An XPS and ToF-SIMS study of the passive film formed on a model FeCrNiMo stainless steel surface in aqueous media after thermal pre-oxidation at ultra-low oxygen pressure.

[Display omitted] • Thermal pre-oxidation effects on passive film properties studied on stainless steel. • Combined surface analytical and electrochemical approach applied with closed transfer. • Thermal pre-oxidation improves corrosion resistance at active/passive transition. • Thermal pre-oxidation promotes the enrichment of molybdenum and chromium. • Thermal pre-oxidation enhances growth of the inner layer after anodic passivation. A combined X-ray photoelectron spectroscopy, time-of-fight secondary ion mass spectrometry and electrochemical approach was used to investigate the mechanisms of molybdenum and chromium enrichment of a passive film formed on a model austenitic stainless steel single crystal in aqueous acid media and the effect that thermal pre-oxidation at 250 ° C plays in the composition, thickness and stratification of the film. A clean FeCrNiMo (1 0 0)-oriented surface, prepared under ultra-high vacuum (UHV), was exposed to ultra-low pressures of oxygen at 250 ° C, forming a thin oxide film, rich in iron and chromium, as well as containing molybdenum. An argon-filled glovebox, directly attached to the UHV system, allowed for electrochemical alterations to be carried out without exposure to ambient air. After being held at open circuit potential for 30 min, hydroxylation of chromium and molybdenum was promoted, with a marked enrichment of both elements in the oxide film. A distinct bilayer film was observed. Anodic polarisation resulted in a significant growth of the inner layer and further enrichment of chromium. Pre-oxidation at 250 ° C, using an ultra-low pressure of oxygen, was found to promote protection against transient active dissolution during anodic passivation, and to cause enrichment of molybdenum in the outer layer of the passive film. [ABSTRACT FROM AUTHOR]