Photoelectrochemical study of nickel base alloys oxide films formed at high temperature and high pressure water

International audienceThe oxide film formed on nickel base alloys at high temperature and high pressure water exhibits semi-conducting properties evidenced by photocurrent generation when exposed to monochromatic light. The use of macro- and micro-photoelectrochemical techniques (PEC and MPEC) aims to identify the different semiconductor phases and their distribution in the oxide film. Three different nickel base alloys were corroded in recirculation loop at 325 °C in pressurised water reactor primary coolant conditions for different exposition durations. PEC experiments on these materials enable to obtain macroscopic energy spectra showing three contributions. The first one, with a band gap around 2.2 eV, was attributed to the presence of nickel hydroxide and/or nickel ferrite. The second one, with a band gap around 3.5 eV, was attributed to Cr2O3. The last contribution, with a band gap in the range of 4.1–4.5 eV, was attributed to the spinel phase Ni1−xFexCr2O4. In addition, macroscopic potential spectra recorded at different energies highlight n-type semi-conduction behaviours for both oxides, Cr2O3 and Ni1−xFexCr2O4. Moreover, MPEC images recorded at different energies exhibit contrasted regions in photocurrent, describing the distribution of nickel hydroxide and/or nickel ferrite and Cr2O3 in the oxide film at a micron scale. It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale

A detailed TEM and SEM study of Ni-base alloys oxide scales formed in primary conditions of pressurized water reactor

International audiencehe oxide film formed on nickel-based alloys in pressurized water reactors (PWR) primary coolant conditions (325 °C, aqueous media) is very thin, in the range of 1–100 nm thick, depending on the surface state and on the corrosion test duration. The nature and the structure of this scale have been investigated by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1−x)FexCr2O4). Moreover, nodules of Cr2O3, with a size about 5 nm, were present at the interface between this spinel and the alloy. No chromium depletion was observed in the alloy, at the alloy/oxide interface. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1−z)Fe(2+z)O4) resulting from precipitation phenomena. SEM and TEM observations showed a link between the nucleation and/or the growth of crystallites of nickel ferrite and the crystallographic orientation of the substrate. A link between the presence of surface defects and the nucleation of the crystallites was also underlined by SEM observations. Partially hydrated nickel hydroxide, was also observed by TEM in the external scale. Based on these results, some considerations about the mechanism of formation of this oxide layer are discussed

The Characterization of the oxide films formed at the surface of Ni-base alloys in pressurized water reactors primary coolant by transmission electron microscopy

International audienceThe oxide film formed on nickel-based alloys in Pressurized Water Reactors (PWR) primary coolant conditions (325°C, aqueous media) has been investigated by Transmission Electron Microscopy (TEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4). Moreover, nodules of Cr2O3 were present at the interface between this spinel and the alloy. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena. The influence of alloy surface defects was also studied underlining two main consequences on the formation of the passive film e.g. the internal layer. On one hand, the growth kinetics of the internal spinel rich in chromium increased with the surface defect density. Besides that, when the defect density increased, the oxide scale became more finely crystallized. This result agrees with a growth mechanism due to a rate limiting process of diffusion through the grain boundaries of the oxide. On the other hand, the quantity of Cr2O3 nodules increased with the number of surface defects, revealing that the nodules nucleated preferentially at defect location

Oxidation kinetics and mechanisms of Ni-base alloys in pressurised water reactor primary conditions: Influence of subsurface defects

International audienceOxidation of Alloy 690 in PWR primary water conditions has been investigated, considering particularly the role played by subsurface structural defects. To simulate a defective surface state, Xe implantation has been set up on samples. Corrosion experiments were thereafter performed in a corrosion loop simulating the PWR medium with durations between 24 h and 1000 h. Microstructural observations and NRA measurements underlined the role played by defects on the crystallinity of the continuous oxide spinel layer, on the nucleation of Cr2O3 nodules and on the oxidation rate. The higher defects concentration seemed to modify the oxygen diffusion in the oxide scale

Corrosion généralisée des alliages à base nickel en milieu aqueux à haute température : Apport à la compréhension des mécanismes

In France, nickel base alloys, such as alloy 600 and alloy 690, are the materials constituting steam generators (SG) tubes of pressurized water reactors (PWR). The generalized corrosion resulting from the interaction between these alloys and the PWR primary media leads, on the one hand, to the formation of a thin protective oxide scale (circa 10 nm), and on the other hand, to the release of cations in the primary circuit, which entails an increase of the global radioactivity of this circuit. The goal of this work is to supply some new comprehension elements about nickel base alloys corrosion phenomena in PWR primary media, taking up with underlining the effects of metallurgical and physico-chemical parameters on the nature and the growth mechanisms of the protective oxide scale. In this context, the passive film formed during the exposition of alloys 600, 690 and Ni-30Cr, in conditions simulating the PWR primary media, has been analyzed by a set of characterization techniques (SEM, TEM, PEC and MPEC, XPS). The coupling of these methods leads to a fine description, in terms of nature and structure, of the multilayered oxide forming during the exposition of nickel base alloys in primary media. Thus, the protective part of the oxide scale is composed of a continuous layer of iron and nickel mixed chromite, and Cr2O3 nodules dispersed at the alloy / mixed chromite interface. The study of protective scale growth mechanisms by tracers and markers experiments reveals that the formation of the mixed chromite is the consequence of an anionic mechanism, resulting from short circuits like grain boundaries diffusion. Besides, the impact of alloy surface defects has also been studied, underlining a double effect of this parameter, which influences the short circuits diffusion density in oxide and the formation rate of Cr2O3 nodules. The sum of these results leads to suggest a description of the nickel base alloys corrosion mechanisms in PWR primary media and to tackle some kinetic aspects which can constitute reflection elements with a view to develop a complete modelization of this corrosion phenomenon.Les alliages à base nickel, et plus précisément les alliages 600 et 690, sont, en France, les matériaux constitutifs des tubes de générateurs de vapeur (GV) des réacteurs à eau pressurisée (REP). La corrosion généralisée résultant de l'interaction entre ces alliages et le milieu primaire des REP conduit, d'une part, à la formation d'une fine couche passive (environ 10 nm) de produits de corrosion, mais également à la dissolution de cations dans le circuit primaire, ces derniers constituant la principale source de contamination de ce circuit. L'objectif de ces travaux est d'apporter de nouveaux éléments de compréhension des phénomènes de corrosion des alliages à base nickel en milieu de type primaire, en s'attachant à décrire l'influence de paramètres métallurgiques ou physico-chimiques, sur la nature et les mécanismes de croissance de la couche d'oxyde protectrice. Dans ce contexte, le film passif formé lors de l'exposition des alliages 600, 690 et Ni-30Cr, dans des conditions simulant le milieu primaire des REP, a été analysé par plusieurs techniques de caractérisation (MEB, MET, PEC et MPEC, XPS). Le couplage de ces méthodes a permis d'obtenir une description fine, en termes de nature et de structure, de l'oxyde multicouche se formant à la surface des alliages à base nickel en milieu de type primaire. Ainsi, la part protectrice de cette couche d'oxyde est constituée d'une couche continue de chromite mixte de nickel et de fer et de nodules de Cr2O3 dispersés à l'interface entre le chromite mixte et l'alliage. L'étude des mécanismes de croissance de la couche protectrice notamment à partir d'expériences mettant en œuvre des marqueurs et des traceurs, a permis de conclure à une croissance de la couche de chromite mixte résultant d'une diffusion anionique, via des courts-circuits de diffusion de type joints de grains. De plus, l'impact du taux de défauts en surface de l'alliage a également été étudié, mettant en évidence un effet double de ce paramètre, qui influence à la fois la densité de courts-circuits de diffusion au sein de l'oxyde mais également la vitesse de formation des nodules de Cr2O3. L'ensemble de ces résultats permet de proposer un mécanisme de corrosion des alliages à base nickel en milieu de type primaire et d'aborder quelques aspects cinétiques susceptibles de constituer des éléments de réflexion en vue du développement d'une modélisation complète de ce phénomène de corrosion

Corrosion des alliages austénitiques inoxydables en milieu primaire des Réacteurs à Eau Pressurisée et absorption d’hydrogène associée : Comment modéliser la cinétique des interactions H /matériau ?

International audienceLes éléments de structure en alliages austénitiques inoxydables (aciers inoxydables, alliages à base nickel) du circuit primaire des Réacteurs à Eau Pressurisée (REP) sont soumis en service à un environnement agressif appelé milieu primaire: de l’eau liquide à environ 325 °C pressurisée sous environ 155 bar, contenant de l’acide borique, de l’hydroxyde de lithium et une légère concentration en dihydrogène dissous. Ces matériaux forment une couche d’oxyde couvrante au contact du milieu primaire, selon des mécanismes maintenant relativement bien identifiés. Au cours de ce processus de corrosion, la réaction cathodique conduit également, par réduction de l’eau, à l’absorption d’hydrogène par les alliages exposés. Cet hydrogène, une fois « inséré » dans l’alliage, peut diffuser en son sein et interagir avec les champs de contraintes, les défauts microstructuraux ou les hétérogénéités cristallines et ainsi modifier le comportement mécanique de celui-ci. Ce phénomène est susceptible de jouer un rôle dans la fissuration de ces matériaux par Corrosion Sous Contrainte (CSC), ce dernier phénomène étant le principal mode de dégradation de ces alliages en service.La présentation s’articulera autour de deux axes d’intérêt pour la modélisation des interactions de l’hydrogène avec les alliages austénitiques inoxydables en conditions de corrosion :- l’absorption d’hydrogène par le matériau au cours du processus de corrosion, qui se focalisera notamment sur le lien entre la croissance de la couche d’oxyde et l’absorption de H,- et le devenir de l’hydrogène dans l’alliage une fois celui-ci absorbé, qui s’intéressera principalement aux interactions H / hétérogénéités cristallines le long du chemin de diffusion.Pour chacun de ces axes, l’état des connaissances relatif aux mécanismes réactionnels gouvernant les phénomènes mis en jeu sera présenté, ainsi que les modèles cinétiques proposés pour rendre compte de ces mécanismes. L’état des développements mathématiques réalisés à l’heure actuelle sera également détaillé. Les limites des approches développées et les difficultés identifiées à ce stade pour aboutir à des modèles mathématiques plus complets et/ou plus robustes seront notamment discutées

Kinetics of hydrogen permeation through a Ni-base alloy membrane exposed to primary medium of pressurized water reactors

International audienceHydrogen permeation kinetics through a Ni-base alloy exposed to simulated pressurized water reactors primary medium at 325 °C has been investigated. In situ measurements exhibit a strong decrease of H flux during the first 50 h of exposure; it has been related to the oxide scale growth at the alloy surface. A kinetic model has been proposed to describe this link. Comparing modelling and experimental results indicates that high H transient activities are generated at the alloy surface during the first hours of oxide scale growth, suggesting that H could play a role in stress corrosion cracking of Ni-base alloys

Corrosion mechanisms of Ni-base alloys in pressurized water reactor primary conditions

International audienceOxidation mechanism of Alloy 690 has been investigated in Pressurised Water Reactor (PWR) primary coolant conditions (325°C, aqueous hydrogenated media). Experiments performed with gold marker and RBS technique reveal that the passive film formation is the consequence of an anionic mechanism. This result is confirmed by experiments achieved with two sequences of corrosion in a H2 16O media and in a mixed H2 16O/ H2 18O media. The localisation of 18O by SIMS analysis in the thin passive layer underlines an oxidation mechanism due to oxygen diffusion by short circuits (like grain boundaries) in the oxide scale. Moreover grain boundary diffusion coefficient in chromite like oxide was estimated to be in the range 2 10-18 - 1 10-17 cm2.s-1 and compared to values extrapolated from higher temperature

Corrosion of nickel-base alloys in primary medium of pressurized water reactors: New insights on the oxide growth mechanisms and kinetic modelling

International audienceThe generalised corrosion of Ni-base alloys exposed to pressurized water reactors primary coolant leads both to the growth of a protective oxide scale and to the release of cations. Based on experimental results previously published, a mechanistic description of this phenomenon is proposed. Then, from a simplified view of theses mechanisms, a kinetic model is proposed, focusing on the link between the alloy subsurface defects density and the shape of oxidation kinetic curves. Comparing calculated curves and experimental data shows this first approach is promising.Nevertheless a better knowledge of some kinetic parameters is necessary to improve the model