The Importance of Oxide Morphology for the Oxidation Rate of Zirconium Alloys

Abstract

The oxide growth rate of zirconium alloys, e.g., Zircaloy-2 and Zircaloy-4, has been proposed to be controlled mainly by the transformation of the zirconium oxide from tetragonal to monoclinic structure at some distance from the metal-oxide interface, leading to cracking. This oxide growth rate model is inconsistent with our results. Zirconium alloys of varying chemical composition but with identical manufacturing process had markedly different oxide growth and hydriding properties in autoclave testing (400°C steam). The materials were characterized by several methods, e.g., electron microscopy (SEM), X-ray diffraction (XRD), and electrochemical impedance (EIS). The SEM and some of the XRD investigations of the oxide were performed on the metal-oxide interface after dissolution of the metal. The oxide growth developed through three different stages with an altered oxide morphology at the metaloxide interface at each stage. The developments of the stages were correlated with the oxide growth rate. Impedance measurements suggested that the oxide film had three layers, the outermost being extensively porous. Relaxation of the oxide film stress showed that the compressive stress in the oxide was not essential for retention of a significant amount of the tetragonal phase.