Passivation-Induced Physicochemical Alterations of the Native Surface Oxide Film on 316L Austenitic Stainless Steel

Abstract

Time of Flight Secondary Ion Mass Spectroscopy, X-Ray Photoelectron\nSpectroscopy, in situ Photo-Current Spectroscopy and electrochemical analysis\nwere combined to characterize the physicochemical alterations induced by\nelectrochemical passivation of the surface oxide film providing corrosion\nresistance to 316L stainless steel. The as-prepared surface is covered by a ~2\nnm thick, mixed (Cr(III)-Fe(III)) and bi-layered hydroxylated oxide. The inner\nlayer is highly enriched in Cr(III) and the outer layer less so. Molybdenum is\nconcentrated, mostly as Mo(VI), in the outer layer. Nickel is only present at\ntrace level. These inner and outer layers have band gap values of 3.0 and\n2.6-2.7 eV, respectively, and the oxide film would behave as an insulator.\nElectrochemical passivation in sulfuric acid solution causes the preferential\ndissolution of Fe(III) resulting in the thickness decrease of the outer layer\nand its increased enrichment in Cr(III) and Mo(IV-VI). The further Cr(III)\nenrichment of the inner layer causes loss of photoactivity and improved\ncorrosion protection with the anodic shift of the corrosion potential and the\nincrease of the polarization resistance by a factor of ~4. Aging in the passive\nstate promotes the Cr enrichment in the inner barrier layer of the passive\nfilm.\n