Hot Corrosion Behaviors of Four Coatings on Ni-Based Superalloy

In order to study hot corrosion behaviors of the high temperature protective coatings on the superalloy, four modified aluminide coatings (NiCr-CrAl coating, Co-Al coating, Al-Si coating and Al-Ti coating) were prepared on the surface of a Ni-based superalloy. The microstructures and the hot corrosion behaviors in the salt of 25 wt. % NaCl + 75 wt. % Na2SO4 at 900 ℃ were investigated. The results show that a large amount of mixed oxides include Cr2O3, α-Al2O3 and NiCr2O4 spinel have been formed on the surface of four coatings except for the NiCr-CrAl coating. The oxides can enhance the hot corrosion resistance of the coatings. Different morphologies appeared on the surface of the four coatings after corrosion for 100 h at 900 ℃. Obviously, discontinuous corrosion products and localized scaling zones occurred on the surface of the NiCr-CrAl coating. The Co-Al coatings were continuous and dense relatively after hot corrosion. However, obvious crack appeared on the surface, which can lead to great degradation and even failure of the coating. As for the Al-Si coating, not only the cracks and spalling arose on the surface, but also the internal oxidation were found in the substrate. The Ti - Al coating were continuously compact and adhered tightly to the substrate after corrosion. Obviously, the Ti-Al coating provided much better protection to the alloy than the other three coatings in this kind of mixed salt.