Material selection for thermal cracking processes of anthropogenic resources is challenging. H2S, HCl are formed and corrosion occurs at high temperatures. Several steels were investigated in the past. In all cases corrosion layers were characteristically structured. On top, large chromium-sulphide crystals are located, followed by a Cr2O3 layer, followed by a layer enriched in chlorine and nickel. In this paper we propose a model on the course of the corrosion reactions. By considering the water-gas shift reaction as well as the influence of H2S, the layer formation can be explained. As an example S31400, 240h corrosion time at 680°C was chosen.