Understanding the stability of surface oxide films
Work has been conducted on model austenitic stainless steel single crystal surfaces on which reactivity towards oxygen was well controlled with sub-monolayer precision in order to better understand, at the nanometric and atomic scales, the mechanisms leading to the Cr enrichment in the surface oxide and their key role in providing passivity and stability to these alloys. This methodology, unconventional in corrosion science, enabled us to show that the pre-passivation oxidation mechanism is at the origin of iron-rich structural/chemical heterogeneities/defects generated at the nanoscale in the surface oxide, which was unknown before. Further work should help establishing that such nanoscale heterogeneities/defects are weak points for passivity in aggressive environments and understanding how they are cured in Mo-bearing alloys or can be reinforced by acting on the pre-passivation mechanisms.
Surface structure of austenitic stainless steel as studied at the nanometric and atomic scales on (100)-oriented FeCrNi single crystalline surfaces by scanning tunneling microscopy. Chromium oxide nucleation at steps pumps Cr atoms from the terraces causing local Cr depletion already in the very first buildup stage of the surface oxide providing corrosion protection.