  Additive manufacturing (AM) by laser powder bed fusion (L-PBF) leads to the
formation of a rich, hierarchical microstructure, including dislocation cell
structures and elemental segregation. This structure has profound impacts on
the corrosion behavior and mechanical properties of printed materials. In this
study, we use in situ liquid cell scanning transmission electron microscope
(STEM) to directly characterize the nanoscale origins of corrosion initiation
in AM 316L stainless steel. Under applied anodic potentials, we found that the
dislocation cellular boundaries were preferentially corroded and that pit-like
features formed along the cellular boundaries. We directly observed the
earliest stages of corrosion by controlling the biasing parameters to
decelerate the corrosion processes. The results show that highly localized
corrosion occurs via inclusion dissolution along dislocation cell boundaries.
More widespread corrosion initiates at the dislocation cell boundaries and
spreads throughout the dislocation networks.
