Steam oxidation of Cr-coated zirconium alloy claddings at 1200 °C: Kinetics transition and failure mechanism of Cr coatings

The microscopic layered structural evolution, the diffusion behavior of O, Cr and Zr elements and oxidation kinetics of the Cr-coated zirconium alloy claddings during steam oxidation at 1200 & DEG;C were systematically studied. At the initial oxidation stage (within 1 h oxidation), the protective Cr2O3 layer formed on the Cr coating, and the interdiffusion between Cr and Zr resulted in a four-layers structure from outer to inner, Cr2O3, Cr, Cr-Zr diffusion layer and Zr substrate. At this stage, the oxidation kinetics followed parabolic law. With increasing oxidation time to 1.8 h, ZrO2 networks formed in the Cr coating due to the occurrence of the redox reaction between Zr and Cr2O3, which provided a pathway for the diffusion of oxygen toward substrate. The layered structure on the Cr coating side transformed from a four-layers to a five-layers structure with the formation of a ZrO2 layer in the substrate. Meanwhile, the oxidation kinetics became linear growth. The failure of Cr coatings resulting in the oxidation of Zr substrate accounts for the parabolic-to-linear kinetics transition.