Characterisation of pre-transition oxides on Zircaloys

The aim of this study was to evaluate the possibility of predicting post-tr ansition oxidation behaviour by studying the properties of pre-transition o xides. Five different zirconium alloys have been tested in autoclave at two different temperatures, 288 degreesC and 360 degreesC. Cross-sectional tra nsmission electron microscopy (TEM) showed that a significant difference in morphology is obtained for the two test temperatures, while the alloy comp osition was found to have no influence. At the lower test temperature the o xide consists of a mixture of columnar and equiaxed grains, while at the hi gher temperature mainly columnar grains are observed and the size of these grains is a factor 3-4 longer. Tetragonal oxide phase was mainly detected b y selected area diffraction in the thicker oxide layers, while the amount o f the tetragonal phase in the thermally formed oxides is too low to be dete cted by X-ray diffraction. Thus, the tetragonal phase is probably stabilise d by other factors than the compressive stress. The impedance measurement s hows that the thinnest oxide layers are compact, irrespective of the alloy composition and thermal or anodic oxide growth. For the thicker oxides, a p orous outer layer is developed. There is a main difference in the morpholog y between thermal and anodic oxide layers of the same size, i.e., the anodi cally formed oxide layer has only equiaxed grains present. Based on the res ult obtained in this investigation, no prediction of the post-transition ox idation can be made. (C) 2001 Elsevier Science B.V. All rights reserved.