Effect of Aging Treatment on the Corrosion Behavior of Selective Laser Melted Fe-30Mn-8Al-1.5C-2.5Ni Lightweight Steel

Selective laser melting (SLM) can effectively suppress metallurgical defects inherent to conventional manufacturing; however, subsequent aging treatments essential for engineering applications introduce secondary precipitates that alter the electrochemical homogeneity of the matrix. The underlying mechanism by which such precipitation behavior governs corrosion resistance remains elusive. In this study, a Fe-30Mn-8Al-1.5C-2.5Ni steel was investigated to elucidate the corrosion morphology and electrochemical behavior of the as-built, 450 °C-aged, and 750 °C-aged specimens during immersion in a 3.5 wt.% NaCl solution. The results demonstrate that the inherent Mn microsegregation and high-density subgrain boundaries induced by SLM trigger preferential localized anodic dissolution on the surface of the as-built sample, culminating in the formation of a loose, porous manganese oxide product layer. Aging treatment at 450 °C induces extensive precipitation of κ-carbides within grain interiors and along grain boundaries, accompanied by localized depletion of Al and Mn at the phase interfaces. A pronounced micro-galvanic coupling established between the κ-carbides and the adjacent Al-depleted zones directly compromises the continuity of the passive film, thereby further deteriorating the corrosion resistance. In contrast, aging at 750 °C relieves the residual stress and eliminates the as-built elemental microsegregation. The resulting compositional homogenization of the matrix reduces the localized electrochemical driving force, which promotes a uniform reaction of Al at the surface to construct a continuous, compact Al-rich passive film, thereby sustaining the highest charge-transfer resistance during long-term immersion. This work elucidates the correlation among the intrinsic defects of SLM, aging-induced solute-depleted zones, κ-carbide precipitation, and localized micro-galvanic corrosion, providing a fundamental basis for tailoring the microstructure and corrosion resistance of additively manufactured lightweight steels.