Characterization of Zircaloy-4 corrosion films using microbeam synchrotron radiation

A study of the oxide layers formed in 360 °C and 316 °C water on Zircaloy-4 samples has been performed in an attempt to help answer fundamental questions about oxide protectiveness, growth mechanisms, and the nature of oxide growth during autoclave corrosion. Two different oxide thicknesses - 12 and 39.5 μm - were investigated. Microbeam synchrotron radiation diffraction and fluorescence techniques with an X-ray beam size of 0.2 μm were used to characterize oxide in cross sections to determine the oxide phase content, grain size, texture, and orientation relationships as a function of through-thickness from the oxide-metal interface. The results confirm that the oxide is comprised primarily of monoclinic ZrO₂, with tetragonal ZrO₂ present in small amounts. The observed diffraction peaks are consistent with monoclinic phases having a strong fiber texture with the 200_m plane aligned with the oxide-metal interface, and with the 011_m plane closely aligned with the transverse-normal (T) plane. The fraction of bulk tetragonal phase increased in the region located within one transition thickness near the oxide-metal interface. A strong periodicity was seen in oxide intensity from both the monoclinic and tetragonal phases corresponding to an oxide transition thickness of 1.8-1.9 μm. The grain size of the tetragonal phase was determined to be smaller than the monoclinic phase, and the grain size for the monoclinic phase decreased starting at a distance of approximately one transition layer from the oxide-metal interface. The relative amounts of monoclinic peak broadening due to strain and grain size were calculated, the former being approximately constant, while the latter decreased with increasing distance from the oxide-metal interface, corresponding to an increase in grain size. These findings are compared to previously performed microbeam diffraction experiments on Zircaloy-4 and other Zr-based alloys.