Synergistic effect of microstructure and etching on surface chemistry, passivity, and corrosion resistance of additively manufactured 316 L stainless steel

The interplay between microstructural features and surface treatments is critical in defining the corrosion resistance of additively manufactured (AM) metals. This study investigates the synergistic effect of microstructure and chemical etching, a relevant surface treatment for AM metal parts, on the surface chemistry, passivity, and corrosion resistance of 316 L stainless steel fabricated by laser-based directed energy deposition (LB-DED). Rapid solidification during LB-DED induces a characteristic cellular microstructure with Cr- and Mo-enriched boundaries. Chemical etching selectively dissolves Fe-rich regions, exposing this interconnected structure and significantly increasing the surface concentration of Cr and Mo. This leads to a localized enhancement in the pitting resistance equivalent number (PREN) and a higher proportion of Cr and Mo oxides in the passive layer. Despite introducing micro-scale roughness, etching improves the passivity range, increases the breakdown potential, and reduces metastable pitting activity. These effects are further reinforced by a reduction in Mn-silicate inclusions at the surface. This study provides new insights into how microstructural characteristics and surface modifications interact to influence corrosion behaviour, offering a foundation for optimizing surface treatments in AM stainless steels.