Abstract
An approach preventing contact to ambient air during transfer from liquid environment for electrochemical treatment to UHV environment for surface analysis by X-Ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry was applied to study the mechanisms of Cr and Mo enrichments in the passive oxide film formed on 316L austenitic stainless steel. Starting from the air-formed native oxide-covered surface, exposures were conducted in aqueous sulfuric acid solution first at open circuit potential and then under anodic polarization in the passive range. At open circuit potential the thickness of the bi-layered oxide film was observed to decrease and the enrichments of both Cr(III) and Mo, mostly Mo(VI), to markedly increase as well as the film hydroxylation. This is due to preferential dissolution of the Fe(III) oxide/hydroxide, not compensated by oxide growth in the absence of an electric field established by anodic polarization. Anodic polarization in the passive domain causes the bi-layered structure of the oxide film to re-grow by oxidation of iron, chromium and molybdenum, without impacting the Cr enrichment and only slightly mitigating the Mo enrichment. De-hydroxylation of the inner layer is also promoted upon anodic polarization. These results show that the treatment of the surface oxide film in acid solution at open circuit potential enhances Cr and Mo enrichments and promotes hydroxylation. Passivation by anodic polarization allows dehydroxylation, yielding more Cr oxide, without markedly affecting the Mo enrichment, also beneficial for the corrosion resistance.
Highlights
Stainless steels (SS) are important technological materials of widespread application that combine excellent mechanical and corrosion properties
The position of the “modified alloy” region between the oxide film and metallic bulk substrate regions, where Ni is found enriched in agreement with previous studies on austenitic stainless steels (Olefjord and Elfström, 1982; De Vito and Marcus, 1992; Olsson and Hörnström, 1994; Hakiki et al, 1998; Maurice et al, 1998, 2015; Bojinov et al, 2001; Yamamoto et al, 2009; Wang et al, 2019), was defined using the Ni−2 ions intensity profile and placed between 70 and 130 s of sputtering
TsthhhooeswNesioOthf−2athtietohnCesrFaOerO−2e −2tahnaednledFaCestOrOi−2n−2teionionsnes.sfoplrloofiwleesdpbeyakthaet different positions, at 20 and 40 s, respectively. This is consistent with the native oxide film having a bilayer structure with iron and chromium oxides more concentrated in the outer and inner layers, respectively, as commonly reported for oxide films on stainless steels (Marcus and Olefjord, 1988; Mischler et al, 1991; Maurice et al, 2015; Wang et al, 2019)
Summary
Stainless steels (SS) are important technological materials of widespread application that combine excellent mechanical and corrosion properties. The maxima of the MoO−2 and FeO−2 ions profiles are still observed at the same sputtering time in the outer layer, with a mtohxaeidrMkeeo(dFOidg−2eucirroeena3ssce)s.uinCggoiennssttieinsntgseintthytleyf,omtrhaetrhkpeerdoFfipeOlrees−2feorifeontnhtsieaclColromOssp−2ao/rFfeedirOot−2no intensity ratio, still increasing from extreme surface to inner part in the oxide film region, is overall higher (∼6 vs ∼3 at saturation in the inner layer), suggesting an increase of the Cr enrichment of the oxide film due to iron oxide dissolution induced by immersion in sulfuric acid at open circuit potential.
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