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 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 band gap 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.

Chloride-induced alterations of the passive film on 316L stainless steel and blocking effect of pre-passivation

“…Figure 5b compares the CrO2and FeO2ions profiles. The profiles do not peak at the same position, which is in agreement with the bilayer structure having iron and chromium oxides more concentrated in the outer and inner layers, respectively, like reported for passivation in the absence of chlorides [7,17,27,46,47]. The interface between outer and inner layers was positioned at 5 s which is the median sputtering position between the two intensity maxima.…”

“…As suggested by recent nanometer scale studies [27,48], the Cr(III) enrichment may not be homogeneous in the passive film, and the Cr enrichment heterogeneities may cause the local failure of the passivity and the initiation of localized corrosion followed by pit growth where the passive film fails to self-repair [49]. The better understanding of the mechanisms governing the Cr (and Mo) enrichment requires to thoroughly investigate the initial stages of oxidation leading to pre-passivation of the SS surface [50,51,52] as well as the alterations brought by electrochemical passivation of the native oxide-covered SS surface [46,47]. successive 1200 and 2400 grades and then with diamond suspensions of successive 6, 3, 1 and 0.25 μm grades.…”

“…Their main effect is to produce a less resistive passive state by poisoning dehydroxylation and further Cr(III) and Mo(IV,VI) enrichments obtained in the absence of chlorides. This detrimental effect can be suppressed by pre-passivation in a Cl-free electrolyte, which blocks the entry of chlorides in the passive film, including in the outer exchange layer, and enables the beneficial aging-induced variations of the composition to take place despite the presence of chlorides in the environment.On AISI 316L, recent surface analytical studies performed by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) [27,46,47] confirmed the bilayer structure of the passive film previously reported [21,22,27,30,32]. The native oxide film formed in air was found to already develop this bilayer structure with both the hydroxide outer and oxide inner layers enriched in Cr(III) and with Fe(III) more concentrated in the outer layer together with Mo(IV,VI).…”

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

“…The results obtained with the single layer model are compiled in Table 3. The thickness values of the oxide layer for the glove box-and air-formed films and for the passive films formed by anodic polarization ranges from 1.2 to 1.6 nm, which is in the lower range of the typical thickness of 1−3 nm for room temperature native oxide films and passive films on stainless steels [11][12][13][14][15]20,21,22,24,38,40,[44][45][46][47]. The oxide film formed in the glove box environment has the same thickness (1.3-1.4 nm) after 5 or 30 minutes but becomes slightly further enriched in chromium as a result of chromium hydroxide formation after prolonged exposure.…”

“…Layered models are often used in the literature to calculate oxide film thickness and composition from XPS data based on the exponential decrease of the photoelectron intensity with increasing depth of emission from the topmost surface plane [13,14,[20][21][22][23][24]40,[41][42][43][44][45]. Here we have applied two types of models in order to discuss the passive layer chemical structure: a single layer model ( Figure 6(a)) and a bilayer model ( Figure 6(b)).…”