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In situ X-ray photoelectron spectroscopy real-time measurements and angular-dependent high resolution core level analysis were used for the first time to investigate the Cr enrichment and oxide growth mechanisms on a model 304 austenitic stainless steel surface in the very initial stages of oxidation leading to pre-passivation. The oxidation kinetics was followed for increasing oxygen exposure and temperature, revealing an early nucleation regime (for exposure <10 L) leading to the formation of a strongly Cr-enriched Cr 3+ /Fe 3+ mixed layer followed by an oxide growth regime where preferential iron oxidation takes over and mitigate the initial chromium enrichment. [ABSTRACT FROM AUTHOR]

High resolution X-ray photoelectron spectroscopy analysis has been used to investigate, in-situ, the mechanisms for oxide nucleation and growth on a FeCrNiMo single crystal alloy. Ultra-high vacuum conditions provided the ideal environment in which an oxide-free surface could be oxidised in a controlled manner, using extremely low pressures and step-wise exposures of oxygen. In-situ snapshot measurements, which records an XPS spectrum every 10 s, provided insight into the mechanisms of chromium and nitrogen segregation during annealing of the sample. High resolution XPS spectra was used to follow the evolution of the key elements at step-wise intervals, allowing for detailed insights into the nucleation and growth mechanisms. Angular XPS analysis provided further information on the in-depth distribution and stratification of the oxide film. [Display omitted] • In-situ XPS indicates two different mechanisms for Cr segregation during annealing. • Surface rearrangement results in CrN behaving as nucleation site for Cr 2 O 3. • Oxidation of Mo is delayed until after formation of Fe and Cr oxide has begun. • Oxide growth became dominated by iron oxide owed to increased Fe cation mobility. • Fe2+ and Cr3+ became enriched at the oxide–metal alloy interface. [ABSTRACT FROM AUTHOR]

• Surface heterogeneities originating from the local supply of metallic Fe and Cr. • Preferred Fe oxide nucleation in the initial stages (exposures < 3 L). • Cr3+ oxide nucleation delayed to exposures above 3 L. • Cr-rich oxide nanogranular film homogeneously covering the surface at saturation. Scanning tunneling microscopy was applied to investigate the topographical and structural alterations at small spatial and temporal scales of a model 304 austenitic stainless steel surface resulting from ultra-low exposures to gaseous oxygen at room temperature. The 2D nucleation and growth mechanisms were observed until surface saturation by a fully covering and nanogranular Cr(III)-enriched oxide film. Local supply of metallic Fe and Cr for oxide nucleation and growth is concluded to be at the origin of heterogeneity developed in the oxide film at the nanoscale. This work provides nanoscale understanding basis for composition and morphology tuning of Cr-enriched oxide film for improved local stability of passivity. [ABSTRACT FROM AUTHOR]

[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]

• Passivation mechanisms studied on Fe-18Cr-13Ni(100) starting from oxide-free surfaces. • Direct transfer between surface preparation/analysis and electrochemical characterization. • Oxidized iron preferentially dissolved in sulfuric acid at open circuit potential. • Oxide film re-growth and Cr(III) dehydroxylation forced by anodic passivation. • Cr(III) enrichment and passivity promoted by UHV pre-oxidation. Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and scanning tunneling microscopy and electrochemical characterization. Starting from oxide-free surfaces, pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron oxidation in water-containing environment. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species. Anodic passivation forces oxide film re-growth, Cr(III) dehydroxylation and further enrichment. ULP pre-oxidation promotes Cr(III) hydroxide formation at open circuit potential, compactness of the nanogranular oxide film and corrosion protection. [ABSTRACT FROM AUTHOR]