Porosity developments within 9Cr-1Mo steel exposed to CO2

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.