Abstract: Hot corrosion of the sputtered nanocrystalline coating and its (Al + Y)-modified coating on N5 with and without different preoxidation treatments under 75 wt.% Na₂SO₄ + 25 wt.% NaCl molten salts at 900 °C was investigated. The results showed both the nanocrystalline coatings without preoxidation suffered from severe corrosion. No intact protective scale could form on the surface due to the fast penetration of the corrosive media through massive grain boundaries in the coating. The effect of different preoxidation treatments on the corrosion varied for the two coatings. The original nanocrystalline coatings with similar compositions to the substrate N5 after preoxidation showed a low corrosion resistance, with a loose defective oxide scale on the surface and severe internal corrosion in the coating. Ta oxide segregation ruined the stability of the preoxide scale and thus accelerated the subsequent corrosion. In comparison, the AlY-modified coating after preoxidation at 1000 °C exhibited the best corrosion resistance. Its oxide consisted exclusively of the Al₂O₃ scale, and the least internal corrosion occurred in the coating. Co-addition of Al and Y retarded the diffusion of Ta, improved the stability of the scale, and provided a sufficient Al supplement to reform the Al₂O₃ scale in the corrosion process, postponing corrosion destruction of the scale. The AlY-modified coating after preoxidation at 1100 °C performed worse than that at 1000 °C. The inferior corrosion resistance could be attributed to the more defective preoxide scale caused by higher thermal stress concentration during preoxidation.

Key words: Superalloys, Coatings, Oxidation, Hot corrosion