Because of higher CO2, and possibly H2O and SO2, levels in the boiler, there are concerns about increased corrosion rates after retrofitting current coal fired boilers from air-firing to oxy-firing to assist in CO2 capture. The oxidation behavior of a combination of commercial and model alloys were investigated both with and without the presence of synthetic coal ash at 600 and 650 °C. At 600 °C, a CO2–H2O environment showed the most rapid oxidation rate for Fe-based alloys with <20% Cr and varying the CO2 content or adding a 0.15% O2 buffer had little effect on the mass change. However, at 650 °C, the O2-buffered CO2–H2O environment showed a similar rate of oxidation as 100% H2O, again requiring more than 20% Cr for a thin protective Cr-rich oxide to form. With synthetic coal ash, increasing the CO2, H2O, and/or SO2 levels in the gas phase tended to show a lower oxide thickness after a 500 h exposure at 600 °C, compared to the base line air-firing condition. At 650 °C, no systematic increase in the reaction rate was observed when switching from the air firing to the oxy-firing gas. These simulations suggest that higher CO2 contents with oxy-firing do not increase the rate of oxidation.