Diffusivity of point defects in the passive film on a Ni16Cr13Co4Mo alloy in molten NaCl-Na2SO4

Passive films were prepared at different film formation potential (Eff) on a Ni16Cr13Co4Mo alloy in NaCl-Na2SO4 eutectic at 700 celcius. The electrochemical property of the passive film was measured and the diffusivity (D0) of the point defects in the passive film was calculated based on point defect model (PDM), which was around 10-14 cm2/s. TEM and XPS proved that passive film was composed of outer Na2CrO4-Cr2O3 mixture, middle Cr2O3-TiO2 mixture, and inner amorphous Al2O3. With increased Eff, the passive film became compact and had better crystallinity. The corrosion mechanism of the alloy under potentiostatic condition was discussed.