Water Vapor Effects on the Oxidation Behavior of Fe-Cr and Ni-Cr Alloys in Atmospheres Relevant to Oxy-fuel Combustion

The oxidation behavior of a number of Fe-Cr- and Ni-Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650 A degrees C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O-2) CO2 + 30%H2O and Ar + 30%H2O gases. Rapidly growing iron oxides appear to be porous and gas permeable. Transition from non-protective to protective oxidation occurs on alloys with higher Cr contents between 13.5 and 22 wt% in H2O. Excess oxygen, usually found in the actual oxyfuel combustion environments, disrupts the selective oxidation of Fe-Cr alloys by accelerating vaporization of early-formed Cr2O3 in combination with accelerated chromia growth induced by the H2O. Rapid Cr consumption leads to the nucleation and rapid growth of iron oxides. On the contrary, Ni-Cr alloys are less affected by the presence of H2O and excess O-2. The difference between Fe-Cr and Ni-Cr alloys is not clear but is postulated to involve less acceleration of chromia growth by water vapor for the latter group of alloys.