The oxidation behaviour of the 9% Cr steel P92 in CO(2)-and H(2)0-rich gases relevant to oxyfuel environments

In oxyfuel plants metallic heat exchanging components will be subjected to service environments containing high amounts of CO(2) 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% CO(2)-30% H(2)0 in the temperature range 550-650'C. The results were compared with the behaviour in air, Ar-CO(2) and Ar-H(2)0. In the CO(2)-and/or H(2)0-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 H(2)0-containing gases and carburisation in the CO(2) gases. The oxide scales allow molecular transport of CO(2) 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 H(2)0-free gas. This has been discussed in terms of the potential for H(2)0 to increase growth stresses and accelerate crack propagation. The oxidation rates in CO(2)-H(2)0 do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.