Passivation-Induced Cr and Mo Enrichments of 316L Stainless Steel Surfaces and Effects of Controlled Pre-Oxidation

Passivation mechanisms and the effects of controlled pre-oxidation, by exposure to oxygen at ultra-low pressure, on Cr and Mo surface enrichments were investigated on polycrystalline AISI 316L stainless steel surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and electrochemistry. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species, which promotes Cr³⁺ and Mo^4+/6+ enrichments. Anodic passivation forces oxide film re-growth and Cr³⁺ dehydroxylation with no loss of Mo^4+/6+ pre-enrichment. Ultra-low pressure pre-oxidation promotes Mo^4+/6+ enrichment in the exchange outer hydroxide layer of the passive film, with no Mo⁰ depletion in the modified alloy region underneath the oxide film at open circuit potential, and under anodic passivation. Mo^4+/6+ enrichment improves protectiveness against transient active dissolution during the active/passive transition.