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

International audienceElectrochemical 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 Cl-ions 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

Passivation-Induced Physicochemical Alterations of the Native Surface Oxide Film on 316L Austenitic Stainless Steel

International audienceTime 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

Marine Microorganisms and Metal interaction: The Start Point of a New Bio solution for Corrosion Protection [Resumo]

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Role of SiC substrate surface on local tarnishing of deposited silver mirror stacks

International audienceThe role of the SiC substrate surface on the resistance to the local initiation of tarnishing of thin-layered silver stacks for demanding space mirror applications was studied by combined surface and interface analysis on model stack samples deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. It is shown that suppressing the surface pores resulting from the bulk SiC material production process by surface pretreatment eliminates the high aspect ratio surface sites that are imperfectly protected by the SiO2 overcoat after the deposition of silver. The formation of channels connecting the silver layer to its environment through the failing protection layer at the surface pores and locally enabling H2S entry and Ag2S growth as columns until emergence at the stack surface is suppressed, which markedly delays tarnishing initiation and thereby preserves the optical performance. The results revealed that residual tarnishing initiation proceeds by a mechanism essentially identical in nature but involving different pathways short circuiting the protection layer and enabling H2S ingress until the silver layer. These permeation pathways are suggested to be of microstructural origin and could correspond to the incompletely coalesced intergranular boundaries of the SiO2 layer

Local Degradation Mechanisms by Tarnishing of Protected Silver Mirror Layers Studied by Combined Surface Analysis

International audienceIn this work, we addressed the local degradation mechanisms limiting the pre-launch environmental durability of thin-layered silver stacks for demanding space mirror applications. Local initiation and propagation of tarnishing were studied by combined surface and interface analysis on model stack samples consisting of thin silver layers supported on light-weight SiC substrates and protected by thin SiO2 overcoats, deposited by cathodic magnetron sputtering and submitted to accelerated aging in gaseous H2S. The results show that tarnishing is locally initiated by the formation of Ag2S columns erupting above the stack surface. Ag2S growth is promoted at high aspect ratio defects (surface pores) of the SiC substrate as a result of an imperfect protection by the SiO2 overcoat. Channels most likely connect the silver layer to its environment through the protection layer, which enables local H2S entry and Ag2S growth. The silver sulfide columns grow in number and size eventually leading to coalescence with increasing H2S exposure. In more advanced stages, tarnishing slows down owing to saturation of all pre-existing imperfectly protected sites of preferential sulfidation. However, it progresses radially at the basis of the Ag2S columns underneath the protection layer, consuming the metallic silver layer and deteriorating the protecting overcoat

Earth Virtualization Engines (EVE)

To manage Earth in the Anthropocene, new tools, new institutions, and new forms of international cooperation will be required. Earth Virtualization Engines is proposed as an international federation of centers of excellence to empower all people to respond to the immense and urgent challenges posed by climate change

Contribution of surface spectroscopic techniques to understanding of conversion coatings formation on aluminium alloys

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Role of Segregated Iron at Grain Boundaries on Mg Corrosion

International audienc

Effect of high temperature oxidation process on corrosion resistance of bright annealed ferritic stainless steel

The corrosion resistance of stainless steels is determined by the thickness, structure, composition and electronic properties of the oxide layers grown on their surface and isolating the metallic substrate from the environment. In the present work, ex situ XPS (X-Ray Photoelectron Spectroscopy) and ToF-SIMS (Time of Flight Secondary Ions Mass Spectrometry) have been combined to in situ PCS (PhotoCurrent Spectroscopy) in order to perform an integrated physicochemical characterization of surface oxide films grown on bright annealed ferritic stainless steel (AISI 434) as-received and after thermal post-treatment in air. The surface oxide film on as-received samples has a bilayer structure with an inner layer close to pure Cr(III) oxide and an outer layer of mixed Fe(III) and Cr(III) oxide enriched in Cr. After thermal post-treatment in air at 350°C and 550°C, the surface oxide films thicken and the outer layers are enriched in Fe(III). Linear voltammetry and electrochemical impedance spectroscopy analysis evidence a decrease of the polarization resistance in the passive range, a cathodic shift of the corrosion potential and an increased cathodic current after thermal post-treatment. The bilayer structure, composition and electronic properties of these surface oxide films and their modifications are discussed to rationalize their corrosion behavior