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 bandgap 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.
Highlights
Stainless steels (SS) are widely used thanks to their excellent mechanical properties and corrosion resistance
The Cr(III) enrichment may not be homogeneous in the passive film, as suggested by recent studies performed at the nanometer scale,[27,41] and the heterogeneities may cause the local failure of the corrosion resistance and of the initiation of localized corrosion.[42]
In the surface oxide region, the most predominant profiles of the oxidized metals are those of the CrO2−, FeO2− and to a lesser extent MoO2− ions while that of the NiO2− ions has much lower intensity, suggesting that only traces of oxidized nickel are present in the native oxide film as supported by the X-Ray Photoelectron Spectroscopy (XPS) analysis discussed below and in agreement with previous data.[20,21,22,27,28,29,30]
Summary
No series of five peaks associated to FeII in the surface oxide layer was needed for reconstruction.[48] In the Mo 3d5/2-3/2 core level spectrum (Fig. 3c), the 5/2-3/2 doublet (Mo1/Mo1’) of the metallic Mo° component is positioned at 227.5–230.6 eV.[7,36] The 5/2-3/2 doublets at 229.5-232.6 eV (Mo2/Mo2’) and 232.6–235.7 eV (Mo3/Mo3’) are assigned to MoIV and MoVI in the surface oxide layer, respectively.[22,27,31,33,36,53] The intensity ratio of the MoVI to MoIV doublets is ∼8, indicating that MoVI species are mostly present.
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