Electrochemical Behavior of Passive Film Formed on Ni-Cr-Mo Alloy 625 in Neutral Sodium Chloride Solution

Abstract The present work investigates the electrochemical behavior of passive film formed on Ni-Cr-Mo alloy 625 in potentials ranging from -100 to 700 mVSCE using various techniques, including potentiodynamic polarization, potentiostatic polarization, Electrochemical Impedance Spectroscopy (EIS), and Mott-Schottky (M-S) analysis in neutral 3.5 wt.% NaCl solution. These comprehensive studies show that the passive film properties might be divided into two potential regions, which the manifestation of Region I likely lasts 200 mVSCE, and Region II dominates at higher potentials. For Region I, where the passive film represents n-type semiconductivity, the passive current-transient is independent of formation potentials, while the total film resistance changes incrementally. However, the passive film represents p-type semiconductivity in Region II, where the passive current-transient is found to be an increasing function of formation potential. In addition, the point defect model (PDM) in conjunction with the results was used to elucidate the corrosion performance mechanism over the two potential regions. These findings are at odds with chloride-catalyzed film dissolution and chloride penetration mechanism for passivity breakdown. Meanwhile, they entirely emphasize the postulate of the PDM envisaging cation vacancy generation at the barrier layer/outer layer interface and subsequent cation vacancy condensation at the metal/barrier layer interface.