Porosity Developments within 9Cr-1Mo Steel Exposed to CO₂

Abstract

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.

The effect of microstructure on the oxidation and carburisation of 9Cr-1Mo steel exposed to CO2

“…These samples, in comparison to samples shown in Figures 7 and 8, have been exposed for a shorter duration. These were chosen as it has been previously [41] been observed that samples which have been exposed and suffered breakaway show very little internal oxidation zone and very thick breakaway oxide which has a very different structure to pre-breakaway oxide.…”

Using a plasma FIB system to characterise the porosity through the oxide scale formed on 9Cr-1Mo steel exposed to CO2

“…This is in contrast to previous work where it was thought that the only way to accurately analyse the porosity though the oxide was by using a FIB [32]. The 2D porosity area fraction has previously been investigated by the authors where various samples are compared, and this showed the development of porosity with exposure, with the area fraction and oxide thickness increasing with exposure time and with the characteristic banding of the Cr-rich spinel clearly visible [28].…”

“…Porosity changes within the oxide of 9Cr-1Mo steel exposed to CO 2 have been previously investigated using SEM imaging techniques [28], but SEM micrographs only give a 2D snapshot into the oxidation processes and porosity development taking place within the oxide scale. In comparison with traditional SEM imaging, focused ion beam microscopy allows for the removal of material, either as a large section or very thin layers (\ 100 nm).…”