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

Abstract

Electrochemical polarization measurements were combined with surface analysis by Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), X-Ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) to study the alterations of the passive film on 316L austenitic stainless steel induced by the presence of chlorides in sulfuric acid electrolyte. The work was performed at a stage of initiation of localized corrosion preceding metastable pitting at the micrometer scale as verified by current transient analysis and AFM. The results show that Clions enter the bilayer structure of the surface oxide already formed in the native oxide-covered initial surface state at concentrations below the detection limit of XPS (< 0.5 at%), mostly in the hydroxide outer layer where Fe(III) and Mo(IV,VI) species are concentrated but barely in the oxide inner layer enriched in Cr(III). 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). Electrochemical passivation in chloride-free sulfuric acid solution did not alter the bilayer structure and thickness of the surface oxide but promoted its Cr and Mo enrichments, owing to the lower stability and preferential dissolution of Fe(III) as previously proposed [4,16,53], and thereby increased the corrosion resistance of the passive state.In the present work, we address the alterations of the surface oxide films brought by electrochemical passivation in Cl-containing sulfuric acid solutions. Potentiodynamic and potentiostatic polarization measurements were used to define the electrochemical conditions best suited to alter the passive state without initiating localized corrosion (i.e metastable pitting) at the micrometer scale. ToF-SIMS and XPS were applied to characterize the bilayer structure, thickness and composition of the passive film and the entry of chlorides. Surface morphology was studied by Atomic Force Microscopy (AFM).The results provide new insight on the effect of pre-passivation on the entry of chlorides in the passive film and increased resistance to metastable pitting at the nanometer scale. ExperimentalThe same polycrystalline AISI 316...

Interplay between the composition of the passive film and the corrosion resistance of citric acid‐passivated AISI 316L stainless steel

“…Metallic chromium (C rmet ) was detected at the lowest binding energy (574 eV), whereas Cr 3+ species appear as Cr 2 O 3 and Cr(OH) 3 . The metallic component is attributed to the substrate beneath the passive film, as reported by other authors 19 . Cr 3+ species are also reported to be the major chromium species in the passive films formed on stainless steels, either passivated or not 20–22 .…”

“…The metallic component is attributed to the substrate beneath the passive film, as reported by other authors. 19 Cr 3+ species are also reported to be the major chromium species in the passive films formed on stainless steels, either passivated or not. [20][21][22] As the main passivating agent, the amount of Cr 2 O 3 in the passive film is critical to its stability against localized corrosion.…”

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

“…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)).…”

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

“…In chloride-rich environments the breakdown of the passive film can lead to a phenomenon known as pitting corrosion [14][15][16][17][18], which can cause structural and aesthetic damage, resulting in the potential endangerment of lives and high costs of repair. However, by alloying with small amounts of molybdenum this effect is strongly combated, with molybdenum found in small amounts in the passive film, without increasing the thickness of the film [19][20][21][22][23][24][25][26][27][28][29]. The exact role of molybdenum in fighting against pitting corrosion is a highly debated issue within the community.…”