XPS studies of the thermal behaviour of passivated Zircaloy‐4 surfaces

Abstract

AbstractThe composition and the chemical states of components of Zircaloy‐4 (zirconium alloy) surfaces were studied in the temperature range between room temperature and 500°C. Each sample was kept at constant temperature (25, 100, 200, 300, 400, 500°C) for up to 16 hours. The changes of composition and chemical states of the Zircaloy‐4 surface during heating were monitored by x‐ray photoelectron spectroscopy (XPS). Originally, the components form well‐defined layers elucidated by angle‐resolved x‐ray photoelectron spectroscopy (ARXPS). In contrast to depth profiling using ion sputtering, ARXPS is non‐destrutive. However, it is applicable for layers of up to a few nanometres thickness only.The experiments showed a decomposition of the ZrO2 coverage above 200°C accompanied by oxygen diffusion into the bulk. These processes lead to the reduction of ZrO2 to metallic zirconium on the surface at 300°C and higher temperatures. The oxygen diffusion into the bulk was indicated by AES depth profiles. The layered structure observed up to a heating temperature of 200°C could not be seen at higher temperatures. After Zr metal appears at the surface during the heating process, a reaction with the adsorbed hydrocarbons takes place, leading to the formation of zirconium carbide.Though the depth resolution of an AES depth profile does not permit identification of layers with thicknesses in the nanometre region, the temperature‐dependent behaviour of oxyen is reflected by its AES profiles, showing features in accordance with the results from ARXPS, especially with respect to the fact that well‐defined layers vanish above 200°C.