Characterisation of pre-transition oxides on Zircaloys

The aim of this study was to evaluate the possibility of predicting post-transition oxidation behaviour by studying the properties of pre-transition oxides. Five different zirconium alloys have been tested in autoclave at two different temperatures, 288°C and 360°C. Cross-sectional transmission electron microscopy (TEM) showed that a significant difference in morphology is obtained for the two test temperatures, while the alloy composition was found to have no influence. At the lower test temperature the oxide consists of a mixture of columnar and equiaxed grains, while at the higher temperature mainly columnar grains are observed and the size of these grains is a factor 3–4 longer. Tetragonal oxide phase was mainly detected by selected area diffraction in the thicker oxide layers, while the amount of the tetragonal phase in the thermally formed oxides is too low to be detected by X-ray diffraction. Thus, the tetragonal phase is probably stabilised by other factors than the compressive stress. The impedance measurement shows that the thinnest oxide layers are compact, irrespective of the alloy composition and thermal or anodic oxide growth. For the thicker oxides, a porous outer layer is developed. There is a main difference in the morphology between thermal and anodic oxide layers of the same size, i.e., the anodically formed oxide layer has only equiaxed grains present. Based on the result obtained in this investigation, no prediction of the post-transition oxidation can be made.