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Water vapor oxidation of ferritic 441 and austenitic 316L stainless steels at 1100 °C for short duration

Abstract : A ferritic 441 and an austenitic 316L steels have been exposed to wet argon at 1100 °C. This study focus on the characterization of the oxide scales formed after different exposure times in the range of 2.5-20 min. Raman spectroscopy, XRD, SEM and XPS have been used. For all exposure times, 316L forms a breakaway type thick oxide scale (rupture of the pre-existing passivating film) with iron oxides on its outer part and a mix of spinels with Fe, Cr and Ni for its inner part. For lower water vapor partial pressure, iron oxides are constituted of wüstite. For higher water vapor partial pressure, iron oxides are constituted of a layer of hematite over a layer of magnetite slightly enriched in chromium. Due to strong oxidation condition, oxide scale is not always homogeneous and iron oxides spallation may occur. For 2.5 min of oxidation on 441, a very thin layer of protective chromium oxide is formed. For longer exposure time, an almost homogeneous and much thicker layer mainly consisting of Cr 2 O 3 is produced. The thickness varies slightly and gradually from 4 to 20 min of oxidation. There are Mn-Cr spinels mixed with the chromium oxide. The most external part is strongly enriched in Mn and Fe in a spinel structure. The diffusivity of chromium is regarded as the main cause of the difference of oxidation behavior. In both cases, the first step is a very thin chromium oxide layer. When the oxidation conditions becomes too strong in terms of exposure time or water vapor partial pressure, this oxide layer breaks. The ferritic steel is able to heal and thicken its protective chromium oxide, preventing the breakaway. The healing would be due to the high diffusivity of chromium. The thickening would be caused by the presence of the Mn-Cr spinels which are a less effective diffusion barrier. The lower diffusivity of chromium in austenite promotes the breakaway.
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Valentin Badin, Entela Diamanti, Pierre Forêt, Evelyne Darque-Ceretti. Water vapor oxidation of ferritic 441 and austenitic 316L stainless steels at 1100 °C for short duration. International Congress of Science and Technology of Metallurgy and Materials, SAM – CONAMET 2014, Oct 2014, Santa Fe, Argentina. pp.48-53, ⟨10.1016/j.mspro.2015.04.006⟩. ⟨hal-01182947⟩

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