Corrosion of 9Cr steel by hot CO2 gas: effects of O2 and H2O

The initial stages of oxidation of 9Cr steel in CO 2 , O 2 , CO 2 -O 2 and CO 2 -O 2 -H 2 O is studied by Gas Phase Analysis (GPA) at 550°C using 13 C 16,16 O 2 , 18,18 O 2 and 2 H 2 16 O isotopic molecules in order to discriminate the reactions of all gas molecules. Protective and non-protective oxide scales are formed on 9Cr steel depending on the exact composition of the gas mixture. In pure CO 2 , 9Cr steel forms a slow growing chromium rich oxide scale without any carburization. Adding O 2 impurities in CO 2 favors the formation of fast growing iron rich duplex oxide scale coupled to strong carburization. Adding several % of O 2 in CO 2 favors again the formation of slow growing oxide scale but with different structure and composition than in pure CO 2 . GPA analyses combined with oxide scale analyses demonstrate that the composition and structure of the transient oxide scale formed on 9Cr surface is determined by the rate at which surface adsorbed oxygen atoms are supplied by the gas phase in the first minutes of exposure. The presence of the very oxidizing O 2 molecules in CO 2 increases drastically the surface oxidation rate and favors the formation of non-protective duplex oxide scale against carburization. Adding water vapor to a CO 2 gas environment slows carburization. Preferential adsorption of water vapor molecules over CO 2 /CO molecules in the inner oxide scale is proposed to explain this result. A unified mechanism for the formation of the transient oxide scale on 9Cr steel in CO 2 /O 2 /H 2 O gas mixtures is described.