Passivation-induced physicochemical alterations of the native surface oxide film on 316L austenitic stainless steel

TL;DR

This study combines advanced spectroscopic and electrochemical techniques to analyze how electrochemical passivation alters the physicochemical properties of the oxide film on 316L stainless steel, enhancing its corrosion resistance.

Contribution

It provides detailed insights into the compositional and electronic changes of the oxide film induced by passivation, revealing mechanisms behind improved corrosion protection.

Findings

Passivation causes enrichment of Cr(III) and Mo(VI) in the oxide layers.

The oxide film becomes more insulating after passivation due to compositional changes.

Passivation increases polarization resistance by approximately four times.

Abstract

Time of Flight Secondary Ion Mass Spectroscopy, X-Ray Photoelectron Spectroscopy, in situ Photo-Current Spectroscopy and electrochemical analysis were combined to characterize the physicochemical alterations induced by electrochemical passivation of the surface oxide film providing corrosion resistance to 316L stainless steel. The as-prepared surface is covered by a ~2 nm thick, mixed (Cr(III)-Fe(III)) and bi-layered hydroxylated oxide. The inner layer is highly enriched in Cr(III) and the outer layer less so. Molybdenum is concentrated, mostly as Mo(VI), in the outer layer. Nickel is only present at trace level. These inner and outer layers have band gap values of 3.0 and 2.6-2.7 eV, respectively, and the oxide film would behave as an insulator. Electrochemical passivation in sulfuric acid solution causes the preferential dissolution of Fe(III) resulting in the thickness decrease of the outer layer and its increased enrichment in Cr(III) and Mo(IV-VI). The further Cr(III) enrichment of the inner layer causes loss of photoactivity and improved corrosion protection with the anodic shift of the corrosion potential and the increase of the polarization resistance by a factor of ~4. Aging in the passive state promotes the Cr enrichment in the inner barrier layer of the passive film.