The role of corrosion-resistant alloying elements in passivity

Passivity of alloys containing corrosion-resistant elements were reviewed. Chromium and valve metals except aluminum form stable oxyhydroxide films even in aggressive hydrochloric acids. Molybdenum forms a passive MoO2 film in the active region of stainless steels and hence decreases the active dissolution current. In the passive region of transition metals and valve metals, molybdenum is generally in the transpassive state and dissolved. However, if the outer oxyhydroxide film is stable the inner MoO2 film is protected by the outer oxyhydroxide film and the MoO2 film acts as the effective barrier against diffusion of matters through the film. Thus the passive current density of 30Cr–2Mo ferritic stainless steel is more than two orders of magnitude lower than that of 30Cr steel without molybdenum in 1 M HCl.