9Cr-1Mo steels have been used extensively in the power generation industry. In this study, a wide range of experimental samples exposed at different times and temperatures in a CO 2 environment were analysed to look at the development of the metal and oxides over time. The main objective of this work was to obtain a better understanding of the carburisation and oxidation behaviour of 9Cr 1Mo steels as a function of temperature/time, with special attention paid to the transition from protective to breakaway oxidation. In addition, experiments were also carried out to investigate any links between oxidation transition and carburisation behaviour of these materials.

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By combining modelling and experimental work, new insights have been gained into the combined oxidation/carburisation of 9Cr-1Mo steels in CO2 rich environments. The breakawayoxidation process is preceded by the rejection of carbon into the scale due to the poor solubility of carbon in the metal. Experimental TEM work reveals the formation of a carbide rich layer near the substrate surface which forms a further barrier against carbon ingress. The eventual oxidation of this layer could contribute to breakaway oxidation. A 2D finite-difference based diffusion model developed in combination with Thermo-Calc software underlines the role of specimen geometry on breakaway initiation.

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