La capture du carbone de combustion implique le transport de gaz riches en CO2 a haute temperature. Cette etude vise a preciser les facteurs controlant l'oxydation d'alliages chromino-formeurs dans ces environnements. Des alliages modeles Fe–Cr et Fe–Cr–Ni ont ainsi ete exposes a des melanges Ar–CO2–H2O a 650 et 800 °C, et les produits de reaction examines a l'aide de techniques de metallographie conventionnelles. La precipitation de carbures sous des couches d'oxyde indique une sursaturation en carbone a l'interface metal/oxyde, par rapport a l'atmosphere exterieure. Sur la base d'un modele d'equilibre thermodynamique local, les vitesses de carburation et fractions volumiques de precipites mesurees sont utilisees pour evaluer l'influence de la composition de l'oxyde et de la presence d'H2O dans le gaz sur le transport du carbone. En analysant la depletion en chrome dans l'alliage sous-jacent, nous montrons que la carburation limitee sous une couche de chromine n'altere pas la stabilite de l'oxyde. L'evolution morphologique des nodules d'oxydes riches en fer formes a la suite de la rupture localisee de la chromine est mise en relation avec la capacite de l'alliage a fournir du chrome a l'interface metal/oxyde. L'application de modeles de germination-croissance aux cinetiques de developement de nodules permet d'evaluer la resistance des couches de chromine via des frequences de germination determinees a partir des taux de recouvrement de nodules et des gains de masse des echantillons. Nous examinons enfin l'importance relative de la germination et de la croissance des nodules dans le controle de la performance globale des alliages en fonction de la temperature de reaction. / Materials to convey hot CO2-rich gases are needed in carbon capture technologies currently being developed. This work is aimed at investigating the factors controlling the oxidation of chromia-forming alloys in these atmospheres. To do so, model Fe–Cr and Fe–Cr–Ni alloys were exposed to Ar–CO2–H2O gas mixtures at 650 and 800 °C,and the reaction products examined using conventional metallography techniques. Carbide precipitation beneath oxide scales reflects a carbon supersaturation at the metal/oxide interface relative to the external atmosphere: as a gradient of oxygen potential is established across the growing scale, an elevated carbon activity results at the interface if the scale transmits carbon. On the basis of a local equilibrium model, measured carburisation rates and precipitate volume fractions were used to evaluate the influence of oxide composition and of the presence of H2O in the gas on carbon uptake/transport in the scales. Limited carburisation beneath Cr2O3 scales was shown by means of an analysis of subscale chromium depletion not to alter the oxide stability. The morphological evolution of Fe-rich oxide nodules formed as a result of localised Cr2O3 failure was studied in relation to the alloy ability to supply chromium to the metal/oxide interface. Application of nucleation-growth models to the kinetics of nodule development allowed the resistance of Cr2O3 scales to be evaluated in terms of nodule nucleation rates determined from experimental nodule surface coverages and specimen weight gains. The relative importance of nodule nucleation and growth in determining the overall alloy performance as a function of reaction temperature is discussed.
| Identifer | oai:union.ndltd.org:theses.fr/2012INPT0060 |
| Date | 31 August 2012 |
| Creators | Gheno, Thomas |
| Contributors | Toulouse, INPT, University of New South Wales, Monceau, Daniel, Young, David |
| Source Sets | Dépôt national des thèses électroniques françaises |
| Language | English |
| Detected Language | French |
| Type | Electronic Thesis or Dissertation, Text |
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