Revisiting the effect of molybdenum on pitting resistance of stainless steels

As an important alloyed element to improve the localized corrosion resistance of stainless steels, Mo has been widely studied by researchers. This article reviews the mechanisms of the effect of Mo on localized corrosion. Two possible effects and corresponding experimental research are described respectively: Mo exists in the passive film as oxide and enhances the film stability (manifesting as lower nucleation probability of localized corrosion and longer delay time of breakdown of the passive film); Mo affects dissolution kinetics and inhibits active dissolution current density (manifesting as smaller metastable pitting current density and size). Then some contradictory results are discussed by considering the pitting model proposedrecently. Strictly speaking, none of the existing experimental results can deny the effect of Mo on the passive film or on the dissolution kinetics. It is reasonable to believe that Mo affects the entire process of localized corrosion rather than a single reaction. Finally, some possible research suggestions are put forward.