Etude des mécanismes et des cinétiques de corrosion aqueuse de l alliage d aluminium AlFeNi utilisé comme gainage du combustible nucléaire de réacteurs expérimentaux

Pour le réacteur nucléaire expérimental Jules Horowitz, l'alliage d'aluminium AlFeNi sera utilisé pour le gainage du combustible nucléaire. Cet alliage (Al 1%Fe 1%Ni 1%Mg) a été développé pour sa bonne résistance à la corrosion aqueuse à haute température. Pourtant à ce jour peu d'études ont porté sur les processus de dégradation en milieu aqueux et sur les effets de l'irradiation sur cet alliage. Or la conception du réacteur nécessite une meilleure compréhension des mécanismes de corrosion. Des expériences de corrosion ont donc été menées en autoclaves à 70C, 165C et 250C sur des tôles en AlFeNi, représentatives du gainage combustible. Différentes techniques ont été utilisées pour caractériser le produit de corrosion : MEB, MET, MASE, DRX, spectroscopie Raman. Les observations associées montrent une structure duplex : une couche dense et amorphe près du métal et une couche cristalline et poreuse au contact de l'eau. Leurs compositions chimiques sont différentes et dépendent des éléments solubilisés dans l eau. Complétées par des expériences de marquage, ces analyses ont démontré que cette structure résulte d'un mécanisme de croissance mixte des couches d'oxyde avec des diffusions cationique et anionique cinétiquement couplées. Des quantifications des vitesses de relâchement en solution, des épaisseurs d'oxyde et de corrosion de l'alliage ont permis de proposer un mécanisme de dégradation de l'alliage AlFeNi associant diffusion cationique et processus de dissolution/précipitation en solution. Une comparaison avec des couches d'oxydes formées en réacteur sur des plaques combustibles a mis en évidence l'importance du régime hydraulique sur les processus de dégradation de l'alliage, l'irradiation semblant être un paramètre de second ordre.For the Jules Horowitz new material-testing reactor (JHR), an aluminium base alloy, called AlFeNi, will be used for the cladding of the fuel plates. This alloy (Al 1% Fe 1% Ni 1 % Mg) which is already used as fuel cladding, was developed for its good corrosion resistance in water at high temperatures. However, few studies dealing with the alteration process in water and the relationships with irradiation effects have been performed on this alloy. The conception of the JHR fuel requires a better knowledge of the corrosion mechanisms. Corrosion tests were performed in autoclaves at 70C, 165C and 250C on AlFeNi plates representative of the fuel cladding. Several techniques were used to characterize the corrosion scale: SEM, TEM, EPMA, XRD, Raman spectroscopy. Our observations show that the corrosion scale is made of two main layers: a dense amorphous scale close to the metal and a porous crystalline scale in contact with the water. More than the morphology, the chemical compositions of both layers are different. This duplex structure results from a mixed growth mechanism: an anionic growth to develop the inner oxide and a cationic diffusion followed by a dissolution-precipitation process to form the outer one. Dynamic experiments at 70C and corrosion kinetics measurements have demonstrated that the oxide growth process is controlled by a diffusion step associated to a dissolution/precipitation process. A corrosion mechanim of the AlFeNi alloy in aqueous media has been proposed. Then post-irradiation exams performed on irradiated fuel plates were used to investigate the effects of the irradiation on the corrosion behaviour in the reactor core.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Impact of ion and neutron irradiation on the corrosion of the 6061-T6 aluminium alloy

International audience• Ion irradiation dehydrates aluminium hydroxide (boehmite and bayerite). • Ion irradiation induces microstructure changes and voids in aluminium hydroxide. • Neutron and ion irradiations increase aluminium alloy corrosion. • Neutron irradiation causes silicon enrichment of aluminium hydroxide

Correlation between quenching rate, mechanical properties and microstructure in thick sections of Al Mg Si( Cu) alloys

International audienceThermal treatment is key for the mechanical behavior of 6000 series aluminum alloys. Numerous studies have therefore been devoted to the impact of aging treatment parameters like time and temperature. However the influence of quenching rate is rather poorly documented while it could be of primary interest for applications requiring thick sections. Using a series of AleMgeSi(eCu) specimens quenched in various fluids, we propose to monitor and describe the quench properly and study the impact of the quenching rate on the microstructure and mechanical properties obtained after aging. Acknowledging the critical quench rate to be about 10°C s −1 , quenching was performed in various quenching fluids (water, air and oil) on 10 cm side 6061 alloy cubes. The quench rate was evaluated by thermocouples placed at several locations and by numerical modeling. For each temper, the microstructure and mechanical behavior has been studied at local and macro scales to highlight some correlations. The combination of transmission electron microscopy imaging and related image analysis, chemical and crystallographic mapping, microhardness maps and tensile tests has pointed out many micro-structural differences at nanoscale and inhomogeneous mechanical behavior we strived to explain and correlate

Effet de la température et du pH sur la corrosion de l'alliage d'Al 6061-T6 et caractérisation des couches d'oxydes

International audienceDuring operation in material testing reactors (MTR), the AA-6061-T6 alloy used for core components is covered by aluminium hydroxide. In this work we investigate the effects of parameters on pitting and uniform corrosion of the aluminium alloy 6061-T6 in the conditions found in MTR for the core structures: temperature (70e100 C) and pH (5 and 7.5). After corrosion experiments, characterisation techniques are used to observe the formed hydroxide, its composition and crystallographic structure (TEM, SEM, EDX, XRD, m-Raman spectroscopy and electron diffraction) and formulate hypotheses on the mechanisms of corrosion at play. At low temperature (70 C), the film is composed of two layers of different aluminium hydroxide phases. These two phases have different corrosion behaviours depending on the pH. The pH has a strong impact of the outer layer but not on the inner layer. PH increase is observed in deaerated medium, this pH variation is due to oxidation of magnesium of the alloy. In addition, corrosion models are used to predict the hydroxide thickness in MTR for a safe and efficient operation. These models are based on empirical correlations. The data obtained from this study allows to discuss the application of these models at low temperature (70 C)