Enhanced steam oxidation resistance of Cr-coated Zr-4 alloy by an evolving Mo-based diffusion barrier: A kinetic and mechanistic study

Cr coatings on Zr alloys suffer from Zr–Cr interdiffusion-induced Kirkendall voiding, blistering, and eutectic-related degradation under high-temperature steam oxidation. Herein, a Mo diffusion barrier was introduced using multi-arc ion plating to retard interfacial diffusion and improve oxidation resistance. The oxidation kinetics, cross-sectional microstructure, interfacial phase evolution and diffusion behavior in the Cr–Mo–Zr system were investigated during steam oxidation. The Mo layer does not change the intrinsic oxidation mechanism of Cr coating because of both coatings exhibiting similar activation energies (∼220 kJ/mol). Compared to the Cr coating, the parabolic oxidation stage of Cr/Mo coating is prolonged, the parabolic rate constant (k_p) is halved in 1200–1400℃, and the effective protection time is extended from 60 min to 180 min at 1200℃. Microstructural and phase analyses reveal that the Mo layer progressively evolves into a continuous Zr(Cr, Mo)₂ interlayer, which acts as a secondary diffusion barrier. The Cr diffusion coefficient in Zr(Cr, Mo)₂ is one order of magnitude lower than that in ZrCr₂, thereby suppressing Zr-Cr interdiffusion and Kirkendall void formation, and further inhibiting blistering and eutectic-like microstructure, which are the primary degradation mechanism of Cr coating. The eventual degradation of the Cr/Mo coating is governed by progressive reduction by outward-diffusing Zr and degraded oxide layer.