Influence of surface finishing on high-temperature oxidation of AISI type 444 ferritic stainless steel used in SOFC stacks

This research is focused on the study of the samples, approximatively 15 × 30 mm2 sized, that were mechanically cut from two sheets (0.4 and 0.2 mm thick, respectively) of AISI 444 Type ferritic stainless steel (FSS) (DIN 1.4521, Eu designation X2CrMoTi18-2); this material was in the 'as-rolled' state. Part of these specimens were treated superficially on one side using abrasive SiC papers with different average grit sizes (i.e., 46.2, 30.2, 18.3 μm) and diamond suspension (up to 1 μm) in order to obtain various surface roughness. Both the 'as-rolled' and superficially treated samples were then aged in a muffle furnace in static air according to a thermal cycle corresponding to the curing phase of an experimental glass used as sealing in the solid oxide fuel cell stacks. After aging, the chemical compositions and morphological peculiarities of the scale formed depending on the thickness of the samples and their surface state were studied by scanning electron microscopy, energy-dispersive spectroscopy, micro-Raman spectroscopy, bright field light optical microscopy. The obtained results show that all scales formed consist of an inner Cr2O3 subscale and an outer (Mn,Cr)3O4 spinel layer; the relationship between FSS grain size and scale microstructural features is consistent on the samples with mirror-like surface only; the scale thicknesses on SiC grinded samples are comparable; the scales covering the 'as-rolled' surfaces are morphologically similar to those grown on the surfaces finished with the 30.2 and 18.3 μm SiC papers, and their thicknesses show an intermediate situation between the abraded and the mirror-like specimens. The last described characteristics depend mainly on the surface and microstructural peculiarities resulting from the rolling process.