Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

International audienceCorrosion of a chromia-forming nickel base alloy, Haynes 230_, has been investigated under impure helium containing a few Pa of CO and H2O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H2O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H2O is the major oxidant of chromia-forming nickel base alloys in impure helium

Mechanism of destruction of the protective oxide layer on Alloy 230 in the impure helium atmosphere of Very High Temperature Reactors.

International audienceAlloy 230 which contains 22wt.% chromium could be a promising candidate material for structures and heat exchangers (maximum operating temperature: 850°-950°C) in Very High Temperature Reactors (VHTR). The feasibility demonstration involves to valid its corrosion resistance in the reactor specific environment namely impure helium. The alloys surface reactivity was investigated at temperatures between 850 and 1000°C. Two main behaviours have been revealed: the formation of a protective Cr/Mn rich oxide layer at 900°C and its following destruction at higher temperatures. Actually, above a critical temperature called TA, oxide is reduced at the oxide/metal interface by carbon in solution in the alloy. To ascribe the scale destruction, a model is proposed based on thermodynamic interfacial data for the alloy (chromium and carbon activity), oxide layer morphology and carbon monoxide partial pressure in helium. The proposed mechanism is then validated regarding experimental results and observations on alloy 230 and model alloys

Carburization of austenitic and ferritic steels in carbon-saturated sodium: preliminary results on the diffusion coefficient of carbon at 873 K

Three commercial steels were exposed to carbon-saturated sodium at 873 K for durations up to 5000 h. Analyses by optical microscopy, infrared-inductive carbon combustion, electron probe microanalysis and glow discharge optical emission spectrometry revealed important carburization of the steels. The carbon concentration at the metal–sodium interface reached equilibrium, and the carbon uptake varied with the square root of time. The carburization kinetics was well described by assuming that diffusion of carbon was coupled with rapid carbide precipitation and equilibrium partitioning of carbon between the metal and precipitates phases

Determination of residual stress gradient in a Ti-stabilized austenitic stainless steel cladding candidate after carburization in liquid sodium at 500 °C and 600 °C

Non-destructive residual stress profile measurement with a micrometric depth resolution within the depth of carburized Ti-stabilized stainless steel cladding candidate was carried out by high-energy X-ray diffraction. The samples were carburized in carburizing nuclear liquid sodium at 500 °C and 600 °C for 1000 h. The full residual stress tensor profile was determined thanks to the estimation of the strain-free lattice parameter evolution using the carbon concentration profile measured by electron probe microanalysis and thermodynamic simulation. For the sample carburized at 500 °C, the residual stress genesis was governed by the carbon concentration within the steel and the formation of expanded austenite. For the sample carburized at 600 °C, the residual stress profile in the austenitic matrix depended on the precipitation of M23C6 carbide. Stress relaxation was observed in the intragranular carburization zone. For the two temperatures, compressive residual stresses developed in the carburized zone and tensile stresses developed in the rest of the sample

Microstructural and Chemical Changes of a Ti-Stabilized Austenitic Stainless Steel After Exposure to Liquid Sodium at Temperatures Between 500 °C and 650 °C

Ti-stabilized austenitic stainless steel was carburized in sodium containing a high carbon activity at three different temperatures, 500 °C, 600 °C, and 650 °C during 1000 hours and 5000 hours. The carbon profile, the carbide volume fraction, and the lattice parameter evolution as function of depth were determined using high-energy X-ray diffraction and electron probe microanalysis. At 650 °C and 600 °C, the carbon precipitated as M23C6 and M7C3 carbides in the sample. The volume fraction of M7C3 carbides was lower than predicted by thermodynamic equilibrium using Thermo-Calc software®. At 500 °C, carbides almost did not form in the steel. Instead, high carbon supersaturation of the austenitic matrix occurred. Both results demonstrate that the carburization profile was strongly influenced by the kinetics of carbide formation at temperatures lower than 650 °C. High-energy X-ray diffraction measurements demonstrated that the austenite and carbide lattice parameters evolved along the carbon profile. Both measured lattice parameter profiles of austenite and M23C6 carbide were compared to the ones predicted from chemical changes of austenite and carbides

Chemical Interaction of Austenitic and Ferritic Steels with B4C Powder in Liquid Sodium at 600°C

In the framework of studies on the control rods’ lifetime for Sodium Fast Reactor, three commercial steels were exposed to B4C powder in liquid sodium at 600°C for durations up to 3,000 h. Analyses by optical and secondary electron microscopy, x-ray diffraction, electron microprobe, and glow discharge optical emission spectrometry revealed the formation of boride layers at the surface of the steels and slight carburization underneath. The growth of the boride layers followed parabolic kinetics. The nature of the formed boride layers was in good agreement with thermodynamic equilibrium predicted by software. The carburization depths were much lower than the ones obtained in carburizing liquid sodium at 600°C. Finally, the carbon penetration depth did not grow with time revealing a transient carburization phenomenon and possible protective character of the boride layers against carbon penetration

Carburization of austenitic and ferritic stainless steels in liquid sodium: Comparison between experimental observations and simulations

Three steels were exposed in carburizing sodium at 600 and 650 °C. The kinetics and extent of carburization were characterized. Numerical simulations using the coupled thermodynamic-kinetic modeling software DICTRA were performed. It was proposed that the observed carbon diffusion profiles were induced by the combined diffusion of carbon in the grains and at grain boundaries coupled with the slow formation of carbides. The blocking effect of carbides on the carbon diffusion was observed to evolve as a function of time and microstructure. Acceptable agreement between experimental and simulated intragranular carbon profiles was achieved by optimizing the labyrinth factor and phases

MÉCANISMES DE FORMATION ET DE DESTRUCTION DE LA COUCHE D'OXYDE SUR UN ALLIAGE CHROMINOFORMEUR EN MILIEU HTR

Haynes 230® alloy which contains 22wt.% chromium could be a promising candidate material for structures and heat exchangers (maximum operating temperature: 850°-950°C) in Very High Temperature Reactors (VHTR). The feasibility demonstration involves to valid its corrosion resistance in the reactor specific environment namely impure helium. The alloys surface reactivity was investigated at temperatures between 850 and 1000°C. We especially focused on the influence of different parameters such as concentrations of impurities in the gas phase (carbon monoxide and methane, water vapour/hydrogen ratio), alloy composition (activities of Cr and C, alloying element contents) and temperature. Two main behaviours have been revealed: the formation of a Cr/Mn rich oxide layer at 900°C and its following reduction at higher temperatures. At 900°C, the water vapour is the main oxidizing gas. However in the initial times, the carbon monoxide reacts at the metal/oxide interface which involves a gaseous transport through the scale; CO mainly oxidizes the minor alloying elements aluminium and silicon. Above a critical temperature TA, the carbon in solution in the alloy reduces chromia. To ascribe the scale destruction, a model is proposed based on thermodynamic interfacial data for the alloy, oxide layer morphology and carbon monoxide partial pressure in helium; the model is then validated regarding experimental results and observations.Le superalliage à 22%mas. en chrome, Haynes 230®, est un matériau candidat pour les échangeurs de chaleur (température maximale 850°C-950°C) des Réacteurs à Caloporteur Gaz, également appelés HTRs (High Temperature Reactors). Dans l'optique de valider les performances de cet alliage, il faut garantir sa résistance à la corrosion dans l'environnement d'hélium impur de ces réacteurs. Dans cet objectif, la réactivité de surface de l'Haynes 230® a été examinée à des températures comprises entre 850 et 1000°C. On s'est attaché à caractériser l'influence de différents paramètres tels que concentrations en impuretés du gaz (monoxyde de carbone, méthane et rapport vapeur d'eau/dihydrogène), caractéristiques de l'alliage (activités en Cr et en C, teneurs en éléments mineurs) et température d'exposition. Deux principaux comportements ont pu être mis en évidence : la formation d'une couche d'oxyde riche en Cr et Mn à 900°C et sa réduction à plus hautes températures. A 900°C, la vapeur d'eau est l'oxydant principal. Toutefois dans les temps initiaux, le monoxyde de carbone réagit à l'interface métal/oxyde ce qui implique un transport de gaz au travers de la couche ; CO semble préférentiellement oxyder les éléments mineurs aluminium et silicium. A partir d'une température critique TA, le carbone en solution dans l'alliage réduit l'oxyde de chrome. Un modèle de destruction de la couche basé sur les grandeurs thermodynamiques interfaciales de l'alliage, la morphologie de la couche et la pression partielle en monoxyde de carbone dans l'hélium est proposé puis validé

Mécanismes de formation et de destruction de la couche d'oxyde sur un alliage chrominoformeur en milieu HTR

Pour une application en tant qu échangeur thermique du Réacteur à Haute Température, la réactivité de surface d un superalliage à base nickel, le Haynes 230® a été examinée à des températures comprises entre 850 et 1000C. On s est attaché à caractériser l influence de différents paramètres tels que concentrations en impuretés du gaz, caractéristiques de l alliage (activités en Cr et en C, teneurs en éléments mineurs) et température d exposition. Deux principaux comportements ont pu être mis en évidence : la formation d une couche d oxyde riche en Cr et Mn à 900C et sa réduction à plus hautes températures. A 900C, la vapeur d eau est l oxydant principal. A partir d une température critique TA, le carbone en solution dans l alliage réduit l oxyde de chrome. Un modèle de destruction de la couche basé sur les grandeurs thermodynamiques interfaciales de l alliage, la morphologie de la couche et la pression partielle en monoxyde de carbone dans l hélium est proposé puis validé.For a heat exchanger application in High Temperature Reactors, the surface reactivity of a nickel base alloy Haynes 230® was investigated at temperatures between 850 and 1000C. We especially focused on the influence of different parameters such as concentrations of impurities in the gas phase, alloy composition (activities of Cr and C, alloying element contents) and temperature. Two main behaviours have been revealed: the formation of a Cr/Mn rich oxide layer at 900C and its following reduction at higher temperatures. At 900C, the water vapour is the main oxidizing gas. Above a critical temperature TA, the carbon in solution in the alloy reduces chromia. To ascribe the scale destruction, a model is proposed based on thermodynamic interfacial data for the alloy, oxide layer morphology and carbon monoxide partial pressure in helium; the model is then validated regarding experimental results and observations.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Intermetallic formation of Al-Fe and Al-Ni phases by ultrafast slurry aluminization (flash aluminizing)

International audienceAluminizing of Fe and Ni-based substrates is a conventional process often made by gas phase related techniques (CVD, pack cementation and above the pack), whereby the coating thickness is controlled by solid-state diffusion. Similar coatings can be achieved by slurry aluminizing where self-propagating high temperature synthesis and solid-state diffusion mechanisms are involved. The present work investigates ultrafast (5 min annealing) aluminization to reduce coating time and compares the results on pure iron and nickel substrates with a conventional slurry aluminizing treatment (2 and 5 h). The use of fast heating ramps (25 and 100 °C/min) led to outwardly diffused coatings even with very high Al slurry activities. On nickel, the coatings were homogeneous while on iron, Kirkendall porosity occurred in the outermost layers due to the great dissolution of Fe in the Al melt and its subsequent outward diffusion towards to the Al source