Influence of feature size and shape on corrosion of 316L lattice structures fabricated by laser powder bed fusion

Laser powder bed fusion (LPBF) has become an established method for manufacturing end-use metal components. Exploiting the geometric freedom of additive manufacturing (AM) offers broad possibilities for part optimization and enables performance enhancements across industry sectors. However, part shape and feature size have been found to locally affect residual stresses, melt pool cooling rates, microstructure, and thus the mechanical properties of components. Even though the mesoscale structure can locally induce microstructural changes, there are no prior studies on how it influences corrosion. Using AM-produced, optimized parts in critical applications necessitates a better understanding of their long-term performance. In this study, lattice structures were used to probe the influence of feature size and shape on corrosion susceptibility and its spatial localization. The susceptibility of submillimeter LPBF-fabricated 316L stainless steel lattice structures to corrosion was investigated by conducting a 21-day immersion corrosion test in an aqueous 3.5 wt% NaCl solution. Schoen gyroid and Schwarz diamond triply periodic minimal surface lattices were manufactured with three unit cell sizes and wall thicknesses (0.867, 0.515, and 0.323 mm). The nominal surface and cross-sectional areas were the same for the two geometries. X-ray microcomputed tomography (microCT) scans before and after the corrosion test were compared for volumetric losses. In addition, the mechanical properties and microstructure of the samples were evaluated. As part of the study, a workflow to register, index, and analyze volumetric changes of consecutive microCT image stacks was developed. The reported method is applicable to any time-lapse studies with microCT. Three out of five of the 0.323 mm wall thickness lattices displayed visually aggressive pitting. Based on the microcomputed tomography data, the mass losses were localized either in the entrapped powder particles or partially melted surface globules. Corrosion did not occur in the dense base material. The total mass losses ranged from 8 to 19 mg. Despite visual indications to support a higher corrosion susceptibility for the smallest lattice sizes, the mass loss values did not confirm this conclusion. The tensile test results did not provide any clear indications of latent corrosion effects on mechanical properties.