Reducing the oxidation rate of Cr-coated Zr alloys under high temperature steam environment: An approach of an outer Zr coating

•Coating structure: 4 μm Zr coating + 9 μm Cr coating.•A Cr/Zr mixed oxide layer formed between the two coatings.•Weight gain rate was reduced by the outer Zr coating.•No cracking or exfoliation was observed on the coating. Cr-coated Zr alloys are widely regarded as promising accident-tolerant fuel (ATF) cladding materials. However, the rapid consumption of the Cr coating through both oxidation and diffusion demands a thick coating, which is detrimental to neutron economic. In this study, a thin Zr coating was fabricated on the top of the Cr coating to act as a protective layer, aiming to reduce the oxidation consumption rate of Cr coatings. In-situ weight gain measurements were performed to determine the oxidation kinetics. The microstructural evolution of the coating/substrate interface and the oxide/coating interface were analyzed. The results show that the weight gain rate of the Zr-Cr coated Zr alloy sample is lower than that of the bare Cr-coated Zr alloy samples. An equiaxed grain structure composed of Cr/Zr mixed oxides was observed in the coating, which helps to inhibit elements diffusion and reduce the oxidation rate of Cr coatings. After steam oxidation at 1000 °C for 4 h, the sample structure consisted of the Zr substrate, a Cr2Zr layer, a residual Cr coating, a Cr2O3 layer, a Cr/Zr mixed oxides layer, and a ZrO2 layer.