transition. It could be confirmed from the SEM observation of the interfacial oxide that the Zr matrix was oxidized earlier than the precipitates. During the oxidation process, the unoxidized precipitates were incorporated in the interfacial oxide, and then the precipitates were oxidized gradually in the oxide layer. From EPMA, synchrotron XRD and SIMS analyses, the alloying elements such as Fe and Cr within precipitates exhibited differences in oxidation rate. It was found that the Fe depletion from precipitates in the initial stage stabilized the tetragonal ZrO2 phase of the surrounding oxide and when the oxidation time increased, the depletion of Fe and Cr initiated the phase transformation of the oxide crystal structure from tetragonal ZrO2 to monoclinic ZrO2. All Fe within precipitates were depleted in the preference to Cr. It is thought that the overall corrosion rate of Zircaloy-4 can be governed mostly by the oxidation of Zr(Fe,Cr)(2) precipitates, and the oxidation rate of Fe was much faster than that of Cr in precipitates. (C) 2002 Elsevier Science B.V. All rights reserved.