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The microstructure of the oxide scale and parent metal in ferritic 9Cr-1Mo steel was observed to explore the oxidation and carburisation mechanisms upon exposure to a CO2 gas environment at high temperature and high pressure. An experimental 9Cr-1Mo steel sample that had been oxidised at 580 °C for more than 165,000 hrs in coolant gas consisting primarily of CO2 was analysed. To elucidate the oxidation characterisation, scanning Raman spectrometry was used to analyse the oxide close to the metal/oxide interface. Carbon was found to be deposited in the spinel layer. The microstructure and the distribution of elemental chemical composition were examined and analysed using optical and scanning electron microscopy combined with energy dispersive X-ray analysis. The results are discussed with relation to understanding the mechanisms of oxidation and carburisation which aim to underpin extension of the service life of components fabricated from this steel. [ABSTRACT FROM AUTHOR]