Passivation mechanisms and pre-oxidation effects on model surfaces of FeCrNi austenitic stainless steel

Abstract

Passivation mechanisms were investigated on (100)-oriented Fe-18Cr-13Ni surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and scanning tunneling microscopy and electrochemical characterization. Starting from oxide-free surfaces, pre-oxidation at saturation under ultra-low pressure (ULP) oxygen markedly promotes the oxide film Cr(III) enrichment and hinders/delays subsequent iron oxidation in water-containing environment. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species. Anodic passivation forces oxide film re-growth, Cr(III) dehydroxylation and further enrichment. ULP pre-oxidation promotes Cr(III) hydroxide formation at open circuit potential, compactness of the nanogranular oxide film and corrosion protection.

An XPS and ToF-SIMS study of the passive film formed on a model FeCrNiMo stainless steel surface in aqueous media after thermal pre-oxidation at ultra-low oxygen pressure

“…It is also interesting to note that we see a resurgence in the amount of iron metal at the surface relative to the amount of oxide, with a metal/oxide ratio of 1.9, which is the same ratio as previously seen after forming the ULP pre-oxidised surface. This is due to the dissolution of iron oxides in acidic media, which is in agreement with previous studies after OCP exposure where a closed system for transfer to surface analysis was used [34,35,38,48]. The main difference between these two oxide layers is that after the electrochemical treatment, we observe a much larger proportion of Fe 3+ to Fe 2+ than after the ULP pre-oxidation treatment.…”

“…however, the ULP pre-oxidised sample has a lower current density, indicating a slightly decreased proton reduction activity in the electrolyte. Similar behaviour in the cathodic branches has been previously reported [35].…”

“…The fitted XPS spectra for each stage are shown in Figure 3, with the corresponding relative intensities shown in [1,30,[33][34][35]. Once again, we observe the decrease in the amount of metal measured at the surface.…”

“…Upon anodically passivating the sample (−0.1 V vs Pt) for 30 minutes (Stage IV), the trend of a decreasing metal concentration is continued, with the of Cr 0 /Cr 3+ now 0.09. However, the ratio of oxide/hydroxide is increased to 1.40, confirming the promoting effect of anodic polarization on dehydroxylation [1,30,[33][34][35]48].…”

“…In recent years, it has been found that the distribution of chromium within the passive film is not homogeneous and that these local heterogeneities, which result from the initial growth mechanisms of the oxide film in the pre-passivation stage, may result in the eventual localised breakdown of the passive film [21,[30][31][32][33][34][35]. It is therefore imperative to better understand the role played by pre-oxidation on the initial stages of passive film formation, as it may improve our knowledge on how we can better combat the localised breakdown and subsequent corrosion.…”