An interesting oxidation phenomenon of Cr coatings on Zry-4 substrates in high temperature steam environment

Cr coatings deposited on zircaloy-4 substrates have been oxidized in high temperature steam environment from 1000 degrees C to 1200 degrees C in this study. It has been reported by many authors that Cr coatings have exhibited excellent oxidation resistance in steam environment because of the formation of a dense and adherent Cr2O3 layer on the surface. However, we have found that in the stage when the Cr coating has completely been oxidized and approached the coating-substrate interface, it is reduced by the Zr substrate and mostly transformed into a metallic Cr layer. The mechanisms of oxidation in this stage is therefore more interesting than generally assumed and we have proposed a new model based on the diffusion and redox reaction between Zr and Cr layers in high temperature steam environment.