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

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