Temperature-time dependence and mechanisms of redox reaction in Cr-coated Zr alloy cladding during steam oxidation at 900–1250 ℃

The structural integrity of the Cr coating can be significantly affected with increasing temperature during steam oxidation. Herein, the elemental diffusion, layered structure, and kinetics evolution of Cr coating in steam environment of 900–1250 ℃ for 0.5 to 3 h were studied. The oxidation behavior of Cr-coated Zr claddings associated with redox reaction between Cr₂O₃ and Zr were investigated in detail. Cr₂O₃ layer forms on Cr coating for steam oxidation at 900 ℃ and Cr-Zr diffusion layer appears at interface between Cr coating and Zr substrate when the samples are subjected to steam oxidation at 1000–1150 ℃ for 3 h. The starting time for redox reaction decreases dramatically when the temperature is higher than 1150 ℃. A prediction modal based on Fick's Law was put forward to predict the starting time of redox reaction for 1100 to 1250 ℃. The prediction is consistent with the experimental results. Furthermore, the initial process of redox reaction was investigated and analyzed in detail. Predominated by ion transportation within residual Cr layer, O^2- from Cr₂O₃ could diffuse along the ZrO₂ particles at Cr coating boundaries to the Cr-Zr layer, resulting in the reduction of Cr₂O₃ and partial oxidation of diffusion layer.