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The failure of transpassive films on transition metal alloys was investigated. The results show that the failure of the transpassive film on 3D printing nickel-based superalloy in flowing NaNO3 solution is dominated by self-induced cracking of the film on niobium-segregated phases and the erosion-induced degradation on the film assisted by cation vacancy condensation at the metal/barrier layer interface. Secondary passivation induces the formation of a transpassive film, the nature of which has been debated for decades. The findings are consistent with a bi-layer structure comprising a point-defective barrier layer that grows into the substrate alloy and a precipitated outer layer.

The influence of an impinging acoustic streaming jet on wall mass transfer is studied both experimentally and numerically. The idea is to show that acoustically-driven jets generated by ultrasounds can be used to enhance transfer phenomena at a distance, by creating localized friction zones. An experimental setup has been developed consisting in a cavity containing an electrolytic solution of [Fe(CN) 6 ] 4 − /[Fe(CN) 6 ] 3 − . A jet forced by an ultrasound beam impinges on the upper wall instrumented with electrodes, at which the mass transfer influenced by the streaming is measured by electrochemical technics. Numerical simulations of the flow and mass transfer in the same configuration are also performed. A significant enhancement of the mass transfer at the electrodes (represented by the Sherwood number Sh ) with the injected acoustic power (quantified by the acoustic Grashof number Gr ac ) is observed. An order of magnitude of the expected Sherwood number and friction coefficient is proposed on the basis of the Leveque law and momentum budget considerations. Scaling laws involving both experimental and numerical mass transfer at the electrodes ( Sh ), numerical wall shear stress and injected power ( Gr ac ) are finally derived.

The effect of dry grinding on 304L stainless steel&rsquo, s passive behavior is compared to two other surface finishing (mechanical polishing down to 2400 with SiC emery paper and 1 &micro, m with diamond paste, respectively). The characterization of the surface state was performed using scanning electron microscopy, transmission electron microscopy, 3D optical profilometer, and X-ray diffraction. Results indicate that each surface treatment leads to different surface states. The ground specimens present an ultrafine grain layer and a strong plastic deformation underneath the surface, while an ultrafine grain layer characterizes the subsurface of the polished specimens. Grinding induces high residual compressive stresses and high roughness compared to polishing. The characterization of the passive films was performed by electrochemical impedance spectroscopy and Mott&ndash, Schottky analysis. The study shows that the semiconductor properties and the thickness of the passive films are dependent on the surface state of the 304L stainless steel.

A combinatorial approach has served as a high-throughput strategy to identify compositional windows with optimized desired properties. Here, ZrCuAg thin-film metallic glasses were deposited by DC magnetron sputtering. For the purpose of using these coatings as biomedical surfaces, their durability in terms of mechanical and physicochemical properties as well as antibacterial properties were characterized. The effect of the chemical composition of thin films was studied. In particular, two key parameters were highlighted: the atomic ratio of Zr/Cu (with three values of 65/35, 50/50, and 35/65) and the silver content (from 1 to 16 at. %). All thin films are XRD amorphous and exhibit a typical veinlike pattern, which is characteristic of metallic glasses. They also show a dense and smooth surface and a hydrophobic behavior. Mechanical properties are found to be deeply influenced by the Zr/Cu ratio and the atomic structure. Although a low Zr/Cu ratio and/or a high silver content is detrimental to corrosion behavior, it favors the bactericidal effect of thin films. For all Zr/Cu ratios, ZrCuAg thin-film metallic glasses with silver contents higher than 12 at % are fully bactericidal. For lower silver contents, the bactericidal effect progressively decreases, which paves the way for a biostatic behavior of these surfaces.

The tribo electrochemical behavior of 2205 duplex stainless steel had been studied under acidic and neutral solutions with control of the dissolved dioxygen. The study focused on the oxide film formation using a galvanic technique approach implemented in a tribometer. Higher wear rates were measured in low pH and aerated solution. In low O2 concentration, a wear reduction was observed for each pH. Analysis of the passivation transients showed the pH influence on the oxide growth mechanism, irrespective of the O2 level. Temporal analysis of the transients confirmed the faster passivation of the wear track in acidic solution. In a controlled O2 environment, the nucleation and the oxide growth were slow down, irrespective of pH. The degradation rate was then correlated to the passivation kinetic and the aging of the passive film.

The passive behavior of a Ni–20Cr binary alloy was studied using an original methodology, based on successive electrochemical impedance spectroscopy (SEIS), over a wide potential range. The proposed method was first validated by comparing the resulting potentiostatic polarization curves with more conventional potentiodynamic polarization curves and determining the effect of the dwell time (30, 120, and 600 s) and potential step size (25 and 50 mV). Using the optimized parameters from this preliminary work, the passive behavior of the Ni–Cr alloy was then studied in solutions of varying pH: 10 g/L Na2SO4 with H2SO4 (pH = 2), 0.01 M Na2SO4 (pH = 5.8), and 0.05 M H3BO3 + 0.075 M Na2B4O7 10H2O (pH = 9.2). The obtained impedance diagrams were interpreted using enhanced graphical representation and fitted using the power-law model to determine the resistivity distribution throughout the passive film during polarizations. The results demonstrated that SEIS provides complementary information to conventional potentiodynamic curves. The evolution of the passive film properties was monitored in situ within the potential range corresponding to stable passivity. With the help of this original method, complementary to the evolution of the current density (measured at each potential step), the thickness, chemical composition, and reactivity changes were also highlighted.

In this work, the effect of substituting Cu with small amounts of Sn and Si on the passive behavior and pitting resistance of the bulk metallic glass Ti40Zr10Cu36Pd14 was investigated. The Sn/Si substitution promoted homogenization of the solid solution and limited the presence of casting defects. In addition, unlike the heterogeneous passive film grown on TiZrCuPd, the passive films of the micro-alloyed materials were homogeneous, implying less sensitivity to film weakening and breakdown. Thus, alloying with small amounts of Sn and Si appears to be a viable approach to drastically improve the corrosion resistance of TiZrCuPd.

Casting of metallic glasses (MG) sometimes induces surface crystallisation despite the fact that the surface is expected to be the region where the cooling rate is the highest. This phenomenon has been observed on various MG, even for those with large critical diameters. Such surface crystallisation can be detrimental when the target applications are focused on surface properties, such as corrosion resistance for biomedical applications. In this paper, a Zr56Co28Al16 bulk metallic glass (BMG) with a large critical diameter was used. We reveal that samples processed using common copper-mould suction casting present surface crystallisation up to 20 µm in thickness, greatly deteriorating corrosion resistance. Using in-house highly reproducible suction casting and injection micro-casting processes, the influence of the processing parameters (mould material and temperature, working atmosphere, applied pressure) were investigated. The origin of surface crystallisation was found to arise from the complex thermal history of the alloy depending on the alloy/mould contact quality. By ensuring a tight contact between the solidifying alloy and the mould, BMG samples without crystalline surface defects were obtained.

The effect of dry grinding on the oxidation features of 304L stainless steel (SS) in simulated primary water of BWR and PWR was studied. The objective is to demonstrate the influence of mechanical surface treatments on surface state and oxidation behavior of the SS. Thus, the oxide formed on a ground surface and two polished surfaces (down to 2400 with SiC emery paper and with colloidal silica) had been compared. Subsurface characterizations were first performed to describe the microstructural modification induced by the surface treatment. Then, the morphology and the composition of the oxides' duplex structure were characterized by either scanning electron microscopy, transmission electron microscopy or X-Ray photoelectron spectrometry. Finally, the reactivity of the oxidized surfaces was determined ex-situ by a recently developed electrochemical technique based on electrochemical Impedance spectroscopy and Mott-Schotcky approach. The mechanical surface treatment mainly influences the thickness and doping density of the inner oxide; and the composition and morphologies of the outer oxide-hydroxide precipitates in terms of shape and size. This work evidences that the thinner the inner oxide, the higher the doping density. Besides, the impact of the considered environment on oxidation features is also discussed.

cited By 0; International audience; The influence of lubricant additives on rolling contact fatigue crack propagation and the mechanisms responsible for the resulting micro-scale damage, was studied via experiments conducted on complete transmissions in a test cell. Bench-scale tribological tests and the exposure of steel surfaces to two different formulated lubricants were also carried out. Scanning and Transmission Electronic Microscopy observations, Electron Dispersive Energy and X-ray Photoelectron Spectroscopy analyses indicated that the sulphur present in the extreme pressure (EP) additives has a positive impact on limiting damage propagation. Thanks to TEM observations of cracks, it was demonstrated that a sulphur rich film is formed at the crack tip. This film can act as both a barrier film towards hydrogen permeation within the metal and / or as an inhibitor of oil decomposition. The latter is associated with the nascent surface's ability to limit hydrogen generation. Without such hydrogen embrittlement, crack propagation is slowed down. © 2017

International audience; he test conditions currently used in biotribocorrosion devices often differ greatly from the physiological conditions of joint replacements, contributing to discrepancies between the simulated and actual life span of joint replacements. In this study, a multidisciplinary biotribocorrosion device was developed based on the limitations of existing tribocorrosimeters. The set-up enables corrosion measurements to be simultaneously performed with real-time visualization of living cells using fluorescence microscopy under dynamic loads and movements. The device was configured to simulate the joint contact of ankle prostheses, and the wear of ultra-high-molecular-weight polyethylene/cobalt alloy (CoCrMo) implants surrounded by murine macrophages was tested. Various characterization techniques (non-contact optical profilometry, scanning and fluorescence electron microscopy and quantitative analyses of metal ions and pro-inflammatory cytokines) were combined in-depth multidisciplinary study. Two experimental conditions were used to promote the production of either polyethylene wear particles or metal ions. The first results indicated two distinct tribocorrosion mechanisms: 1) adhesive wear coupled with slow ionic depassivation of the cobalt alloy. The main degradation products were micrometric spherical polyethylene particles that seem to have little impact effect on the metabolic activity of the macrophages. 2) Ionic wear with the production of small, fibrillar polyethylene particles was observed. The production of metal ions, mainly chromium, was the predominant degradation process. The cytotoxicity of the chromium ions was evaluated based on the secretion of pro-inflammatory cytokines (prostaglandin E2). Our findings indicate that simulated conditions that result in low mechanical wear but high ions release appear to be more harmful to cells.

Oxidation-assisted cracking (OAC) is a particular type of damaging process called “dynamic embrittlement”. Dynamic embrittlement is a fragile rupture process which implies the ingress, diffusion and segregation of embrittling elements at the grain boundaries leading to their debonding. These embrittling elements may be provided by the material itself, as in the case of sulfur in steels or tin in copper alloys, or by the material's environment, notably oxygen's partial pressure. One of the most-studied cases is OAC for nickel-based alloys for which the penetration and segregation of oxygen, often associated with intergranular oxidation, change the crack propagation mode from transgranular to intergranular and significantly increase the crack propagation rate. In the aeronautical industry, as in the nuclear industry, this phenomenon is responsible for numerous cases of in-service cracking. To study the propagation rate's susceptibility to the chemical environment, numerous mechanical tests are conventionally performed in laboratory air or a vacuum. The propagation rates measured during low-frequency fatigue-creep tests (trapezoidal mechanical cycle: 10s-300s-10s) in the case of alloy 718 tested at 650°C in different environments.

The constant-phase-element behavior measured for passive films was analysed using the power law model. It allows to obtain the variation of the resistivity through the passive film thickness. To avoid many unknown parameters required for the model, a methodology to analyze the impedance diagrams has been described. It is illustrated with several examples. First, the resistivity profiles were obtained for a Ni-20Cr alloy immersed in solutions with different pH values to validate the proposed methodology. The dielectric properties of the passive films were discussed from the duplex character of these passive layers. It was shown that the shape of the resistivity profile could be attributed to the contribution and the thickness of the inner dense oxide and the outer hydroxide layers, respectively. Then, the second example provided information on the evolution of the behavior of the passive film formed on 316L stainless steel under irradiation.

Passive film (PF) could be considered a particular nanomaterial with its growth, chemical composition, microstructure, performance and evolution of its performance. The understanding of the film construction, its maintenance and its resistance is the result of a total complementarity between characterization from enhanced physico-chemical analysis techniques, especially those developed by P. Marcus and his group[1], and electrochemical measurements, mainly the electrochemical impedance spectroscopy (EIS). EIS gives access to the capacitive and resistive properties of the passive film. The first, complemented by the Mott-Schottky theory, allows a quantitative approach of electronic properties (semi-conductivity) and the maintenance process of passive films. The second involves the corrosion resistance of the film. Nevertheless, it is important to make a rigorous adjustment of the parameters resulting from the impedance diagrams. An adjustment that takes into account the physical meaning of parameters, especially for the analysis of the constant phase elements (CPE) used in the treatment of the diagrams that present one depressed semicircle [2]. The knowledge of the oxi-hydroxide character of the PF, evidenced from physicochemical characterizations, allows considerations of the spatial distribution of the heterogeneities (3D) within the thickness of the PF from the variation of its resistivity obtained from the Power Law Model (PLM)[3,4]. In fine, the resistivity profiles give information about interface reactivity and distribution of species along the thickness of the passive layer. The use of the PLM requires the assessment of several parameters, especially a and Q defining the CPE or the thickness of the PF, which is generally assessed by physicochemical measurements. Most of them can be determined by advanced graphical analysis of impedance diagrams[5,6]. It can then be used to guide the adjustment of the diagrams by inputting known parameters into the model. To exemplify the interdependency between physicochemical analysis and electrochemical impedance measurements, two studies illustrate the contribution of in-depth analyses of the resistivity profiles for two passive materials. That will be the opportunity to highlight which information can be drawn and finally, to propose reaction mechanism. First of all, the passivity of the Ni-20Cr alloy has been studied in different solutions[7]. In this work, the electrochemical impedance diagrams have been successively performed under applied potential in passive region. From PLM analysis, it was observed that the evolution of the resistivity at the Ni-20Cr/PF interface explains the evolution of the current density during polarization. The resistivity profiles obtained for increasing applied potentials on the passive plateau showed a decrease of the resistivity plateau at the substrate/PF interface and an increase of the resistivity at the PF/electrolyte interface, highlighting the modification of the passive film according to the anodic polarization. The second example concerns 316L stainless steel immersed in a 0.02M sodium sulphate solution and under proton irradiation[8]. It was shown that the impedance diagrams obtained before and after irradiation were almost superimposed, suggesting that the system was reversible. However, the resistivity profiles showed that under irradiation, the resistivity at the PF/electrolyte interface decreased and reached its initial value after irradiation. On the other hand, the length of the plateau at 316L/PF interface disappeared during irradiation and was reduced after irradiation by comparison with before irradiation. The case studies presented show that the advances proposed in the analysis of impedance diagrams, combined with physicochemical analyses, provide a global knowledge of passivation mechanisms. Maurice, P. Marcus, Current opinion in solid state & material science, 22 (2018) 156-167. Tribollet, V. Vivier, M.E. Orazem, in Encyclopedia of Interfacial Chemistry (2018), Ed. Elsevier, 93-107. B Hirschorn, M. E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, Journal of the Electrochemical Society 157 (2010) C452-C457. B Hirschorn, M. E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, Journal of the Electrochemical Society 157 (2010) C458-C463. E. Orazem, N. Pébère, B. Tribollet, Journal of the Electrochemical Society 153 (2006) B129-B136. Benoit, C. Bataillon, B. Gwinner, F. Miserque, M. E. Orazem, C. M. Sánchez-Sánchez, B. Tribollet, V. Vivier, Electrochimica Acta 201 (2016) 340-347. Zhang, B. Ter-Ovanessian, S. Marcelin, B. Normand, submitted to Electrochimica Acta. Normand, N. Bererd, P. Martinet, S. Marcelin, M. Moine, J. Ferreira, D. Baux, T. Sauvage, N. Moncoffre, submitted to Corrosion Science.

cited By 1; International audience; The susceptibility of Ni-Cr alloys to pitting corrosion is known to be closely related to the Cr content in the alloy. However, the role of Cr in the different stages of the pitting process is not totally understood. To determine the role Cr in the different stages of the pitting process, the passive films formed on different nickel-base alloys (with 16, 20, 24 and 28 wt% Cr) in a sulfate solution were characterized using electrochemical techniques and X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy (EIS) analyses are discussed in terms of the migration of point defects. Metastable and stable pitting was then studied using polarization curves in a 0.1 M NaCl solution. The alloys studied exhibit a wide range of susceptibility. The 16 wt% Cr alloy was the most susceptible to metastable and stable pitting while the 28 wt% Cr alloy was almost unaffected by both phenomena. By studying the chemical composition of the passive film, it was shown that nickel incorporated into the passive film was responsible for deleterious interactions between the surface and chloride ions leading to metastable pit events. Migration resistance is a good indicator of the passivity breakdown process based on the EIS results. Low migration resistance values imply the presence of a vacancy condensation process while high values imply a thinning process. As a result, a step by step mechanism is proposed showing chloride ion interaction with passive films at stable pit initiation. © 2016 The Electrochemical Society.

International audience; The development of biocompatible metallic glasses with enhanced ductility that are free of potentially toxic elements such as aluminium and nickel remains a challenge. Although Cu is associated with cytotoxicity levels above a safe threshold, the mechanical properties of Cu-rich metallic glasses are particularly attractive. Herein, the effect of minor additions of yttrium to the Cu–Zr–Ti system is studied. Rods of composition (Cu60Zr30Ti10)(100−x)Yx (with x = 0, 1, 2) were prepared, and the critical diameter was improved from 2 to 3.5 mm with the addition of 1 at% Y. The compressive fracture strength was greater than 2000 MPa for all of the compositions, and the plastic strain increased from 0.9% to 2% with the addition of 1 at% Y. Transmission electron microscopy analysis revealed the presence of yttria precipitates surrounded by nanocrystallized areas, suggesting that the precipitates improved the plasticity. The addition of yttrium also led to the formation of a protective layer, which enhanced both the thermal stability and corrosion resistance of the metallic glass. Finally, the biocompatibility improved with minor additions of yttrium. Our findings demonstrate the wide range of beneficial properties of Cu-based metallic glasses, which may open the door for their potential application as biomaterials.

cited By 0; International audience; Corrosion tests of metallic materials for biomedical applications are generally carried out in highly simplified solutions and are often disapproved by direct or indirect cytotoxicity tests. Cell culture medium is then the better alternative to get robust biocompatibility assessments. The specific intrinsic reactivity of a Roswell Park Institute Medium cell culture medium (RPMI-1640) was studied here using a platinum electrode inside and outside a dedicated CO2 incubator. Electrochemical tests and surface analyses revealed that organic compounds of the RPMI-1610 are forming an organic adsorbed layer on the surface which is electrochemically active in a wide potential range. Precipitations of inorganic salts (calcium phosphate) are also promoted by organic species. The effect of the 5% content of CO2 in the incubator was also discussed, relative to the fact that carbonate and bicarbonate ions are involved in the charge transfer and mass transport processes. All these features were taken into account when performing biocompatibility tests for material design and selection. © 2018 Elsevier Ltd

cited By 22; International audience; In the presence of chloride ions, passive films grown on Nickel-Chromium alloys may be sensitive to chemically induced breakdown. As chloride ion adsorption on the passive film/electrolyte interface and their penetration through the film are the main reasons for passivity breakdown, their mechanisms need to be assessed and understood. In the present study, the passivity of pure Ni, pure Cr and different Ni-Cr alloys (with 16, 20, 24 and 28 wt% Cr) was characterized for two film growth conditions by electrochemical impedance spectroscopy (EIS) and the Mott-Schottky analysis. EIS spectra, obtained after the immersions, were analyzed from an Equivalent Electrical Circuit taking into account migration phenomena through the passive film (transport resistance, apparent diffusivity). Mott-Schottky analyses were performed to obtain the semiconductor properties of the passive films, such as semiconducting type, charge carrier density and flat band potential. All of these results were compared as a function of substrate composition and film-forming conditions. The relation between chloride anions adsorption or penetration and the electronic and transport properties of the passive films is then discussed. Particular attention is paid to the migration parameters, the flat band potential and the charge carrier density. © 2014 Elsevier Ltd.

At high anodic potentials, two phenomena may generally occur on the surface of a passivated electrode: the oxygen evolution and the transpassive dissolution (at grain boundaries or generalized). For some materials, this anodic dissolution is postponed by the structural or chemical modification of the passive film. This phenomenon is characterized by a second plateau on anodic polarization curves. This latter informs about a limited further dissolution. This feature is commonly called the secondary passivation. Generally, it is assumed that the passive film grown on Ni-Cr based alloys is mainly composed of chromium (Ⅲ) oxide. However, in certain circumstances, the formation of oxide with higher valence of chromium is related to the secondary passivation. It is of interest to investigate the role of alloying elements in the process of the secondary passivation to solve industrial applications or alloys design issues. The aim of the present work is to study the electrochemical behaviour, especially the secondary passivation behaviour, of different Ni-Cr based alloys and pure Cr. This was mainly achieved by using successive electrochemical impedance spectroscopy (EIS) measurements. After 24h of immersion in borate buffer solution, EIS diagrams for different samples were successively recorded at different potentials from cathodic to anodic direction on a wide potential range. The step-by-step polarization curves (potentiostatic polarization) obtained from successive electrochemical impedance measurements was used as a complementary method to conventional polarization curves. Besides, each impedance diagram was firstly analysed by enhanced graphical representation and complex capacitance representation to assess electrolyte resistance, constant phase element (CPE) parameters (α and Q), capacitance and thickness of the passive film. Since in most of the cases the CPE behavior observed for passive films could be attributed to a distribution of the resistivity through the film thickness. The power law model was used to adjust the experimental impedance diagrams. This model allows to study the trend of the resistivity profiles within the passive film as a function of the potential. Thus, the variation of the determined parameters related to the potential was presented for the different alloys. The use of this analytical approach in the secondary passivation domain was discussed. To conclude, the role of the alloying elements on the process of the secondary passivation and especially on the chemical composition of the passive film was discussed.

The electronic properties of the passive film formed on pure chromium in an acidic sulfate medium at 30 °C was studied by using the multi-frequency Mott–Schottky approach and the point defect model. Conversely to common Mott–Schottky experiments, the relevance of the multi-frequency Mott–Schottky analysis relies on the determination of the space charge capacitance from a large frequency range impedance diagram. To analyze the Constant-Phase-Element behavior commonly observed for the impedance diagrams which characterize the passive film, the power-law model was used and allowed to determine the space charge capacitance. The consistency of the results showed that it was relevant to combine the power-law model to the predictive point defect model. The aim of this study is to extract the semi-conductive parameters of a passive film from impedance measurements using a sensitive analytical method. Keywords: Passive films, EIS, Mott–Schottky theory, Power law model, Point defect model

cited By 2; International audience; One of the most popular models taking into account the semiconductive behavior of passive film is the Point Defect Model (PDM) which assumes that growth and breakdown kinetics are mainly governed by the migration of point defects under the high electric field within the film. Based on these PDM approaches and employing electrochemical technics such as a multi-frequency Mott–Schottky analysis, the role of Cr addition in synthetic Nickel-based alloys is investigated in this work. Particular attention is paid to cation vacancy diffusivity and flux. The accuracy of the transport coefficient of point defects as a good parameter for assessing the kinetics of film growth is then debated. Moreover, a beneficial synergic alloying effect on cation vacancy diffusivity for Nickel alloy with 24 wt% Cr is also observed and discussed. © 2015, Springer-Verlag Berlin Heidelberg.

International audience; At high temperatures, alloy 718, like many other nickel-based superalloys, is sensitive to an oxidation-assisted intergranular crack (OAIC) growth mechanism. Former studies have pointed out that even if intergranular oxidation still occurred, intergranular crack initiation was inhibited due to specific mechanical loadings which were identified as Portevin-Le Châtelier (PLC) plastic instabilities. In the present work, key parameters triggering crack initiation or PLC instabilities in the [550-700°C] temperature range were determined for the studied grade by means of tensile tests on smooth specimens. Then, in order to assess the applicability of such a finding at a scale compatible with the material microstructure, a dedicated tensile V-shaped specimen was designed to generate different surface and sub-surface strain and strain rate histories. Thanks to a dual approach based on the observation of crack initiation location on this specific experimental specimen together with associated FE calculations, the critical mechanical loading paths inducing OAIC initiation have been specified. Thus, assuming that the metallurgical state is homogenous at the structure scale, a mapping of intergranular crack initiation is then obtainable.

cited By 3; International audience; The present study investigated the oxidation behavior of Ni-xCr model alloys (14 ≤ x ≤ 30 wt pct) in steam at 973 K (700 °C), with particular emphasis on the influence of chromium content. Mirror polishing was employed to reduce the hardened layer, thereby avoiding the synergetic effect with dislocations in the subsurface. To elucidate the oxidation behavior, weight gain measurements and the meticulous characterization of the oxide scale were performed. The optimal Cr content (24 wt pct) corresponded to an oxidation rate slower than the parabolic rate due to the growth of a protective Cr oxide layer. Increasing the Cr content above the optimal amount led to the degradation of oxidation resistance. © 2015, The Minerals, Metals & Materials Society and ASM International.

cited By 13; International audience; Abstract Passive metals play a key role in many fields. The numerous phenomena affecting the behavior of the film are linked to the main issue that help us to understand the processes of passivation. The description of Electrochemical Impedance Spectroscopy (EIS) data allows us to evaluate kinetic parameters, but there are many models offered and it is difficult to select one to fit EIS data. Mass transport is a significant factor when characterizing the passive film and it is a good way of identifying models. This study proposes a methodology allowing us to choose the most appropriate model and to determine objectively if the mass transport has to be taken into account. The mass transport coefficient of the point defect through oxide is determined through the Point Defect Model. Knowing the thickness of the oxide, through X-ray Photoelectron Spectroscopy (XPS) analysis, the time constant of the transport is determined. Based on this value, a descriptive model is selected avoiding the over-parameterization of the EIS data. This methodology is used in acidic solutions at 30°C and 80°C on pure chromium considered as model material and demonstrates that the mass transport has to be taken into account at 80°C. © 2015 Elsevier Ltd. All rights reserved.

cited By 5; International audience; Adsorption of sulphate anions onto polycrystalline pure copper, pure nickel and two copper-nickel alloys was explored by performing zero charge potential E PZC measurements. E PZC measurements were performed by differential capacitance analysis using single-frequency electrochemical impedance spectroscopy. E PZC and surface charge of the studied materials were analysed for different surface states: bare - immediately after cathodic polarisation and after 5 h of immersion in a 0.1 M Na 2SO4 solution. According to the surface state and nature of the studied materials, the choice of the appropriate measuring frequency was investigated. Specific methodologies dedicated to the study of electrode/electrolyte interfaces are then presented for bare metal, metals covered by corrosion products and metals covered by passive film. In the presence of passive film, the space charge capacitance interferes with the differential capacitance measurements, and its influence is discussed in the framework of semiconductor electrochemistry. © 2013 Springer Science+Business Media Dordrecht.

International audience; Two nickel-based alloys, alloy 718 and alloy 600, known to have different resistances to IGSCC, were exposed to a simulated PWR primary water environment at 360 °C for 1000 h. The intergranular oxidation damage was analyzed in detail using an original approach involving two characterization methods (Incremental Mechanical Polishing/Microcopy procedure and SIMS imaging) which yielded a tomographic analysis of the damage. Intergranular oxygen/oxide penetrations occurred either as connected or isolated penetrations deep under the external oxide/substrate interface as far as 10 μm for alloy 600 and only 4 μm for alloy 718. Therefore, assessing this damage precisely is essential to interpret IGSCC susceptibility.

International audience; The exposure in high-temperature water (300–360°C) of Alloy 600 is known to induce not only generalized corrosion, but also degradation at the underlying base metal such as intergranular penetrations of oxygen and\or oxides and Cr-depletion. Possible consequences of Cr-depletion are the creation of local stresses in the underlying metal due to local variations of the lattice parameter, and the formation of physicals properties gradient, like the elastic modulus or thermal expansion coefficient. In order to assess the effects caused by the Cr-depletion on an exposed Alloy 600, finite element (FE) calculations and physical properties characterisations on synthetic alloys with a chemical composition representative of different Cr-depleted layers, were performed. The levels of the calculated stresses were then discussed regarding to the other features of oxidation involved in the high temperature water stress-corrosion cracking mechanism.

Many studies have emphasized the beneficial effect of niobium on the physical metallurgy of Ni-Cr-Fe alloy 718. Among the different strengthening actions of niobium, such as solid solution hardening and carbide precipitation, the precipitation of niobium with nickel in a strengthening phase γ” (Ni3Nb) during the aging heat treatment has the largest influence on the mechanical properties of alloy 718. The improvement of the niobium distribution and diffusion in the Ni-matrix may allow a more homogenized repartition of γ” precipitates and seems then to be an effective way to upgrade the mechanical properties. As γ” precipitates decompose to the stable δ phase at very long aging times, the study of the effect of carbon, nitrogen and oxygen concentrations on precipitation and dissolution of the δ phase may give information on γ” precipitation and on niobium distribution. It is the purpose of the present work to examine the role that the alloy content of interstitial species plays with the niobium-rich δ phase evolution in alloy 718. Alloy 718 samples were heat treated under hydrogenated argon at 980°C for 0 to 96 hours in order to gradually curb the content of interstitial species by reaction with the reducing atmosphere. Chemical analyses realized by glow discharge mass spectrometry (GDMS) confirmed the reduction of the concentration of these species. Specimens were solution-treated for 1h at 1050°C in an inert atmosphere and furnace cooled. Some of the samples were then aged at 920°C for times ranging from 10 min to 1 hour. The precipitation was measured quantitatively in terms of volume fraction and the morphology of the precipitates was appreciated using scanning electron microscopy (SEM). The differences in the precipitation kinetics and in the microstructure evolution for each interstitial concentration are then discussed.

Two Ni-Fe-Cr ternary alloys have been oxidized in simulated pressurized water reactor primary water at 360°C for 1000 h. The chemical composition of those alloys were chosen in order to be representative of the one of chromium depleted areas under the oxide scale of industrial alloys (e.g. alloy 600) exposed in the same conditions. The resulting oxidized structures (corrosion scale and underlying metal) were characterized using complementary analytical methods (FEG-SEM, TEM, SIMS, optical microscopy). On the one hand, the characterized external oxide layer is very close to the one observed on industrial nickel-base alloys, hence validating the use of such model alloys. On the other hand, both free oxygen and oxides have been detected at grain boundaries several micrometers under the metal/oxide interface. Implications of such a finding on the involved transport mechanisms for oxygen and the intergranular stress corrosion cracking resistance of nickel-base alloys are then discussed.

Alloy 718 is known to be sensitive to oxidation assisted intergranular cracking. It is also demonstrated that the occurrence of jerky flow (also called Portevin-Le Châtelier effect) stops the intergranular damaging mechanism. As dynamic strain ageing is known to be linked with the alloy content of interstitial species, the aim of the present work is to study the effect of carbon, nitrogen and oxygen concentrations on the mechanical behaviour of thin tensile specimens tested under oxidation conditions close to those encountered industrially for turbo machine disks. Thanks to heat treatments performed under reducing atmosphere, the content of interstitial species in tested alloy 718 samples is gradually curbed. Tensile specimens were then tested between 550 and 700°C for the strain rate range [10-5, 10-1] s-1. The key point of this work is that, for a given testing temperature, the tensile tests clearly demonstrated that the transition from an intergranular fragile fracture mode to a transgranular ductile one was always linked with the occurrence of Portevin-Le Châtelier phenomenon but for slower strain rates in comparison with what was observed on the as received aged material tested in the same conditions. This shift of the transition of fracture mode through the lower strain rates remained true until a threshold value of the heat treatment time under reducing atmosphere. Specimens heat treated over this value systematically exhibited a fully transgranular ductile fracture mode, whatever the plastic flow regime was. Implication of such a finding on the intergranular embrittlement of alloy 718 by high temperature oxidation is then discussed.

Passive materials are working in more and more severe conditions. By example, the development of the Proton Exchange Membrane Fuel Cell – PEMFC, leads to the used of stainless steel in higher temperature to improve the energetic efficiency. The main issue understanding the passivation process lies with the multiplicity of the phenomena modifying the behavior of the film. As describe into the Point Defect Model - PDM, the electrochemical reactions occurs at the metal/oxide interface and the oxide/solution interface. The reactivity is limited by mass transport through the oxide. Although, this phenomenon is slow because occurring into a solid phase, it could be observable by electrochemical impedance spectroscopy at regular frequencies because of a very thin oxide. A methodology was developed to study this transport phenomenon combining the use of the PDM with a descriptive model to analyze experimental data from electrochemical impedance spectroscopy. The evolution of the main charge carrier density vs. the formation potential of the oxide is studied and according to the PDM the diffusion coefficient of the point defect through the oxide is determined. Knowing the thickness of the oxide, by means of X-ray Photoelectron Spectroscopy, the time constant of the transport is determined. Based on this value, a descriptive model is selected avoiding the over-parameterization of the EIS data. In this way the combination of a predictive model and a descriptive one is used to objectively take into account the transport phenomena without using an over-parameterized model. This methodology is used in acidic solutions at several temperatures on two materials. Pure chromium is studied as model material and 316L as an industrial one. The conditions which leads to the consideration of the mass transport in the electrochemical impedance analysis are discussed. At higher temperature, the study of the mass transport seems to be necessary.

International audience; Niobium is one of the major alloying elements, among the refractory elements, contributing to the strengthening of superalloys. Consequently, data about its behavior and its migration mechanism in fcc-Ni are essential knowledge to understand and control the strengthening in such alloys. We present in this work Nb interactions, solubility and diffusion in Ni performed by using the GGA approximation of the density functional theory. The substituted site is found to be the most favorable configuration in comparison to the tetrahedral and octahedral sites. The effect of temperature on solubility is discussed taking into account the thermal expansion of the lattice parameter and the vibrational contribution. Its diffusion mechanism is also discussed and compared to the literature. We finally discuss the segregation of Nb atoms on a 65-(012) symmetric tilt grain boundary.

Ni-based alloys are widely used in various engineering applications, such as oil and gas, nuclear power plant and chemical process, due to their excellent corrosion resistance and good mechanical properties at elevated temperature. To extend the sustainability and the versatility of Ni-based alloys, development of new grades or improvement of the existing ones is still ongoing. To carry out the alloy design, the study on the role of alloying elements on nickel-based alloys properties and corrosion resistance remains the best scientific approach. Within this framework, the main objective of this thesis is to develop a methodology based on electrochemical characterization and analysis for understanding the role of the alloying elements on the corrosion behavior and on the passive film properties of Ni-based alloys. The proposed methodology relies on successive electrochemical impedance spectroscopy measurements and advanced analyses of the impedance data, to follow the evolution of the passive film properties grown on different binary (Ni-Cr) and ternary (Ni-Cr-Fe) alloys at different potentials. The obtained impedance diagrams were interpreted with enhanced graphical method and fitted with Power-Law Model for giving physical meanings to passive film. This experimental procedure provides supplementary data from conventional polarization curves, such as evolution of the passive film thickness and resistivity profiles through the film. In this work, the role of alloying elements, especially chromium and iron, on the passive film properties and reactivity is studied. Understanding how alloying elements act on the passive film nature, structure and properties will provide some guidelines to engineers and scientists for designing or choosing the more adapted alloys. Finally, the proposed experimental method is also applied on certain commercial alloys. Their responses are compared with that of the closest binary and ternary alloys to evidence the discriminating factors that control the passive behavior., Les alliages base nickel sont largement utilisés dans divers domaines d’application, tels que le pétrole et le gaz, le nucléaire et la chimie, grâce à leur excellente résistance à la corrosion et leurs bonnes propriétés mécaniques à haute température. Dans le but de prolonger la durée de vie de certains composants stratégiques, de nombreuses études sur l’influence des éléments d'alliage sur les propriétés et la résistance à la corrosion des alliages base nickel ont été réalisées ou sont toujours en cours. Dans ce cadre, l'objectif principal de cette thèse est de développer une méthodologie basée sur la caractérisation et l'analyse électrochimique permettant de comprendre l’effet des éléments d'alliage sur les propriétés du film passif des alliages base nickel. La méthodologie proposée se base sur des mesures successives de spectroscopie d'impédance électrochimique et sur une analyse approfondie des diagrammes d’impédance, permettant de suivre l'évolution des propriétés du film passif formé sur différents alliages binaires (Ni-Cr) et ternaires (Ni-Cr-Fe) à différents potentiels. Les diagrammes d’impédance obtenus ont été interprétés avec une méthode graphique avancée et ajustés par le modèle en loi de puissance fournissant ainsi des informations caractéristiques du film passif. Cette méthodologie permet ainsi de déterminer et d’obtenir des informations complémentaires aux traditionnelles courbes de polarisation, telles que l’évolution de l’épaisseur du film et des profils de résistivité au travers de son épaisseur. Cette méthodologie a aussi été appliquée à des alliages commerciaux. Leurs réponses ont été comparées à celles obtenues pour les alliages binaires ou tertiaires dont les compositions sont les plus proches afin de mettre en évidence quels sont les paramètres microstructuraux ou chimiques les plus discriminants. Dans ce travail, le rôle des éléments d'alliage, en particulier le chrome et le fer, sur les propriétés et la réactivité du film passif a été étudié. Comprendre comment les éléments d'alliage agissent sur la nature, la structure et les propriétés du film passif fournira quelques lignes directrices aux ingénieurs et scientifiques pour la conception ou le choix des alliages les plus adaptés.