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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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When 9Cr-1Mo steel is exposed to CO2-rich advance gas-cooled reactor (AGR) gases it forms a duplex oxide which consists of an outer Fe rich layer and an inner Cr rich spinel which provides oxidation resistance allowing the steel to resist the corrosive atmosphere of the plant. The oxide scale develops, growing both into the substrate and outwards from the initial metal/air interface. The spinel develops porosity through the coalescence of Fe vacancies which over time alters the properties of the oxide and potentially allows a transport network to form within the oxide. The porosity of the duplex oxide was measured using scanning electron microscopy of oxides on 9Cr steel samples oxidised in a CO2 atmosphere. Results show that samples which have suffered breakaway oxidation show larger oxide scales with alternating Fe/Cr bands whereas samples which have yet to suffer from breakaway show higher peak porosity values but thinner oxide scales. Furthermore the samples which are currently under protective oxidation show a high max porosity peak in comparison to those which have suffered breakaway.

Focused ion beam microscopy and scanning electron microscopy have been used to characterise the porosity of the oxide scale of an experimental 9Cr–1Mo steel sample exposed for 4580 h in a CO2-rich environment. The magnetite shows a high frequency of spherical pores (~ 1 µm3) with no interconnectivity. The Cr-rich spinel layer shows greater interconnectivity, but no single pore spans the total oxide scale. A mechanism for the formation of the different morphologies observed across the scale is proposed, linking porosity changes across the oxide scale to the carburisation and elemental segregation of Cr within the substrate. Graphical abstract