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Etude des mécanismes d'oxydation des alliages FeCrAl et de l'acierAISI 316L à haute température (700-1300°C). Influence du mode d'introduction du lanthane et du Cerium. Protection par des couches d'oxydes thermiques contre la corrosion aqueuse

Abstract : This work highlights the role of the reactive elements such as lanthanum, zirconium and magnesium during oxidation of FeCrAl alloys at high temperature. The role of molybdenum was studied during the oxidation of an AISI 316L steel. The lanthanum, present in the initial composition of the alloy, has a beneficial effect on the oxidation behaviour until 1300°C. Its location allows us to think that its action can be described within the framework of the model of dynamic segregation by blocking the cation external diffusion along grain boundaries. The effect of magnesium cannot be described in terms of dynamic segregation because it diffuses too fast towards the external alumina interface to form the spinel phase MgAl2O4. This oxide leads to the formation of surface porosities, which can be useful in the case of surgical implants asking for colonization by the neighbouring bone tissue. The zirconium blocks partially the diffusion of aluminium at the beginning of oxidation. Then, the oxide becomes richer in chromium and the oxidation rate is increased. On the other hand, the presence of zirconium favours the formation of pegs, which improves the scale adherence on the metal. In the case of the AISI 316L steel, molybdenumcontaining oxides forms along the alloy grains boundaries as well as at the internal interface. It appears that the NiMoO4 mixed oxide blocks the external iron diffusion. The presence of molybdenum improves the chromia scale adherence by a pegging effect due to internal oxidation. In aqueous corrosion, the pre-oxidized samples of FeCrAl or AISI 316L show the best behaviour with the finest polishing grade. A pre-oxidation at 1100°C for FeCrAl alloys and at 800°C for AISI 316L are the best temperatures promoting a protective oxide layer against pitting corrosion.
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Samira El Messki. Etude des mécanismes d'oxydation des alliages FeCrAl et de l'acierAISI 316L à haute température (700-1300°C). Influence du mode d'introduction du lanthane et du Cerium. Protection par des couches d'oxydes thermiques contre la corrosion aqueuse. Chimie organique. Université Blaise Pascal - Clermont-Ferrand II, 2007. Français. ⟨NNT : 2007CLF21765⟩. ⟨tel-00718195⟩

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