Non-metallic inclusions are preferential sites for pitting corrosion initiation in austenitic stainless steels. This study investigates the effect of magnesium addition on inclusion evolution and localized corrosion behavior of austenitic stainless steel using SEM–EDS, immersion testing, scanning Kelvin probe force microscopy (SKPFM), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Magnesium addition modifies the inclusion population from coarse (Mn,Al)-O into finer MgO·Al₂O₃ spinel-based inclusions with improved distribution. Immersion testing in FeCl₃–HCl solution reveals delayed inclusion dissolution and reduced localized pitting attack in the Mg-treated steel. SKPFM maps, together with immersion observations, provide qualitative evidence of changes in the local electrochemical contrast between inclusions and the surrounding matrix. Electrochemical measurements in 3.5 wt.% NaCl further demonstrate improved corrosion resistance after Mg addition, as evidenced by an increase in pitting potential from 0.28 V to 0.52 V vs. SCE and charge-transfer resistance. The improved corrosion resistance is associated with inclusion refinement and modification of the local surface-potential distribution, which may influence inclusion-related localized corrosion processes. Overall, Mg microalloying modifies the inclusion characteristics of austenitic stainless steel and enhances its resistance to chloride-induced localized corrosion, demonstrating the potential of inclusion engineering as a strategy for improving stainless-steel corrosion performance.

Overcoming the strength–ductility trade-off at cryogenic temperatures is a key bottleneck for austenitic stainless steels. This study investigated the effects of Nb additions (0, 0.03, and 0.06 wt.%) on the mechanical properties and fracture behavior of 304 steel from room temperature (RT) to −196 °C. Within the investigated range, the 0.06 wt.% Nb steel produced the largest measured response. Compared with the Nb-free steel, the RT yield strength and elongation after fracture increased by ~5% and 2.4%, respectively; at −196 °C, they increased by 11.1% and 4.1%, and impact toughness by 31.8%, mitigating the low-temperature strength–ductility trade-off. Fractography revealed a transition from brittle cleavage/mixed fracture to dimple-dominated ductile fracture. The improvement in mechanical properties is consistent with the refined microstructure and the modified inclusion distribution, with the underlying mechanisms likely involving grain refinement and the solid-solution strengthening of the austenite matrix by Nb. It should be noted that, owing to experimental limitations, the deformation-induced α′-martensite was not quantitatively characterized in the present study, and no rigorous quantitative correlation has yet been established between inclusion characteristics and low-temperature impact toughness. Consequently, the relative contribution of each contributing factor to the observed performance enhancement remains unresolved. Nevertheless, this work provides experimental support for the microalloying design of ultra-low-temperature austenitic stainless steels.