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

Abstract

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 Mo4+/6+ enrichments. Anodic passivation forces oxide film re-growth and Cr3+ dehydroxylation with no loss of Mo4+/6+ pre-enrichment. Ultra-low pressure pre-oxidation promotes Mo4+/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. Mo4+/6+ enrichment improves protectiveness against transient active dissolution during the active/passive transition.