In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO₂ and water vapour. In the present paper, the oxidation behaviour of the ferritic/martensitic 9% Cr steel P92 was studied in a model gas mixture containing 70% CO₂-30% H₂O in the temperature range 550–650°C. The results were compared with the behaviour in air, Ar–CO₂ and Ar–H₂O. In the CO₂- and/or H₂O-rich gases, the steel formed iron-rich oxide scales which possess substantially higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich oxide scales are formed as a result of a decreased flux of chromium in the bulk alloy toward the surface. This is the result of enhanced internal oxidation of chromium in the H₂O-containing gases and carburisation in the CO₂ gases. The oxide scales allow molecular transport of CO₂ towards the metallic surface, resulting in carburisation of the alloy. The presence of water vapour induced buckling in the outer haematite layer, apparently as a result of compressive oxide growth stresses. Buckling did not occur in the H₂O-free gas. This has been discussed in terms of the potential for H₂O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO₂–H₂O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.

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