Oxidation and degradation of Cr-coated Zr-based alloy up to 1500°C: Effect of Cr thickness

This study systematically investigates the influence of chromium coating thickness (5–25 µm) on the oxidation and degradation behavior of Cr-coated Zircaloy-4 under high-temperature steam conditions up to 1500°C. Using thermogravimetry, hydrogen release monitoring, and extensive post-test microstructural analyses, the results show that thicker Cr coatings significantly extend the protective effect below the Zr–Cr eutectic temperature (∼1330°C). The degradation processes are governed by different diffusion mechanisms that gradually lead to a loss of coating protectiveness, with transition times strongly dependent on coating thickness and temperature. Above the eutectic point, however, all coatings fail almost instantaneously, regardless of thickness, with the oxidation kinetics switching from chromia to zirconia formation. The findings confirm the effectiveness of Cr coatings under normal operation, transients, and design-basis accidents, but demonstrate their intrinsic limitations in beyond-design-basis accident scenarios exceeding the eutectic temperature.