Discussion on the presence condition of suboxide ZrO beneath the oxide in zirconium alloys

Characteristics related to the oxide-metal interface of oxidized (corroded) zirconium alloys in an autoclave have been extensively explored using both experimental and theoretical methods during the last ten years. A thin intermediate or suboxide layer with an approximate 1:1 ZrO stoichiometry was observed at the oxide-metal interface in several zirconium alloys. Through investigating its chemical composition, morphology, grain size and orientation relationship with the substrate α-Zr, some researchers assumed that the suboxide layer is related to the oxidation (corrosion) rate, whereas others did not. The majority of earlier studies used aqueous corrosion samples that were exposed to high pressure and low temperature, simulating in-service conditions. However, the interfacial characteristics at higher temperature have been scarcely reported. In this study, we focused on the microstructure and micro-chemistry of the oxide and suboxide ZrO in Zircaloy-4 and Zr-1.0Nb-0.01Cu oxidized at 650–1000 °C by combining Backscattered Electron (BSE) mode in Scanning Electron Microscope (SEM), Transmission Electron Backscatter Diffraction (t-EBSD) and (Scanning) Transmission Electron Microscope fitted with an Energy Dispersive X-ray Spectroscope ((S)TEM-EDS). The results show that the suboxide ZrO in oxidized Zircaloy-4 and Zr-1.0Nb-0.01Cu alloys has different morphologies. With increasing oxidation temperature, the suboxide ZrO distribution varies significantly, from a continuous layer to a few occasional blocky grains, until any ZrO grains are undetectable at the ZrO ₂ /α-Zr(O) interface. The hexagonal crystallographic structure of ZrO has also been confirmed. According to discussions combining our investigations and earlier reports, the presence of suboxide ZrO and their morphology are probably related to the oxidation temperature, chemical composition of alloys and complex stress accumulation adjacent to the interfacial region.