Abstract
Hot corrosion of the N5 nanocrystalline coating with 0.5 wt% Y addition with and without preoxidation under 75 wt% Na2SO4 + 25 wt% NaCl molten salts at 900 °C was investigated, in comparison with that of the NiCrAlY coating under the same condition. The results showed that the nanocrystalline coating with nano-columnar structure suffered more severe corrosion than NiCrAlY coating. The high density of grain boundaries in the nanocrystalline coating facilitated inward penetration of the corrosive medium and accelerated the destruction of the scale. Preoxidation significantly enhanced the corrosion resistance of the nanocrystalline coating by forming an exclusive Al2O3 scale on the surface that could prevent molten salt from accessing the coating surface. The coating preoxidized at 1000 °C exhibited the highest corrosion resistance with the least internal corrosion. Y stabilized Ta and/or S during preoxidation and subsequent corrosion process, improving the purity and stability of the scale. The coating preoxidized at 1100 °C performed worse than that at 1000 °C. The inferior corrosion resistance was attributed to the more Ta oxides segregation and faster consumption of Al at the higher preoxidation temperature, accelerating scale destruction during hot corrosion.
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