Etude par diffraction des rayons X in situ des mécanismes d'oxydation de l'acier AISI 304 entre 800°C et 1000°C. Influence des dépôts sol-gel de lanthane et de cérium. Apport de la spectroscopie infrarouge à l'identification des oxydes mixtes

Abstract : This work presents a study on the AISI 304 oxidation mechanism within the temperature range of 800 to 1000°C, in air. We have closely examined the effect of Lanthanum and Cerium sol-gel coating on the oxidation process. IR spectroscopy enables us to better identify the mixed oxides FeCr2O4 and Mn1,5Cr1,5O4 and the corundum type oxides Fe2O3 and Cr2O3. The combination of various analytical techniques such as: in situ XRD, IR spectroscopy, MEB, EDS and MET, lead us to propose a new oxidation mechanism of AISI 304 oxidation at 1000°C. The corrosion process involves a great contribution of internal oxidation after a transient stage. The presence of lanthanum and cerium sol-gel coatings leads to a decrease of oxidation rate constants. With Lanthanum, the in situ XRD results show that the growth of LaCrO3 mixed oxide occurs at the beginning of oxidation process. This oxide is localised at the alloy/oxide internal interface. It blocks the external iron diffusion and slows down the external diffusion of manganese leading to a chromia scale formation. Either Lanthanum can act with silicon to block the external iron diffusion and favour the chromia scale formation or Lanthanum can also act by blocking the external diffusion of metallic cations through the chromia layer according to a "dynamic segregation process" along grain boundaries. With a cerium sol-gel coating, a linear regime has been observed at all temperatures. It corresponds to a growth process limited by the oxygen diffusion through the CeO2 layer (showing a constant thickness) situated at the external interface. In this case, the absence of iron containing oxides in the scale leads us to conclude to a synergistic effect with silicon, then, blocking the external iron diffusion. Thermal cycling tests show that the adherence of oxide layers formed, in the presence of Cerium and Lanthanum sol-gel coatings, is not improved compared to blank specimens.
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Theoretical and/or physical chemistry. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21782>


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Noureddine Karimi. Etude par diffraction des rayons X in situ des mécanismes d'oxydation de l'acier AISI 304 entre 800°C et 1000°C. Influence des dépôts sol-gel de lanthane et de cérium. Apport de la spectroscopie infrarouge à l'identification des oxydes mixtes. Theoretical and/or physical chemistry. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21782>. <tel-00718341>

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