Corrosion behavior of selective laser melted NiTi shape memory alloy

The present work explores corrosion response of Selective Laser Melting (SLM) prepared NiTi shape memory alloys with an aim to study processing-induced defects and surface-specific microstructural features inherent to SLM, influence on alloy corrosion response. The study systematically investigated corrosion behavior within the defect-minimized region of established SLM printability maps by comparing representative low- and high-power/scan speed conditions (P-80: 80 W, 330 mm s⁻¹; P-200: 200 W, 1080 mm s⁻¹) and by varying hatch spacing (64 μm and 80 μm) under constant laser power and scan speed. Corrosion behavior analysis was carried out on four distinct surfaces following a 72-h immersion in Hank's Balanced Salt Solution (HBSS): the internal and external surface of P-80 and P-200 specimens sectioned parallel to build direction (BD), and top and bottom surface of h-64 and h-80 specimens sectioned perpendicular to BD. Unlike prior studies that compared SLM NiTi against wrought material or varied parameters broadly, this work systematically isolates surface-specific defect populations within the defect-minimized region of an established printability map at matched volumetric energy density. The parallel to BD, P-80 specimens, showed the highest porosity of 5.4% and a corrosion current density of 140 nA/cm², with localized pits reaching up to 110 μm in depth, initiating primarily at porosity defects thereby disrupting the passive film formation. In contrast, perpendicular to BD specimens, particularly h-80 with larger hatch spacings, showed enhanced corrosion resistance on a single-specimen basis, attributed to a more effective passive film formation. Surfaces with higher defect density developed effective thicker but defect-rich passive films with poor protective performance, whereas Ti-enriched, low-defect surfaces formed effective thinner yet compact and highly resistive films. These results indicate that within the defect-minimized region of the printability map, surface-specific defect populations and surface chemistry are the dominant factors governing corrosion response, rather than nominal process parameters. The findings imply that build-orientation selection and surface-specific post-processing-beyond bulk-density optimization-are required for corrosion-critical SLM NiTi components. Direct passive-film characterization, replicate testing, and Ni-ion-release quantification are identified as essential follow-on work.