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• Post corrosion imaging shows corrosion initiation at NbC inclusions. • Evidence that pH 13 NaOH pretreatment dissolves and degrades NbC inclusions. • In-situ time lapse microscopy technique tracks NbC modification by NaOH. • Pre-treatment indicates no benefit as regards long-term corrosion inhibition. Niobium-stabilised austenitic stainless steel (20Cr-25Ni-Nb) has been immersed in sodium hydroxide, which is used as a corrosion inhibitor. The work shows how NbC precipitates may be degraded by use of pH 13 NaOH. Initial electrochemical measurements indicate that there is no benefit to this pretreatment as regards long-term corrosion inhibition, and post corrosion imaging shows the initiation of pitting corrosion at Nb-rich precipitates still present in the microstructure. [ABSTRACT FROM AUTHOR]

• SKPFM highlights nobility of niobium carbides. • Evidence of intergranular pitting corrosion at niobium carbide inclusions. • EC-AFM provides evidence that intergranular pits form around inclusions. The role of niobium carbide (NbC) inclusions in directing the initiation of localised corrosion in sensitised 20Cr-25Ni-Nb stainless steel is investigated using a range of in-situ scanning techniques, allowing visualisation of corrosion behaviour over multiple length scales. Volta potential mapping shows NbC inclusions are up to 30 mV noble to the matrix, while chromium-depleted grain boundaries are up to 65 mV less noble. Corrosion initiation, shown by scanning vibrating electrode technique to comprise highly localised anode sites, is observed at grain boundaries, adjacent to the location of NbC inclusion clusters which remain unaffected by anodic dissolution of the surrounding matrix. [ABSTRACT FROM AUTHOR]