Using a Plasma Focused Ion Beam System to Characterise the Porosity Through the Oxide Scale Formed on a Martensitic...

Focused ion beam (FIB) microscopy, in combination with scanning electron microscopy (SEM) has been used to characterise the porosity through the oxide scale of an experimental martensitic 9Cr-1Mo steel exposed for 3325 h to a CO₂-rich environment at 600 °C. A typical magnetite outer layer forms with a small fraction of spherical pores, but no interconnectivity between them. A complex mixed middle layer is observed which consists of larger magnetite grains, smaller spinel grains and some pore interconnectivity, but typically the porosity is present in localised regions, inhibited from coalescence by the different grain types. A spinel layer also shows greater pore interconnectivity with three large pores spanning the total distance through this layer, but no single pore spans the total oxide scale thickness. A mechanism linking the substrate microstructure to the variation in oxide structure between ferritic and martensitic material is proposed which explains the formation of the magnetite, the complex oxide and spinel oxide layers.