Influence of Trace Magnesium Addition on Inclusion Modification and Localized Corrosion Behavior of 304 Austenitic Stainless Steel

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.