Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O2 and 10% H2O (pO2 = 0.02 and 10−7 atm, respectively) at 800 °C for times up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O2 atmosphere. This layer buckles and spalls off after exposure to 10% H2O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H2O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower. Graphic Abstract

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Insights into early damage mechanisms during high‐temperature gas corrosion provide important aspects for the prediction of the long‐term stability of high‐temperature materials exposed to a hot and corrosive environment. The current work presents a real time study concerning the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere using energy dispersive X‐ray diffraction. The applied high temperature reactor allows the transmittance of high energetic X‐rays in order to collect X‐ray diffraction pattern of the sample surface in situ during the reaction with the reactive environment. The results revealed that the first phases crystallizing on iron with 2 wt% chromium and with 9 wt% chromium are oxides. The aging experiments at T = 650° C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide‐sulfide layer on top of iron with 2 wt% chromium. On the higher alloyed material the external scale consists of iron oxides and the inner scale of mixed (Fe,Cr)‐oxides and chromium‐sulfides. The oxide content continuously increases parallel to an increase of the sulfide amount. Thus, the initially formed (Fe,Cr)‐oxides do not have a protective character and support the transport of sulfur through the growing oxide scale.