DISSOLUTION OF ZIRCONIUM-OXIDE FILMS IN 300-DEGREES-C LIOH

Zircaloy-2 specimens were preoxidised in 300-degrees-C, pH 7 water and changes in the topography of these preformed oxides were followed by the successive examinations of electron microscope replicas of the same areas of surface following continued exposures in either water or LiOH. Few changes in topography were detected during exposures in water. However, the transfer of specimens from water (3 days, 300-degrees-C) to 0.1 molar LiOH solution showed that porosity rapidly developed all over the initial water-formed oxide. The pores reached a maximum number density, suggesting formation by preferential dissolution of some features of the oxide surface. Small numbers of large (30-50 nm) pores appeared to be related to small second phase particles, but the small (5-10 nm) pores are thought to arise from preferential dissolution of cubic or tetragonal crystallites in the oxide surface. Laser Raman spectra; change of impedance with time of immersion in an electrolyte; changes in surface microtopography after each successive autoclave exposure; and the observation of zirconium in spinel crystals that deposited on the surface from autoclave corrosion (following an increase in LiOH concentration to 1.0 molar), all support an hypothesis for preferential oxide dissolution and reprecipitation as the mechanism for pore formation.