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 °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 TiNiO 3 , NiCr 2 O 4 , continuous Cr 2 O 3 , discontinuous TaTiO 4 , continuous NiAl 2 O 4 , and Al 2 O 3 layer. In the salt-coated zone, the film was an external continuous TiO 2 doped by NiCr 2 O 4 , NiAl 2 O 4 , an intermediate continuous Cr 2 O 3 , and an inner Al 2 O 3 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 Ti 4+ determined the reactions, while Cr 3+ was responsible for the oxidation-bared zone. Moreover, TiO 2 was easier to dissolve in alkaline salt than Cr 2 O 3 , accelerating the inward transport of Cl 2 and promoting nucleation of corrosion pits, significantly weakening the alloy's corrosion resistance.