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.

This study focuses on the characterization of the oxide scales formed after different exposure times in the range of 2.5-20 min. A commercially available ferritic steel grade AISI 441 was exposed to wet argon at 1100°C with 5, 9 and 13% H 2 O. Raman microspectroscopy, XRD, EDS and XPS were used to fully characterize the oxide scale. For all samples exposed for over 4 min, the scale was constituted of three layers in this order: a thin top layer of spinel phases (Fe,Cr,Mn) 3 O 4 with local outgrowths; a second and main layer of Cr 2 O 3 ? (Mn,Cr) 3 O 4 ; and finally a bottom layer of SiO 2 . The uncommon presence of Fe in the top layer was also observed.

International audienceA new method to create porous surfaces on stainless steel by reducing oxide scales with hydrogen at 1100 °C has been investigated. Mercury Intrusion Porosimetry (MIP) along with Scanning Electron Microscopy (SEM) have been used to successfully study the porosity of the surfaces. Two different sets of parameters led to different morphologies. The first type of surface results from a 5 min reduction of a wüstite FeO surface oxide layer and provides smooth micrometer scale pores with a Gaussian distribution size. The second type of surface results from a 3 h reduction of a chromium rich oxide layer and provides three different micrometer scale pore size distributions with burst morphology. The volume of the porosity has been compared to its precedent oxide scale volume. The non-stoichiometry of wüstite is believed to be the main factor influencing the difference in the pore creation mechanism as compared to the mechanism of reduction from chromium-rich oxide

International audienceA 10-µm-thick oxide scale covering an austenitic stainless steel has been reduced with hydrogen at 1373 K (1100 °C) to study the resulting porosity. Multiple methods of characterization have been employed on samples with the reduction time ranging from 5 seconds to 30 minutes. Focus has been placed on short durations to highlight the different steps occurring during the reduction. Three main steps have been observed, leading to micrometer scale porosity. The process is regarded as a potential new method for creating metal porous surfaces