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

Abstract

Abstract The initial stages of oxidation of 9Cr steel in CO2, O2, CO2-O2 and CO2-O2-H2O is studied by Gas Phase Analysis (GPA) at 550°C using 13C16,16O2, 18,18O2 and 2H216O 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 CO2, 9Cr steel forms a slow growing chromium rich oxide scale without any carburization. Adding O2 impurities in CO2 favors the formation of fast growing iron rich duplex oxide scale coupled to strong carburization. Adding several % of O2 in CO2 favors again the formation of slow growing oxide scale but with different structure and composition than in pure CO2. 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 O2 molecules in CO2 increases drastically the surface oxidation rate and favors the formation of non-protective duplex oxide scale against carburization. Adding water vapor to a CO2 gas environment slows carburization. Preferential adsorption of water vapor molecules over CO2/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 CO2/O2/H2O gas mixtures is described.