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 °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.