Recent studies have shown that melt pool boundaries (MPBs) in selective laser melted (SLM) stainless steels (SS) can serve as preferential sites for pit initiation and corrosion propagation. In this study, we utilize electrochemical polarization experiments combined with detailed microstructural analysis to elucidate the role played by MPBs on localized corrosion in chloride media. Potentiodynamic polarization of artificial 1-D pits constructed from SLM SS316L grown to sufficient depth showed a sudden current drop below the salt-film dissolution potential on the downward scan, which was followed by an active Tafel-like behavior until the zero-current potential (ZCP) was reached. The observed current drop marks a critical potential for pit stability and characterizes the partial repassivation of the pit, as the current density value after the drop was significantly greater than the reported current density at complete repassivation. When the same 1-D pit was polarized upwards from a potential below the ZCP, it showed characteristic re-activation before salt film formation. This behavior was unique to SLM SS316L and was not found in conventional wrought SS316L. Moreover, 1-D artificial pit study on SLM SS316L using a modified ASTM G192 sequence showed a similar partial repassivation behavior. Scanning electron microscopy of SLM SS316L pit bottom morphology after long-term potentiostatic polarization at a potential between the partial repassivation potential and the ZCP showed selective, crystallographic attack along MPBs. The morphology also revealed melt pool fall-out due to the continued attack of the MPBs. Scanning transmission electron microscopy characterization of the SLM 316L material revealed Cr and Mo depletion at the MPBs. The observation of passivating element depletion at MPBs is in line with calculations of solute partitioning during melt pool solidification for the material under study. Based on our results, we conclude that localized corrosion under conditions at the edge of passivity can lead to selective attack of Cr- and Mo-depleted MPBs. The implications of inter-melt-pool corrosion on component structural integrity and protective repassivation potentials will be discussed.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-4016 A