Mechanisms of steam oxidation-induced degradation of chromium coating on zirconium alloys at high temperatures

Abstract

Steam oxidation experiments combined with extensive material characterizations were conducted to investigate the comprehensive microstructural mechanisms and kinetics responsible for the degradation of chromium coatings on zirconium alloys under high-temperature steam oxidation conditions. A mechanistic framework was developed to integrate key degradation phenomena, potentially enabling quantitative prediction of coating protection loss onset and subsequent oxidation behaviors. The findings reveal a general phenomenological framework comprising four critical factors: transient Cr-coating thickness, grain boundary density, Zr diffusion along grain boundaries, and Cr₂O₃ layer reduction and oxygen ingress. Transient Cr-coating thickness, influenced by oxidation and diffusion losses, determines the Zr diffusion length and, thus, protection duration. Significant grain growth at high temperatures was observed, varying substantially with coating fabrication methods and initial microstructural conditions. Zr diffusion primarily occurred along high-angle grain boundaries, forming intergranular ZrO₂ phases and associated defects such as twins and dislocations. Parameters governing protection loss were established. The first transition time, marking Zr reaching the Cr/Cr₂O₃ interface, was proportional to the square of the initial coating thickness. The second transition, characterized by the loss of Cr₂O₃ layer protectiveness and a sharp increase in oxidation rate, occurs when voids formed by Cr₂O₃ reduction interconnect across a substantial portion of the oxide layer. This interconnected void structure provides direct steam access to Zr diffusion paths, significantly accelerating oxygen uptake and resulting in complete coating degradation. Post-protection oxidation kinetics were controlled by oxygen diffusion rate along ZrO₂ path. This framework provides a robust basis for predicting Cr-coating degradation on zirconium by steam oxidation.