The oxidation behaviour of the 9 % Cr steel P92in CO₂- and H₂O-rich gases relevant to oxyfuel environments

Abstract

Abstract 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 O in the temperature range 550 – 650 °C. The results were compared with the behaviour in air, Ar–CO 2 and Ar–H 2 O. In the CO 2 - and/or H 2 O-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 O-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 O-free gas. This has been discussed in terms of the potential for H 2 O to increase growth stresses and accelerate crack propagation. The oxidation rates in CO 2 –H 2 O do not seem to be higher than those observed in flue gases of conventional fossil fuel fired power plants.