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

This study investigates the influence of plastic strain and sensitisation interaction on the corrosion behaviour of 316L austenitic stainless steel in artificial seawater. Bipolar electrochemistry (BPE) was used alongside standard potentiodynamic polarisation to examine samples strained from 0% to 25%. Results show a clear, strain-dependent rise in corrosion severity for both sensitised and non-sensitised states, though the mechanisms differ significantly. For the sensitised samples, a high initial density of micropits at 0% strain was observed. However, as the applied strain increased up to 25%, this micropit density actually decreased. This indicates that under heavier deformation, the anodic attack shifts away from forming new scattered pits and instead focuses into continuous weakened paths, which accelerates crevice growth. On the other hand, non-sensitised samples responded to plastic deformation by developing more micropits. This contrast suggests that in the regular material, strain breaks down the passive film and creates preferential pitting sites right at the slip bands or dislocation clusters. Under the bipolar potential field of the BPE setup, this degradation resembled stray-current corrosion, where deformation-induced microgalvanic coupling further localised the attack. These results show that while strain increases the overall driving force for dissolution, the sensitisation state governs the ultimate morphology of the failure. These insights are particularly relevant for predicting the long-term reliability of 316L components in chlorine containing environments where both mechanical loading, residual stresses and thermal sensitisation are common.