Mécanismes d'absorption de l'hydrogène en milieux aqueux dans des aciers austénitiques Fe-Mn-C (conséquences sur l'endommagement)

Dans un contexte industriel en évolution permanente, les aciers austénitiques Fe-Mn-C sont développés afin d obtenir une résistance mécanique élevée tout en conservant une ductilité considérable. Cependant, ces aciers présentent une sensibilité à différentes formes d'endommagement par l'hydrogène, notamment la corrosion sous contrainte. L'objectif de cette étude est de caractériser l'influence de la composition chimique et de l'état microstructural de ces alliages sur leur sensibilité aux phénomènes de fragilisation par l hydrogène (FPH), lorsqu ils sont soumis à des phénomènes de corrosion aqueuse. Ce travail est introduit par une présentation générale de la métallurgie des aciers austénitiques Fe-Mn-C ainsi que des méthodes expérimentales utilisées. Le traçage isotopique de l hydrogène avec du deutérium par analyse SIMS a permis d étudier les mécanismes de diffusion de l hydrogène dans ces alliages. L étude du comportement en corrosion aqueuse de ces alliages, par des tests électrochimiques et des immersions au potentiel libre dans des environnements aqueux deutérés, a mis en évidence l influence des éléments d addition sur la prise d hydrogène associée à la corrosion.La sensibilité à la FPH de ces aciers a été caractérisée par des essais de traction in situ. Ils montrent que les interactions hydrogène-plasticité ont un rôle essentiel sur les mécanismes régissant la FPH. Cette étude a également montré une forte influence des éléments d addition sur la FPH. Finalement, les résultats de cette étude nous ont permis de discuter des mécanismes associés à l influence des éléments d addition sur la sensibilité à la FPH de ces aciers.The automotive industry is a sector in constant evolution, in which the lightening of structures by the use of new alloys, in order to save energy, is one of the main objectives. In this context, austenitic Fe-Mn-C steels are developed in order to obtain high mechanical strength associated with considerable ductility. However, these steels are sensitive to different forms of hydrogen damage, in particular stress corrosion cracking. The objective of this study is to characterize the influence of the chemical composition and the microstructural state of these alloys on their sensitivity to hydrogen embrittlement (HE) phenomena associated with corrosion process in aqueous media. This work starts with a general presentation of the metallurgical properties of austenitic Fe-Mn-C steels and of the experimental techniques. Then, the isotopic tracing of hydrogen with deuterium by SIMS analysis allowed studying hydrogen diffusion mechanisms in these alloys. The corrosion of these steels in aqueous media have been studied by electrochemical tests and immersions at the rest potential in deuterated solution ; the influence of alloying elements on the hydrogen absorption during corrosion war characterized in detail. In situ tensile tests were used to characterize the HE susceptibility of these steels. They show that hydrogen-plasticity interactions play a predominant role in the HE mechanisms. This study also showed a strong influence of alloying elements on HE. Finally, the results of this study allowed discussing the mechanism involved in the role of alloying elements on the HE susceptibility of these steels.EVRY-Bib. électronique (912289901) / SudocSudocFranceF

Corrosion mechanism of a Ni-based alloy in supercritical water: Impact of surface plastic deformation

Ni–Fe–Cr alloys are expected to be a candidate material for the generation IV nuclear reactors that use supercritical water at temperatures up to 600 °C and pressures of 25 MPa. The corrosion resistance of Alloy 690 in these extreme conditions was studied considering the surface finish of the alloy. The oxide scale could suffer from dissolution or from internal oxidation. The presence of a work-hardened zone reveals the competition between the selective oxidation of chromium with respect to the oxidation of nickel and iron. Finally, corrosion mechanisms for Ni based alloys are proposed considering the effects of plastically deformed surfaces and the dissolution

Corrosion mechanisms of 316L stainless steel in supercritical water: The significant effect of work hardening induced by surface finishes

The oxidation of 316 L stainless steel in hydrogenated supercritical water at 600 °C is strongly dependent on the effects of work hardening induced by surface finishes. The oxide scale formed under these conditions is always double-layered with an external layer of Fe-rich oxides. However, when a hardening threshold is reached, a switch in oxidation mechanisms leads to a considerable thinning of the oxide scale. This thinning results from the formation of a Cr-rich internal oxide layer that acts as a diffusion barrier against ionic species responsible for its growth but also against Fe cations implied in the external layer growth

Corrosion généralisée des alliages à base nickel en milieu aqueux à haute température (apport à la compréhension des mécanismes)

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 ( 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.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 ( 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.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

XPS study of Ni-base alloys oxide films formed in primary conditions of pressurized water reactor

International audienceThe oxide scales formed on alloy 690 during its exposure to pressurized water reactor primary simulated media at 325 °C for different periods, ranging between 24 and 858 h have been characterized through XPS analysis. These results have been compared with the data obtained on Cr2O3, NiCr2O4, NiFe2O4, and Ni(OH)2 reference compounds. This comparison leads to the following conclusions about the scale formed on alloy 690: (i) the deconvolution of Cr-2p3/2 core level spectra seems to show that the internal oxide scale is made of a mix of chromite and chromia; (ii) the evolution of the height ratio between satellite and principal peaks, on the Ni-2p3/2 core level, has highlighted the presence of nickel hydroxide for all corrosion durations; and (iii) XPS analysis can be used to study the kinetics of nickel ferrite precipitation. In order to obtain a more complete description of such oxide scales based upon XPS measurements, reference data, obtained on nickel ferrites and chromites in which Fe is partially substituted to Ni, are needed. The comparison of published data and reference spectra performed in this work also leads to conclude that core level fits are dependent on the energy resolution, which can be achieved with specific spectrometer and analysis procedures. Therefore, reference compound spectra need to be recorded in the very identical analytical configuration before dealing with results obtained on corroded specimen

Numerical modeling of thermal desorption mass spectroscopy (TDS) for the study of hydrogen diffusion and trapping interactions in metals

International audienceDeriving the kinetic reaction constants associated with hydrogen diffusion and trapping in metals from thermal desorption mass spectroscopy (TDS) spectra proves to be complicated and the existing analysis methods are subject to debate. This article will provide a brief background of several commonly employed analysis techniques and discuss the necessity of a more complex and rigorous analysis method for the determination of the kinetic constants associated with hydrogen trapping interactions. Furthermore, a numerical simulation method will be proposed using the McNabb & Foster equations to fit experimental TDS spectra in order to derive both diffusion and trapping/detrapping parameters, including the respective pre-exponential constants and activation energies associated with these interactions in metals