Passivation-Induced Cr and Mo Enrichments of 316L Stainless Steel Surfaces and Effects of Controlled Pre-Oxidation

Abstract

Passivation mechanisms and the effects of controlled pre-oxidation, by exposure to oxygen at ultra-low pressure, on Cr and Mo surface enrichments were investigated on polycrystalline AISI 316L stainless steel surfaces with direct transfer between surface preparation and analysis by X-ray photoelectron spectroscopy and electrochemistry. Exposure to sulfuric acid at open circuit potential causes preferential dissolution of oxidized iron species, which promotes Cr 3+ and Mo 4+/6+ enrichments. Anodic passivation forces oxide film re-growth and Cr 3+ dehydroxylation with no loss of Mo 4+/6+ pre-enrichment. Ultra-low pressure preoxidation promotes Mo 4+/6+ enrichment in the exchange outer hydroxide layer of the passive film, with no Mo 0 depletion in the modified alloy region underneath the oxide film at open circuit potential, and under anodic passivation. Mo 4+/6+ enrichment improves protectiveness against transient active dissolution during the active/passive transition.

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.…”

“…XPS fitting parameters, in line with those used in previous work [38], for the Mo 3 Spin orbit splitting between Mo 3 d 5/2 and Mo 3 d 3/2 was found to be 3.2 eV for all peaks except for Mo 4+ ((oxy)hydroxide), which was 3.3 eV. These values are in line with those reported in the literature [33,41,42].…”

“…The anodic branch shows that the current density of the ULP prepared surface is comparatively lower at the active-passive transition than that of the native oxide covered surface prior to passivation and with clear suppression of the active peak. Similar observations have previously been made when comparing native oxide covered and room temperature pre-oxidised ULP surfaces on polycrystalline 316L samples [38]. We can therefore deduce that the ULP pre-oxidised sample, prepared at 250 • C and exposed to an acidic electrolyte, is fully protected against active dissolution, much like the ULP pre-oxidised samples prepared at room temperature, whilst the native oxide covered samples, formed in air or an Ar-filled glovebox, remain only partially protected.…”

“…By taking into account that the thickness of the (100) plane for the fcc structure of the alloy is 0.18 nm, and that the atomic density is 1.55×10 15 at•cm −2 , we arrive at a value of 0.07 nm, not taking account the stoichiometry of dissolution, which corresponds to less than one monolayer (0.38 ML) of additionally consumed metal. In the case of the polycrystalline sample [38], the charge transfer difference at the active-passive transition was reported as 0.38 mC•cm −2 , twice the amount of the present case, which yields a value of 0.76 ML of additionally consumed metal for the native oxide-covered sample, assuming a (100) orientation for the calculation. Therefore, we can say that the presence of ULP and native oxide films provides protection and pre-passivation at OCP on both FeCrNiMo single and 316L poly crystalline samples.…”

“…The passive oxide films were formed by performing anodic passivation in an electrochemical cell controlled by a Picoscan potentiostat and the corresponding Picoscan software from Agilent Technologies. All treatments were carried out in a home-made Kel-F electrochemical cell [38,39] The spectra were recorded at take-off angles of 45 • and 90 • . Only data for the former are shown.…”