Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO2 and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9-12%Cr steels in a model gas mixture containing 70% CO2-30% H2O was studied in the temperature range 550-700 degrees C. The results were compared with the behaviour in air, Ar-CO2 and Ar-H2O. It was found that in the CO2- and/or H2O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H2O-containing gases and carbide fori-nation in the CO2-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO2(-H2O)-rich gases appeared to be permeable to CO2 molecules resulting in substantial carburization of the steel.