  Corrosion is a ubiquitous failure mode of materials in extreme environments.
The more localized it is, the more difficult it is to detect and more
deleterious its effects. Often, the progression of localized corrosion is
accompanied by the evolution of porosity in materials, creating internal
void-structures that facilitate the ingress of the external environment into
the interior of the material, further accelerating the internal corrosion.
Previously, the dominant morphology of such void-structures has been reported
to be either three-dimensional (3D) or two-dimensional (2D). Here, we report a
more localized form of corrosion, which we call 1D wormhole corrosion. Using
electron tomography, we show multiple examples of this 1D and percolating
morphology that manifests a significantly high aspect ratio differentiable from
2D and 3D corrosion. To understand the origin of this mechanism in a Ni-Cr
alloy corroded by molten salt, we combined energy-filtered four-dimensional
scanning transmission electron microscopy (EF-4D-STEM) and ab initio density
functional theory (DFT) calculations to develop a vacancy mapping method with
nanometer-resolution, identifying a remarkably high vacancy concentration in
the diffusion-induced grain boundary migration (DIGM) zone, up to 100 times the
equilibrium value at the melting point. These vacancy supersaturation regions
act as the precursors of wormholes, and lead to the asymmetrical growth of
voids along GBs. We show that similar 1D penetrating corrosion morphologies
could also occur in other materials or corrosion conditions, implying the broad
impact of this extremely localized corrosion mechanism. Deciphering the origins
of 1D corrosion is an important step towards designing structural materials
with enhanced corrosion resistance, and also offers new pathways to create
ordered-porous materials for functional applications.
