Microstructural aspects of the scales formed on FeMnAl and FeMnAlCr alloys in SO₂/O₂ atmospheres at elevated temperatures

Fe-30Mn-8Al and Fe-30Mn-8Al-2Cr alloys were exposed to mixed oxidant gaseous environments containing both oxygen and sulfur dioxide at 900°C. The microstructure of the reaction products formed on these alloys were examined by scanning electron microscopy (SEM) and x-ray mapping techniques. The scales formed on the Fe-30Mn-8Al alloy exposed to oxidizing/sulfidizing environments consist of duplex oxides structure with an outer (FeMn)₂O₃ layer and an inner α-Al₂O₃ layer at the oxide-alloy interface. This alloy does not undergo severe degradation by sulfidation after prolonged exposure in SO₂/O₂ gas mixture for 24 h. On the other hand, addition of Cr to the Fe-30Mn-8Al alloy leads to a substantial change in scale morphology. The results show that Cr addition promotes the formation and growth of the external fibrous (Mn, Cr) S sulfide and internal MnS sulfide within the substrate alloy. © 1990.