Effect of calcium addition on inclusion refinement, passive film chemistry and pitting corrosion resistance of S30403 austenitic stainless steel

The effect of calcium addition on inclusion characteristics, passive film chemistry, and corrosion behavior of S30403 austenitic stainless steel was systematically investigated in 3.5 wt.% NaCl solution. Calcium treatment significantly refined non-metallic inclusions, transforming complex (Mn,Cr,Al)O x and MnS inclusions into fine, globular (Ca,Mg,Al,Si) composite oxides. Immersion tests and scanning Kelvin probe force microscopy revealed that Ca-modified inclusions exhibit a reduced dissolution tendency and a markedly lower Volta potential difference relative to the surrounding matrix, thereby weakening micro-galvanic coupling and delaying pit initiation. Electrochemical measurements showed that corrosion current density decreased from 2.85 × 10 -6 to 0.88 × 10 -6 A·cm -2 . X-ray photoelectron spectroscopy further demonstrated that modification promotes the formation of a compact passive film enriched in Cr 2 O 3 and Fe 2 O 3 with a higher O 2- /OH - ratio. These results demonstrate that calcium addition enhances corrosion resistance through the synergistic effects of inclusion modification and passive film stabilization, offering an effective strategy for improving the durability of austenitic stainless steels in chloride-containing environments.