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

摘要

Stainless steels are widely used thanks to their excellent mechanical properties and corrosionresistance, the latter resulting from self-protection provided by a continuous and protective surfaceoxide/hydroxide layer, the passive film, markedly enriched in Cr(III) and only a few nanometersthick when formed at ambient temperature. The Cr(III) enrichment may not be homogeneous in thepassive film, as suggested by recent studies performed at the nanometer scale, and theheterogeneities may cause the local failure of the corrosion resistance and the initiation oflocalized corrosion. These heterogeneities of Cr enrichment would find their origin in themechanisms of pre-passivation leading to the initial formation, most often in air, of the native oxideand in the subsequent modifications of the oxide film induced by immersion in solution.In this work, the passivation-induced modifications of the native surface oxide film formed on 316Laustenitic stainless steel and their effects on the corrosion protection properties were studied bycombining XPS (X-Ray Photoelectron Spectroscopy) and ToF-SIMS (Time of Flight Secondary IonMass Spectroscopy) for surface analysis, in situ PCS (Photocurrent Spectroscopy) forcharacterization of the electronic properties and electrochemical analysis of the corrosionresistance. The results show that the as-prepared 316L SS surface is covered by an ultra-thin (~2nm), mixed (Cr(III)-Fe(III)) and bi-layered hydroxylated oxide. The inner layer is highly enriched inCr(III) and the outer layer less so. Molybdenum is concentrated, mostly as Mo(VI), in the lessCr(III)-enriched outer layer. Nickel is only present at trace level. These inner and outer layers haveband gap values of 3.0 and 2.6-2.7 eV, respectively, and the oxide film behaves as an insulatorwith an inversion potential of ~ - 0.4 V vs. Ag/AgCl at pH ~ 9.5. Electrochemical passivation insulfuric acid solution causes the preferential dissolution of Fe(III) resulting in the thicknessdecrease of the outer layer, its increased enrichment in Cr(III) and Mo(IV-VI) and the further Cr(III)enrichment of the inner layer. A loss of photoactivity in the energy region associated with the innerlayer. The electrochemical data evidenced improved corrosion protection with the anodic shift wasobserved the corrosion potential and the increase of the polarization resistance by a factor of ~4.