Comparative study on high-temperature air and steam oxidation of Cr-coated Zr-4 alloy through experimental and DFT calculation

The high-temperature oxidation performance of Cr-coated Zr-4 alloy in air and steam atmosphere is comparatively studied and the mechanism of steam promoted oxidation is revealed by density functional theory (DFT) calculation. The experimental results show that there are significant differences in surface and cross-sectional microstructures after oxidation in the two atmospheres. Dense irregular polyhedral oxides accompanied by randomly occurring micro-cracks are developed after high-temperature air oxidation. While mackerel scale-like or worm-like particles with whisker-like structures accompanied by defects such as pores and micro-cracks are developed after high-temperature steam oxidation. In the high-temperature steam atmosphere, the more vigorous atomic diffusion leads to a thicker Cr-Zr diffusion layer and higher O content, so that after exposure at 1100 °C for 3 and 4 h, the Zr-4 alloy adjacent to the Cr-Zr diffusion layer is oxidized to ZrO₂. All the experimental results demonstrate that Cr-coated Zr-4 alloy experiences more severe oxidation in high-temperature steam atmosphere. The DFT calculation results reveal the main reason of steam promoted oxidation is that the interstitial H protons boost the formation of Cr and O vacancies and vacancy pairs in the Cr₂O₃ oxide scale.