Effects of Al and Si elements on oxidation behavior in 1000–1200 °C steam for PEO/CrAlSi composite coating on Zr alloy

A PEO/CrAlSi composite coating of ~10 μm thick on Zr–2 alloy was prepared by duplex plasma electrolytic oxidation (PEO) and filtered cathode vacuum arc deposition (FCVAD) techniques. The high temperature steam oxidation behavior of bare and PEO/CrAlSi–coated Zr alloys was performed in 1000–1200 °C. Their morphologies, microstructures, compositions and phase components were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X–ray diffraction (XRD) and glow discharge optical emission spectrometer (GDOES). The effects of Al and Si elements on multilayers microstructure evolution of coated Zr–2 alloy at steam atmosphere were discussed. It was found that the composite coating significantly improved the high–temperature steam oxidation resistance of Zr–2 alloy. The mass gains of the PEO/CrAlSi–coated Zr–2 alloy after 3600 s exposure in 1000 °C, 1100 °C and 1200 °C steam were 28 %, 29 % and 44 % of the bare alloy. Through the oxidations, Al diffused both inward and outward to the substrate and the surface regions, respectively. Formation of a top oxide layer (Al2O3, Cr2O3) greatly suppressed the inward oxygen diffusion. The Si enrichment in the PEO interlayer formed a Zr2Si layer, which inhibited the Cr-Zr interdiffusion at 1000 °C and 1100 °C. At 1200 °C, the Zr2Si layer was oxidized and the CrZr2 phase was formed in the Zr alloy substrate especially at the β–Zr/α–Zr(O) interface.