Protection of Stainless Steels by Mo against Cl Attack: A DFT Study

We report a DFT study of (0001)-oriented Cr₂O₃ and Fe₂O₃ surfaces addressing the beneficial effects of Mo on the passivity breakdown of stainless steels in Cl-rich environments. Compared to Cr-rich zones, Fe-rich zones of the oxide barrier layer of the passive film are more prone to adsorbing Cl ions. Mo substitutes preferentially in these Fe-rich zones and favors the cationic vacancy formation to promote selective dissolution, thus curing the weak sites, sensitive to localized corrosion attack. In the Fe-rich weak sites with adsorbed Cl, Mo increases the barrier of O vacancy formation, thus mitigating passivity breakdown by inhibiting Cl penetration.