STRUCTURAL-ANALYSIS OF OXIDE SCALES GROWN ON ZIRCONIUM ALLOYS IN AUTOCLAVES AND IN A PWR

The analysis of the water-side corrosion of zirconium alloys has been a field of research for more than 25 years, but the details of the mechanism involved still cannot be put into a coherent picture. Improved methods are required to establish the details of the microstructure of the oxide scales. A new approach has been made for a general analysis of oxide specimens from scales grown on the zirconium-based cladding alloys of PWR rods to analyse the morphology of these scales, the topography of the oxide/metal interface and the crystal structures close to this interface. - The method applied to isolate the oxide scales from the supporting alloy can also be used to reveal the alloy grain structure near the oxide scale. - The layer structure of the oxide scales has been confirmed. - A quantitative description of the topography of the oxide/metal interface has become possible. - By using the particular properties of the oxide scales the corrosion-rate determining innermost part of the oxide layer at the oxide/metal interface can be separated from the rest of the oxide scale and then analysed by SEM, STEM, TEM and electron diffraction after dissolution of the alloy. Examples are given from oxides grown on Zr-alloy claddings in a pressurized-water reactor and in an autoclave.