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

Abstract

The oxidation behavior of a number of Fe–Cr- and Ni–Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650 °C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O2) 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 O2. 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.