Unraveling the mechanistic role of trace Nb microalloying in inclusion evolution and pitting corrosion resistance of 304 austenitic stainless steel

: Trace Nb microalloying is a promising strategy for mitigating inclusion-mediated pitting in austenitic stainless steels, but the mechanisms linking Nb-induced inclusion evolution to localized corrosion remain unclear. The effect of 0.05 wt.% Nb on 304 austenitic stainless steel was systematically investigated using thermodynamic calculations, microstructural characterization, electrochemical and immersion tests, XPS and by surface chemical composition analysis. Nb refined the austenitic microstructure, reducing the area-weighted equivalent circular diameter from 19.2 to 11.4 μm and increasing the low-angle grain-boundary fraction from 35.2% to 61.7%. More importantly, Nb modified the inclusion population, reducing inclusion density and size, promoting finer Nb-rich oxides, and suppressing sulfide-rich phases. The corrosion current density decreased by more than one order of magnitude, accompanied by increased charge-transfer resistance and substantially suppressed stable pit growth. XPS revealed Nb-O species and a more oxide-rich passive film, while AFM/SKPFM showed reduced local electrochemical heterogeneity after Nb addition. These findings indicate that trace Nb enhances pitting-corrosion resistance through a coupled mechanism involving inclusion stabilization, reduced micro-galvanic activity, and improved passive-film stability, thereby delaying passive-film breakdown and inhibiting the transition from pit initiation to stable propagation.