Depletion and voids formation in the substrate during high temperature oxidation of Ni-Cr alloys

A numerical model to treat the kinetics of vacancy annihilation at the metal/oxide interface but also in the bulk metal has been implemented. This was done using EKINOX, which is a mesoscopic scale 1D-code that simulates oxide growth kinetics with explicit calculation of vacancy fluxes. Calculations were performed for high temperature Ni-Cr alloys oxidation forming a single chromia scale. The kinetic parameters used to describe the diffusion in the alloy were directly derived from an atomistic model. Our results showed that the Cr depletion profile can be strongly affected by the cold work state of the alloy. In fact, the oversaturation of vacancies is directly linked to the efficiency of the sinks which is proportional to the density of dislocations. The resulting vacancy profile highlights a supersaturation of vacancy within the metal. Based on the classical nucleation theory, the possibility and the rate of void formation are discussed

Thermodynamique des agrégats bimétalliques : surface, volume et effet de taille finie

The finite size of the nanoparticles confers specific characteristics, both in the surface and in the core. In this context, we studied the thermodynamics of bimetallic clusters using an Ising model by both Monte Carlo simulations and also the mean-field approximation. We considered the Cu-Ag system, a prototype of systems with a strong tendency to phase separate. For a cuboctahedron of thousands of atoms, Monte Carlo simulations show that the different facets of the surface undergo a dynamical equilibrium at a sufficiently low temperature. We can then predict, in the same cluster, the coexistence of copper-rich and silver-rich (001) facets, similar results are obtained for (111) facets, and even edges. Also a dynamical equilibrium is observed for the bulk layers, analogous of the phase separation for the infinite system. By studying the linear chain we determined the parameters that control the competition between the wetting (Ag enrichment from the surface that progressively reachs the core) and the installation of a dynamical equilibrium initiated in the core. The influence of the chain size on this competition was investigated, yielding the draft of the phase diagram of a finite structure that takes into account superficial segregation. This rigid-lattice study was later completed taking in account atomic relaxations, via a N-interatomic potential, for the icosahedron case. We illustrate the stress effects inherent in this structure, that induce strong heterogeneities of the intra - and inter-layer composition.La taille finie des nanoparticules leur confère des caractéristiques spécifiques, tant en surface qu'en leur coeur. Dans ce contexte, nous avons étudié la thermodynamique des agrégats bimétalliques à l'aide d'un modèle d'Ising traité par simulations Monte Carlo et également dans l'approximation de champ moyen. Nous avons considéré le système Cu-Ag, prototype des systèmes à forte tendance à la démixtion. Pour un cuboctaèdre de quelques milliers d'atomes, les simulations Monte Carlo ont montré que les différentes facettes de la surface sont le lieu d'un équilibre dynamique à suffisamment basse température. Ainsi, au sein d'un même agrégat, nous prédisons la coexistence de facettes (100) riches en cuivre et riches en argent, ce résultat s'étendant aux facettes (111) et mêmes aux arêtes. Un équilibre dynamique similaire est également observé pour les couches de coeur, équivalent de la démixtion pour le système infini. L'étude de la chaîne linéaire nous a permis de déterminer les facteurs régissant la compétition entre le mouillage (enrichissement en Ag partant de la surface et gagnant progressivement le coeur) et l'établissement d'un équilibre dynamique initié au coeur. L'influence de la taille de la chaîne sur cette compétition a été spécialement étudiée, ce qui conduit à l'ébauche d'un diagramme de phase de milieu fini intégrant la ségrégation superficielle. Nous avons complété cette étude sur réseau par la prise en compte des relaxations atomiques, via un potentiel interatomique à N-corps, dans le cas de l'icosaèdre. Nous illustrons les effets des contraintes propres à cette structure, qui se manifestent par de fortes hétérogénéités de compositions intra et inter couches

Impact des ions chlorures : interaction entre le matériau et son environnement

RAPPORT CEA-R-6580Natural and complex, the corrosion phenomenon is an industrial problem that can take various forms, ranging from a simple loss of uniform material to more complex aspects. In atmospheric corrosion, the presence of pollutants plays a key role in the corrosion phenomenon, particularly chloride-based pollutants. If, in the first place, we think of the seaside with the presence of sodium chloride NaCl, chloride ions are also widely used in industry. What makes the difference is the cation with which the anion is associated. The objective of the study is to understand the corrosion phenomenathat can appear when a material is exposed to a humid environment in the presence of chlorinated pollutants such as HCl or NaCl. If corrosion phenomena are at the heart of the problem, the study of the electrolyte is an essential point. From its creation to its disappearance, including its finite size, the electrolyte plays a key role in atmospheric corrosion phenomena because its presence ensures the transport of ionic species, and therefore of current. This HDR shows how factors inherent in the medium such as aeration, acidity, or salinity of the medium, can influence the course of electrochemical processes ; and how external parameters such as temperature or hydrodynamics of the solution can also be a source of significant variations in corrosion phenomena.Naturel et complexe, le phénomène de corrosion est un problème industriel pouvant revêtir des formes variées allant d’une simple perte de matière uniforme à des aspects plus complexes. En corrosion atmosphérique, la présence de polluants joue un rôle clé dans le phénomène de corrosion et particulièrement les polluants à base de chlorures. Si en tout premier lieu, on pense au bord de mer avec la présence de chlorure de sodium NaCl, les ions chlorures sont aussi très utilisés dans l’industrie. Ce qui fait la différence c’est le cation avec lequel l’anion est associé. L’objectif de l’étude est de comprendre les phénomènes de corrosion qui peuvent apparaître lorsqu’un matériau est exposé à un environnement humide en présence de polluants chlorurés tels que HCl ou NaCl. Si les phénomènes de corrosion sont au coeur de la problématique, l’étude de l’électrolyte est un point essentiel. De sa création à sa disparition en passant par sa taille finie, l’électrolyte joue un rôle clé dans les phénomènes de corrosion atmosphérique car sa présence assure le transport des espèces ioniques, et donc du courant. Cette Habilitation à Diriger des Recherches (HDR) montre comment des facteurs inhérents au milieu comme l’aération, l’acidité, ou la salinité du milieu, peuvent influer le déroulement des processus électrochimiques ; et comment les paramètres externes tels que la température ou l’hydrodynamique de la solution peuvent également être une source de variations importantes des phénomènes de corrosion

Impact des ions chlorures : interaction entre le matériau et son environnement

RAPPORT CEA-R-6580Natural and complex, the corrosion phenomenon is an industrial problem that can take various forms, ranging from a simple loss of uniform material to more complex aspects. In atmospheric corrosion, the presence of pollutants plays a key role in the corrosion phenomenon, particularly chloride-based pollutants. If, in the first place, we think of the seaside with the presence of sodium chloride NaCl, chloride ions are also widely used in industry. What makes the difference is the cation with which the anion is associated. The objective of the study is to understand the corrosion phenomenathat can appear when a material is exposed to a humid environment in the presence of chlorinated pollutants such as HCl or NaCl. If corrosion phenomena are at the heart of the problem, the study of the electrolyte is an essential point. From its creation to its disappearance, including its finite size, the electrolyte plays a key role in atmospheric corrosion phenomena because its presence ensures the transport of ionic species, and therefore of current. This HDR shows how factors inherent in the medium such as aeration, acidity, or salinity of the medium, can influence the course of electrochemical processes ; and how external parameters such as temperature or hydrodynamics of the solution can also be a source of significant variations in corrosion phenomena.Naturel et complexe, le phénomène de corrosion est un problème industriel pouvant revêtir des formes variées allant d’une simple perte de matière uniforme à des aspects plus complexes. En corrosion atmosphérique, la présence de polluants joue un rôle clé dans le phénomène de corrosion et particulièrement les polluants à base de chlorures. Si en tout premier lieu, on pense au bord de mer avec la présence de chlorure de sodium NaCl, les ions chlorures sont aussi très utilisés dans l’industrie. Ce qui fait la différence c’est le cation avec lequel l’anion est associé. L’objectif de l’étude est de comprendre les phénomènes de corrosion qui peuvent apparaître lorsqu’un matériau est exposé à un environnement humide en présence de polluants chlorurés tels que HCl ou NaCl. Si les phénomènes de corrosion sont au coeur de la problématique, l’étude de l’électrolyte est un point essentiel. De sa création à sa disparition en passant par sa taille finie, l’électrolyte joue un rôle clé dans les phénomènes de corrosion atmosphérique car sa présence assure le transport des espèces ioniques, et donc du courant. Cette Habilitation à Diriger des Recherches (HDR) montre comment des facteurs inhérents au milieu comme l’aération, l’acidité, ou la salinité du milieu, peuvent influer le déroulement des processus électrochimiques ; et comment les paramètres externes tels que la température ou l’hydrodynamique de la solution peuvent également être une source de variations importantes des phénomènes de corrosion

What happens when a Pb–Sn coating deposited on low carbon steel is exposed in an HCl‐polluted wet environment? Development of a corrosion mechanism

International audienceThis article synthesizes all the results obtained to establish a global corrosion mechanism when a Pb–Sn coating deposited on low carbon steel is corroded in an HCl-polluted wet environment. The successive stages of the process are shown. In HCl-polluted environment, coupled with water, acid chlorides provide an aggressive electrolyte particularly favorable to corrosion. In contact with this electrolyte, lead products are created as PbCO3 and PbCl2, showing the impact of the surrounding atmosphere. In parallel, the electrolyte concentrates in Sn2+ until saturation, and tin precipitates under different forms. Due to the lead consumption, the lead corrosion products layer breaks, and the coating thins, diffusion pathways are created and the steel oxidizes. Iron corrosion products are similar with or without a coating except for the presence of a tin-rich filament. Lead is no longer present. The different stages of the mechanism are compared to corrosion phenomena observed during the use of Pb–Sn alloys to provide effective solutions to minimize or even avoid the phenomenon of corrosion

The corrosion mechanism initiation of a 75Sn–25Pb coating on a low‐carbon steel sample in HCl environments

International audienceThe corrosion of a 75Sn–25Pb coating on low-carbon steel exposed to a vapor/condensate phase over a 1 mol/L HCl solution has been studied. The corrosion mechanism initiation was based on several steps. The presence of CO2 and lead involves the precipitation of cerussite PbCO3 crystals. Overtime, the cerussite evolved and transformed into PbCl2. This transformation was not direct and involved the creation of reaction intermediates whose chemical composition was variable. In parallel, tin formed oxides that were not stable in the HCl solution. These oxides dissolved and led to the formation of an electrolyte concentrated in Sn2+. When tin ions' saturation was reached, tin precipitated to form abhurite, Sn21Cl16(OH)14O6, or hydrated tin chloride, SnCl2.2H2O. This study highlighted the particularity of the corrosion mechanism. It also showed that the presence of tin promoted the oxidation of lead and consequently the formation of cerussite

Thermodynamique des agrégats bimétalliques (surface, volume et effet de taille finie)

La taille finie des nanoparticules leur confère des caractéristiques spécifiques, tant en surface qu en leur cœur. Dans ce contexte, nous avons étudié la thermodynamique des agrégats bimétalliques à l aide d un modèle d'Ising traité par simulations Monte Carlo et également dans l'approximation de champ moyen. Nous avons considéré le système Cu-Ag, prototype des systèmes à forte tendance à la démixtion. Pour un cuboctaèdre de quelques milliers d atomes, les simulations Monte Carlo ont montré que les différentes facettes de la surface sont le lieu d un équilibre dynamique à suffisamment basse température. Ainsi, au sein d un même agrégat, nous prédisons la coexistence de facettes (100) riches en cuivre et riches en argent, ce résultat s étendant aux facettes (111) et mêmes aux arêtes. Un équilibre dynamique similaire est également observé pour les couches de cœur, équivalent de la démixtion pour le système infini. L étude de la chaîne linéaire nous a permis de déterminer les facteurs régissant la compétition entre le mouillage (enrichissement en Ag partant de la surface et gagnant progressivement le cœur) et l établissement d un équilibre dynamique initié au cœur. L influence de la taille de la chaîne sur cette compétition a été spécialement étudiée, ce qui conduit à l ébauche d un diagramme de phase de milieu fini intégrant la ségrégation superficielle. Nous avons complété cette étude sur réseau par la prise en compte des relaxations atomiques, via un potentiel interatomique à N-corps, dans le cas de l icosaèdre. Nous illustrons les effets des contraintes propres à cette structure, qui se manifestent par de fortes hétérogénéités de compositions intra et inter couches.The finite size of the nanoparticles confers specific characteristics, both in the surface and in the core. In this context, we studied the thermodynamics of bimetallic clusters using an Ising model by both Monte Carlo simulations and also the mean-field approximation. We considered the Cu-Ag system, a prototype of systems with a strong tendency to phase separate. For a cuboctahedron of thousands of atoms, Monte Carlo simulations show that the different facets of the surface undergo a dynamical equilibrium at a sufficiently low temperature. We can then predict, in the same cluster, the coexistence of copper-rich and silver-rich (001) facets, similar results are obtained for (111) facets, and even edges. Also a dynamical equilibrium is observed for the bulk layers, analogous of the phase separation for the infinite system. By studying the linear chain we determined the parameters that control the competition between the wetting (Ag enrichment from the surface that progressively reachs the core) and the installation of a dynamical equilibrium initiated in the core. The influence of the chain size on this competition was investigated, yielding the draft of the phase diagram of a finite structure that takes into account superficial segregation. This rigid-lattice study was later completed taking in account atomic relaxations, via a N-interatomic potential, for the icosahedron case. We illustrate the stress effects inherent in this structure, that induce strong heterogeneities of the intra - and inter-layer composition.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Corrosion initiation of Hastelloy C‐2000 in an HCl‐polluted atmosphere

In this study, welded and non-welded Hastelloy C-2000 samples are tested at room temperature in HCl environments, with two experiments: one in hydrochloric acid solutions and one in HCl vapor/condensate phases. In the hydrochloric acid solutions, there is no corrosion after more than 900 hr of exposure. In the humid air polluted by HCl, corrosion occurs in the form of an intergranular attack at the metal grain boundaries. In the weld bead zone, areas rich in molybdenum and depleted of nickel seem to be less susceptible to corrosion than the zones depleted of molybdenum and rich in nickel. This study highlights that Hastelloy C-2000, exposed to HCl vapor/condensate phases, is more penalizing than when immersed in an HCl solution, which is due to the presence of oxygen

Numerical study of evaporating sessile droplets and corrosion

International audienceIn France, some of nuclear wastes are packaged in stainless steel containers and stored in above-ground storage facilities. These waste packages are likely to remain in interim storage for a periodof several decades before being moved to an underground facility for permanent disposal. During theinterim period of storage, atmospheric aerosols will deposit on the container surfaces, in particularin marine atmosphere, where sprayed droplets are carried by the wind from the sea. The dropletdeposits on the exposed surface and becomes saline sessile droplet. This creates an electrolyte offinite size that can react with the metallic surface, leading to an atmospheric corrosion phenomenon.Moreover, the day/night cycles induce a variation of the relative humidity and the temperature: thisleads to a constant repetition of evaporation and condensation phases which can increase the corrosionphenomena [1].The aim of the study is to enrich and develop a numerical analysis of the corrosion under droplet, soas to accurately predict its impact on the lifetime of the materials. We propose a numerical modelof evaporation dynamics of salty sessile drops, built with Basilisk, a free software which implementsfinite volume methods for the Navier-Stokes equations [2]. Our numerical results will be validate withlitterature and experimental data

Numerical model using a Volume-Of-Fluid method for the study of evaporating sessile droplets in both unpinned and pinned modes

International audienceThis article focuses on the numerical study of the evaporation of sessile dropletsusing a Volume-Of-Fluid (VOF) method in the free-software Basilisk. We con-sider pure liquid droplets forming a spherical-cap onto a smooth or rough andnon-corrosive substrate and we investigate two different modes of evaporation:the unpinned mode where the contact angle is constant and the pinned modewhere the wetting area is constant. The numerical method used to implementthe contact angle and for the reconstruction of the interface is fully described,especially for the pinned mode where we propose a new VOF implementation.In the unpinned mode, we perform parametric studies and we show the influenceof the relative humidity and the contact angle on the evaporation process. Forall the explored parameters, the simulations predict that the volume decreaseslinearly with time, which matches the signature behaviour of evaporating un-pinned droplets, irrespective of the geometrical parameters. In the pinned mode,the contact angle analysis indicates a linear decrease in time which was expectedaccording to the theory and validated with some experiments we performed