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X‐ray photoelectron spectroscopy analysis was applied to investigate the thermal stability under ultra‐high vacuum environment of the surface oxide film formed by electrochemical passivation of a newly designed Cr15Fe10Co5Ni60Mo10 (at.%) multi‐principal element alloy and providing the alloy superior localized corrosion resistance compared to conventional stainless steels and alloys. A spectral decomposition methodology involving the subtraction of Auger peaks overlapping the Fe 2p and Co 2p core level regions was applied for quantification of the oxide film composition and thickness. The results show that, at 100°C, the passive oxide film is mainly dehydrated and dehydroxylated. Obvious loss of Ni hydroxide and conversion of Mo (VI) to Mo (IV) species are observed at 200°C, with further reduction of Mo species to Mo (III) observed at 300°C. In this temperature range, the total cation quantity in the oxide film remains stable despite the compositional alteration. At 400°C, Cr (III) oxide forms at the expense of Fe and Mo oxides, resulting in an oxide film essentially consisting of chromium oxide. At 500°C, Cr (III) oxide is eliminated, making the passive film unstable at this temperature. Possible Cr oxide removal mechanisms are discussed. [ABSTRACT FROM AUTHOR]

X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry were combined to characterize the surface oxides (native oxides and passive films) formed on Ni-based alloys containing chromium and molybdenum. Two alloys were studied, Ni-20Cr and Ni-20Cr-10Mo (in wt.%). For Ni-20Cr, both native oxide and passive films formed in acidic medium present a duplex structure comprising a Ni and Cr outer hydroxide layer and a Cr inner oxide layer. The Ni-20Cr-10Mo alloy presents a similar bilayer structure, but with Mo oxide located at the outer layer/inner layer interface. Cr enrichment is observed after passivation for both alloys. The corrosion resistance in acidic solution containing chlorides is enhanced by Mo, and by electrochemical pre-passivation in Cl free solution. [ABSTRACT FROM AUTHOR]

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 Cr3+ and Mo4+/6+ enrichments. Anodic passivation forces oxide film re-growth and Cr3+ dehydroxylation with no loss of Mo4+/6+ pre-enrichment. Ultra-low pressure preoxidation promotes Mo4+/6+ enrichment in the exchange outer hydroxide layer of the passive film, with no Mo0 depletion in the modified alloy region underneath the oxide film at open circuit potential, and under anodic passivation. Mo4+/6+ enrichment improves protectiveness against transient active dissolution during the active/passive transition [ABSTRACT FROM AUTHOR]

The composition and structure of the native and passive oxide films formed on 316 L stainless steel have been studied in situ by ToF-SIMS. High temperature re-oxidation experiments in isotopic 18O2 gas have also been done to assess the ion transport mechanisms in the native and passive oxide films. Duplex oxides with an inner Cr rich layer and an outer layer rich in Fe and Mo oxide have been observed on native and passive oxide films. Exposure of the oxide films to isotopic 18O2 tracer at 300 °C reveals that the outward cationic diffusion governs the inner oxide growth. The outer Mo-rich layer prevents the continued transport of Cr to the outermost surface. The passive film, due to its composition and structure, exhibits a markedly lower oxidation rate compared to native oxide films. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]

Highlights • Current developments of nanoscale insight into passive oxide films. • Cr(III) enrichment and its homogeneity at the nanoscale in passive films. • Corrosion properties of grain boundaries in early intergranular corrosion. • Interaction of organic inhibitor molecules with passivated metallic surfaces. Abstract Oxide passive films are a key for the durability of metals and alloys components as well as a central issue in corrosion science and engineering. Herein, we discuss current developments of the nanometer and sub-nanometer scale knowledge of the barrier properties and adsorption properties of passive oxide films brought by recent model experimental and theoretical investigations. The discussed aspects include (i) the chromium enrichment and its homogeneity at the nanoscale in passive films formed on Cr-bearing alloys such as stainless steel, (ii) the corrosion properties of grain boundaries in early intergranular corrosion before penetration and propagation in the grain boundary network, and (iii) the interaction of organic inhibitor molecules with incompletely passivated metallic surfaces. In all three cases, key issues are highlighted and future developments that we consider as most relevant are identified. [ABSTRACT FROM AUTHOR]

The oxidation of chromium at 300 °C was investigated in situ by ToF -SIMS for three different oxygen pressures ( $$P_{{{\text{O}}_{2} }} = 2.0 \times 10^{ - 7}$$ , 6.0 × 10 and 2.0 × 10 mbar). Sequential exposure to the O isotopic tracer was performed to reveal the governing transport mechanism in the oxide film. The evolution of the oxide thickness was monitored. Volatilization of CrO was evidenced. A model was used to describe the kinetics resulting from the measurements. Both the parabolic and volatilization constants showed a dependence on oxygen partial pressure like $$P_{{{\text{O}}_{2} }}^{ - 1/n}$$ , with n = 1.9 ± 0.1, indicating a defect structure mainly consisting of oxygen vacancies. The re-oxidation in O shows a growth of the oxide layer at the metal/oxide interface, demonstrating an oxidation process governed by anionic transport via oxygen vacancies. The diffusion coefficient of oxygen in the oxide was determined by fitting the ToF -SIMS depth profiles. It is 2.0 × 10 cm s. [ABSTRACT FROM AUTHOR]

We modelled with Density Functional Theory (DFT) an Al-Cu alloy covered with a passive film, with several Cu concentrations (from the limit of the isolated atom to the monolayer) at the interface with the oxide, as well as Guinier-Preston 1 (GP1) zones. At low (respectively high) concentration, Cu segregates in the first (respectively second) metal layer underneath the passive film. The Cu monolayer is the most stable configuration (-0.37 eV/Cu atom). GP1 zones were modelled, with a three-copper atom cluster in the alloy. The GP1 zone is slightly favoured with respect to the Cu monolayer under the oxide film. A low (respectively high) Cu concentration induces an electronic workfunction increase (respectively decrease) by 0.3 eV (respectively -0.4 to -0.6 eV) as compared to pure Al. In contrast, without oxide, Cu segregation at the Al surface induces no workfunction change at low concentration and an increase of 0.3 eV of the workfunction at high concentration. Thus, the presence of oxide modifies the expected tendency of workfunction increase by adding a more noble metal. For the studied models, no spontaneous electron transfer occurs to the O2 molecule. [ABSTRACT FROM AUTHOR]

A number of passive film growth models have been advanced in the past, each founded on different assumptions and providing different rate laws. A new, generalized growth model has been developed and the numerical simulation of the transient behavior of oxide film growth on metals and alloys is reported here. This model is based on the transport of anions and cations (in a Cr2O3 film for stainless alloy). In contrast to most models up to date, it describes the time-variant behavior of film and film/solution potential differences. Because these potential differences are expressed as exponential functions of film thickness, asymptotic behaviors can be studied and it is shown that a square root law results for very thin and very thick films. Growth rate for intermediate films and during transient periods, on the other hand, is determined by numerical integration. Parameters which were introduced by the model and whose values are unknown are conveniently regrouped into only four lumped parameters, which are easily estimated by numerical optimisation. It is shown that experimental oxide film thickness measurements on stainless alloys in high-temperature aqueous environments can be reproduced. It is further shown that the diffusional rate expressions of all the species contributing to growth are analogous, explaining why it is possible to fit growth kinetics with a single type of point defect. [ABSTRACT FROM AUTHOR]

Oxide layers play a crucial role in the corrosion resistance of metals and alloys and the growth kinetics of these films is of major interest. To express the oxide growth kinetics, three main models are available: the Cabrera-Mott model, the Fehlner-Mott model and the Point Defect Model (PDM). These models are reviewed in the first part of the paper. Among these models, the PDM is the only one that takes into account the interfacial potential drops during the oxide growth. However, in this model: (i) no parameters relative to the substrate alloy are taken into account, and (ii) the growth is limited by the flow of oxygen vacancies through the film (transport via both cation interstitial and vacancy positions are not taken into account). Here we present a "generalized model" for the kinetics of oxide growth in which the evolution of the interfacial potential drops during oxide growth is included, as well as the variation of the electric field in the oxide during film growth. This new model allows us to describe the growth of oxide films on alloys under non-steady-state conditions. The link between oxide growth and cation release into the solution is also included. [ABSTRACT FROM AUTHOR]

Stainless steels are widely used as metal components owing to self-protection in aggressive environments, provided by an extremely thin surface oxide film enriched in chromium oxide. Yet, despite decades of research, the mechanisms distributing the chromium enrichment at small length scale are poorly understood, although it may cause loss of stability and local failure of the corrosion resistance. Here, we apply high resolution surface analysis to investigate at small time and length scales the nucleation and growth mechanisms of the surface oxide on a model stainless steel. Starting from an oxide-free surface, we report the direct observation of the oxide nucleation and local oxidation of chromium, which governs the nanoscale heterogeneity of the growing surface oxide by chromium pumping from the atomic terraces to the steps for preferential Cr(III) oxide nucleation and subsequently by segregation from the atomic planes below to grow the Cr(III) layer incompletely saturating the stainless steel surface. This work provides new insight on corrosion chemistry, by evidencing local chemical and structural defects self-generated at the nanoscale by the building process of the protective oxide barrier, and affecting the passive film stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interfacial passivation mechanisms studied on Cr- and Mo-containing alloy surfaces. • D 2 O labelling applied to study the growth of nanometer-thick passive oxide film. • D-labelled species mostly detected in outer hydroxide layer of passive film. • Minor inward hydroxyl diffusion from electrolyte during anodic passivation. • Key role of hydroxyls initially in native oxide in forming Cr hydroxide. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with deuterium labelling (D 2 O), and X-ray photoelectron spectroscopy (XPS) were applied to investigate the interfacial transport mechanisms of hydroxyls between aqueous electrolyte and outer part of the protective oxide film during passivation of Cr 15 Fe 10 Co 5 Ni 60 Mo 10 multi-principal element alloy surfaces. A ToF-SIMS signal treatment methodology was developed to retrieve the in-depth distribution of deuterated hydroxides in the nanometer-thick surface oxide film. After immersion at free potential or anodic passivation in acidified D 2 O, the initial bilayer structure is retained. The use of heavy water electrolyte has no effect on the composition alterations induced by passivation. Deuterated species were detected mostly in the outer layer of the surface oxide film. Pure inward hydroxyl diffusion from electrolyte/oxide to oxide/metal interface is excluded from being primarily responsible for oxide film growth induced by anodic passivation. The hydroxyls from the dissolving hydroxide species are retained in the oxide film and participate in forming Cr hydroxide, thus reducing the hydroxyl surface uptake from the electrolyte. This work provides deeper insight into the mechanisms of oxide growth and corrosion protection induced by anodic passivation. [ABSTRACT FROM AUTHOR]

Substitution of a metal cation by Mo and its effects on vacancy formation on (0001)-oriented chromia and hematite surfaces were studied using DFT calculations to provide atomistic insights on the experimentally observed Mo-enhanced passivity. The substitution is favoured energetically, resulting in Mo more enriched in the Fe-rich than in the Cr-rich oxide zones in passive oxide films. The presence of substitutional Mo disfavours the formation of O vacancies and favours metal vacancies, more significantly in Fe 2 O 3 than in Cr 2 O 3 , suggesting a prevention of passivity breakdown in chloride-rich environment and a cure of the Fe-rich weak sites of passivity by Mo. • Mo-enhanced passivity studied by DFT calculated (0001) Cr 2 O 3 and Fe 2 O 3 surfaces. • Substitution by Mo always exothermic, more favoured in Fe 2 O 3 than in Cr 2 O 3. • Formation of O vacancies disfavoured (and metal vacancies favoured) by Mo. • Mo effect on vacancy formation more significant on Fe 2 O 3 than on Cr 2 O 3 surfaces. • Beneficial effects for the resistance to localised corrosion of stainless steels and other alloys are discussed [ABSTRACT FROM AUTHOR]

• Surface oxide layers formed on 2304 duplex stainless steel are investigated. • The native surface and the passivated surface are compared. • An innovative coupling of XPS and ToF-SIMS is used. • The oxide layers formed on austenite and ferrite phases are locally characterized. • The effect of passivation on the surface oxide layer is discussed. This work reports an innovative methodology based on the coupling of XPS and ToF-SIMS in order to study the surface oxide layer formed on austenite and ferrite phases of duplex stainless steels. The native oxide film obtained after mechanical polishing and the passive film obtained after electrochemical passivation in 0.05 M H 2 SO 4 were investigated. Chromium and molybdenum contents were found to be higher in the oxide layer formed on ferrite phase. Nickel and nitrogen enrichments were found to be more pronounced under the oxide layer formed on austenite phase. The effect of passivation on the surface oxide layer is discussed. [ABSTRACT FROM AUTHOR]

The mechanisms of initial oxide growth on the equiatomic CoCrFeMnNi high entropy alloy were studied in situ by X-ray photoelectron spectroscopy on clean, oxide-free surfaces exposed stepwise to low-pressure gaseous oxygen. Cr(III), Mn(III), Fe(III), Co(II) but no Ni(II) oxides were formed at 100 °C, and Mn(II) at 200 °C. Data quantification revealed a transition mechanism from selective initial oxidation to 3D kinetically-controlled growth, leading to stratification of the oxide film. Formation of the Co-rich outer layer above the Cr-rich inner layer first formed implies predominant cationic outward oxide growth supported by the overall stoichiometry of the film. • Initial oxide growth studied in situ starting from clean, oxide free surface. • Valence of oxides formed on CoCrFeMnNi HEA at 100 and 200 °C characterized. • Transition from selective initial oxidation to kinetically-controlled oxide growth revealed. • Stratification mechanism during early build-up of the surface oxide unravelled. • Predominant cationic outward oxide growth supported by overall stoichiometry. [ABSTRACT FROM AUTHOR]

The oxidation behaviours of polycrystalline and monocrystalline Ni-base alloys (Ni-16Cr-8Fe (wt%)) were investigated in situ by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The oxide layer formed is duplex: an inner chromium-rich layer and an outer layer which is rich in nickel and iron. Time evolution of the composition of inner layer from pure Cr 2 O 3 towards spinel (NiCr 2 O 4 ) was observed. The oxidation kinetics was accelerated by alloy grain boundaries, which can favour the diffusion of chromium. Both the parabolic and volatilization constants increased with increasing temperature, with k p varying from 1.9×10 −3 to 1.6×10 −2 nm 2 .s −1 , and k v from 1.0×10 −3 to 2.9×10 −3 nm.s −1 . [ABSTRACT FROM AUTHOR]

Ion transport mechanisms in growing oxide films on Ni-20Cr and Ni-20Cr-10Mo alloys have been investigated by ToF-SIMS with in situ re-oxidation using 18O isotopic tracer. Whereas the growth is controlled by pure anionic diffusion on Ni-20Cr alloy, the behavior is drastically modified by Mo addition. A mixed cationic and anionic growth mechanism and a reduced diffusion rate of moving species are observed on the alloy containing molybdenum. The passivation in sulfuric acid, which increases the inner Cr oxide barrier layer thickness, also reduces the diffusion rate of the moving species for both Ni-20Cr and Ni-20Cr-10Mo alloys. Significantly higher re-oxidation temperatures are needed to observe the ion transport mechanisms in passive oxides formed on Ni-20Cr and Ni-20Cr-10Mo alloys, as compared to native oxides. Mo addition and electrochemical passivation have synergistic effects that reduce the diffusion rate of moving species through the surface oxide films and thus reinforce the corrosion resistance of the oxide films formed on Ni alloys. [ABSTRACT FROM AUTHOR]

Sublayers grown with filtered cathodic arc deposition (FCAD) were added under atomic layer deposited (ALD) oxide coatings for interface control and improved corrosion protection of low alloy steel. The FCAD sublayer was either Ta:O or Cr:O-Ta:O nanolaminate, and the ALD layer was Al2O3-Ta2O5 nanolaminate, AlxTayOz mixture or graded mixture. The total thicknesses of the FCAD/ALD duplex coatings were between 65 and 120 nm. Thorough analysis of the coatings was conducted to gain insight into the influence of the FCAD sublayer on the overall coating performance. Similar characteristics as with single FCAD and ALD coatings on steel were found in the morphology and composition of the duplex coatings. However, the FCAD process allowed better control of the interface with the steel by reducing the native oxide and preventing its regrowth during the initial stages of the ALD process. Residual hydrocarbon impurities were buried in the interface between the FCAD layer and steel. This enabled growth of ALD layers with improved electrochemical sealing properties, inhibiting the development of localized corrosion by pitting during immersion in acidic NaCl and enhancing durability in neutral salt spray testing. [ABSTRACT FROM AUTHOR]

Highlights: [•] Passive films grown on stainless steel in high temperature water analysed by C-AFM. [•] Electrical resistance of grains and grain boundaries measured at nanometre scale. [•] Local electrical resistance influenced by changes in resistivity of the oxide film. [•] Local resistivity assigned to balance in Fe(II)/Cr(III) composition of inner layer. [•] Grain boundaries of outer layer would promote Cr(III) enrichment of inner layer. [Copyright &y& Elsevier]

[Display omitted] • Thermal stability of Ni20Cr and Ni20Cr10Mo is investigated by XPS and in situ ToF-SIMS • The chemical modifications are studied during heating of surface oxides in vacuum • The highest thermal stability is observed for the pre-passivated Ni20Cr10Mo alloy. • The findings are useful to improve corrosion protection strategies. Thermal stability of surface oxides on Ni-20Cr and Ni-20Cr-10Mo alloys, including air formed native oxides and passive films formed by electrochemical passivation, was investigated by stepwise heat treatments in ultra-high vacuum up to 300 °C. The modifications of the surface oxides were analyzed by XPS and in situ ToF-SIMS. The analytical surface characterization at the nanoscale revealed modifications of the protective surface oxides that depend on temperature and nature of the alloy. A loss of Ni(OH) 2 in the outer hydroxide layer and a decrease of film thickness are observed on both alloys above 100 °C. At 200 °C, Ni hydroxide in the outer layer is no longer stable while MoO 3 decreases and MoO 2 increases in the oxide film. At 300 °C no native oxide film remains at the surface of Ni-20Cr alloy, whereas a thin Cr oxide film partially covering the surface remains on passivated Ni-20Cr and for both native and passivated surfaces of Ni-20Cr-10Mo alloy. It is shown that the thermal stability of the surface oxide is significantly improved by electrochemical passivation on both alloys and is further enhanced by the presence of Mo in the oxide on the Mo-containing alloy. Such findings are useful for the development of corrosion protection strategies. [ABSTRACT FROM AUTHOR]

High resolution X-ray photoelectron spectroscopy analysis has been used to investigate, in-situ, the mechanisms for oxide nucleation and growth on a FeCrNiMo single crystal alloy. Ultra-high vacuum conditions provided the ideal environment in which an oxide-free surface could be oxidised in a controlled manner, using extremely low pressures and step-wise exposures of oxygen. In-situ snapshot measurements, which records an XPS spectrum every 10 s, provided insight into the mechanisms of chromium and nitrogen segregation during annealing of the sample. High resolution XPS spectra was used to follow the evolution of the key elements at step-wise intervals, allowing for detailed insights into the nucleation and growth mechanisms. Angular XPS analysis provided further information on the in-depth distribution and stratification of the oxide film. [Display omitted] • In-situ XPS indicates two different mechanisms for Cr segregation during annealing. • Surface rearrangement results in CrN behaving as nucleation site for Cr 2 O 3. • Oxidation of Mo is delayed until after formation of Fe and Cr oxide has begun. • Oxide growth became dominated by iron oxide owed to increased Fe cation mobility. • Fe2+ and Cr3+ became enriched at the oxide–metal alloy interface. [ABSTRACT FROM AUTHOR]

• Surface heterogeneities originating from the local supply of metallic Fe and Cr. • Preferred Fe oxide nucleation in the initial stages (exposures < 3 L). • Cr3+ oxide nucleation delayed to exposures above 3 L. • Cr-rich oxide nanogranular film homogeneously covering the surface at saturation. Scanning tunneling microscopy was applied to investigate the topographical and structural alterations at small spatial and temporal scales of a model 304 austenitic stainless steel surface resulting from ultra-low exposures to gaseous oxygen at room temperature. The 2D nucleation and growth mechanisms were observed until surface saturation by a fully covering and nanogranular Cr(III)-enriched oxide film. Local supply of metallic Fe and Cr for oxide nucleation and growth is concluded to be at the origin of heterogeneity developed in the oxide film at the nanoscale. This work provides nanoscale understanding basis for composition and morphology tuning of Cr-enriched oxide film for improved local stability of passivity. [ABSTRACT FROM AUTHOR]

• Passivity of CoCrFeMnNi high entropy alloy surfaces investigated by ToF-SIMS. • Bilayer structure formed in aqueous acid solution thermally stable in vacuum. • Further oxidation in low pressure O 2 gas studied in situ with isotopic 18O tracing. • Outward cation diffusion faster than inward oxygen diffusion at 170 °C. • Parabolic law of re-oxidation established and parabolic constant determined. The ion transport mechanisms in passive films pre-formed on CoCrFeMnNi high entropy alloy surfaces in acid solution were studied in-situ by time-of-flight secondary ion mass spectrometry upon oxidation in 18O 2 gas. The bilayer structure of the passive film, with outer Fe, Co-rich and inner Cr, Mn-rich layers, is thermally stable, but with reduction of Fe and Co oxides and formation of Mn oxide in the outer layer above 150 °C. At 170 °C, the transport mechanism is mainly outward cation diffusion with slower inward oxygen diffusion. The re-oxidation growth rate follows a parabolic law and the value of parabolic constant was determined. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thermal pre-oxidation effects on passive film properties studied on stainless steel. • Combined surface analytical and electrochemical approach applied with closed transfer. • Thermal pre-oxidation improves corrosion resistance at active/passive transition. • Thermal pre-oxidation promotes the enrichment of molybdenum and chromium. • Thermal pre-oxidation enhances growth of the inner layer after anodic passivation. A combined X-ray photoelectron spectroscopy, time-of-fight secondary ion mass spectrometry and electrochemical approach was used to investigate the mechanisms of molybdenum and chromium enrichment of a passive film formed on a model austenitic stainless steel single crystal in aqueous acid media and the effect that thermal pre-oxidation at 250 ° C plays in the composition, thickness and stratification of the film. A clean FeCrNiMo (1 0 0)-oriented surface, prepared under ultra-high vacuum (UHV), was exposed to ultra-low pressures of oxygen at 250 ° C, forming a thin oxide film, rich in iron and chromium, as well as containing molybdenum. An argon-filled glovebox, directly attached to the UHV system, allowed for electrochemical alterations to be carried out without exposure to ambient air. After being held at open circuit potential for 30 min, hydroxylation of chromium and molybdenum was promoted, with a marked enrichment of both elements in the oxide film. A distinct bilayer film was observed. Anodic polarisation resulted in a significant growth of the inner layer and further enrichment of chromium. Pre-oxidation at 250 ° C, using an ultra-low pressure of oxygen, was found to promote protection against transient active dissolution during anodic passivation, and to cause enrichment of molybdenum in the outer layer of the passive film. [ABSTRACT FROM AUTHOR]

• Surface oxide layers formed on CoCrFeMnNi High Entropy Alloy are investigated by XPS and ToF-SIMS. • An XPS methodology based on analysis of the 3p core level spectra was developed. • The native oxide and passive films are compared. • The oxide film on HEA is duplex comprising a Cr/Mn inner oxide layer and a Cr/Fe/Co outer oxide/hydroxide layer. • The effect of electrochemical passivation on the surface oxide layer is discussed. XPS and ToF-SIMS analysis have been combined with electrochemical measurements to determine the corrosion behavior of the equiatomic CoCrFeMnNi high entropy alloy. An XPS methodology based on analysis of the 3p core level spectra was developed to determine the composition and thickness of oxide films. Both native oxide (∼1.4 nm) and passive films (∼ 1.6 nm) formed in acidic medium are duplex, comprising Cr and Mn inner layer and Cr/Fe/Co mixture outer layer. No nickel is observed in the oxide layer. The effects of exposure of the native film to sulfuric acid and passivation under anodic polarization have been investigated. [ABSTRACT FROM AUTHOR]

• Passivation mechanisms studied on Fe-18Cr-13Ni(100) starting from oxide-free surfaces. • Direct transfer between surface preparation/analysis and electrochemical characterization. • Oxidized iron preferentially dissolved in sulfuric acid at open circuit potential. • Oxide film re-growth and Cr(III) dehydroxylation forced by anodic passivation. • Cr(III) enrichment and passivity promoted by UHV pre-oxidation. 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. [ABSTRACT FROM AUTHOR]