Unveiling the salt effect on the corrosion film structure and mechanism of a directionally solidified Ni-based superalloy

The effect of molten NaCl salt on the corrosion film structure and mechanism was investigated in a Ni-based superalloy at 950 degrees C/200 Mpa. The results showed that molten salt significantly altered the film structure. In the oxidation-bared zone, five layered structures were formed in the following order: discontinuous TiNiO3, NiCr2O4, continuous Cr2O3, discontinuous TaTiO4, continuous NiAl2O4, and Al2O3 layer. In the salt-coated zone, the film was an external continuous TiO2 doped by NiCr2O4, NiAl2O4, an intermediate continuous Cr2O3, and an inner Al2O3 layer. Compared with the oxidation-bared zone with defects-free, recrystallization occurred due to the synergistic effect of salt and deformation, which resulted in the flux of Ti via grain boundaries increasing sharply. Thus, the outward diffusion of Ti4+ determined the reactions, while Cr3+ was responsible for the oxidation-bared zone. Moreover, TiO2 was easier to dissolve in alkaline salt than Cr2O3, accelerating the inward transport of Cl2 and promoting nucleation of corrosion pits, significantly weakening the alloy's corrosion resistance.