Mechanism of adding rhenium to improve hot corrosion resistance of nickel-based single-crystal superalloys

Hot corrosion behavior in sulfate salt at 950 °C of Rene N5 single-crystal superalloys with 3 wt% rhenium (NSR) was investigated compared with that of nickel-based single-crystal superalloys without rhenium (NS). After 30-h corrosion, the surface of the NS superalloy is seriously corroded. Many holes and exfoliation appear on the surface. The NSR superalloys exhibit better hot corrosion resistance than the NS superalloys. After 30-h corrosion, a continuous and compact Al₂O₃ film is observed on its surface. The Al₂O₃ film with dense structure formed on the surface provides protection for the matrix. The characterization results show that Al is aggregated in the γ′ phase, while Re is aggregated in the γ phase during the formation of oxide scale. Considering that Re can inhibit the diffusion of Al in the nickel matrix, it is inferred that Re can inhibit the outward diffusion of Al and prevent the decrease of Al concentration in the γ′ phase. High concentration of Al hinders the decomposition of Al₂O₃ due to the reaction of acid and basic dissolution. Al₂O₃ keeps its structure intact and provides protection for the matrix.