Differences between 3D, 2D, and 1D corrosion: (a) schematic drawing of a typical bulk bicontinuous dealloying corrosion morphology; (b) schematic drawing of a typical pitting corrosion morphology with the volume set as transparent; the grain boundaries (GBs) were not displayed because both intergranular and intragranular pitting can occur; (c) schematic drawing of a typical intergranular corrosion morphology with the volume set as opaque; the sides of the cube display the cross-sectional views; (d) schematic drawing of 1D wormhole corrosion in a polycrystalline material; the upper half is a cross-sectional view showing discontinuous dots (i.e., voids filled by molten salt) along the GBs; the bottom half is a volumetric cutaway along the GBs where the 1D percolating network of tunnels on the grain surfaces is clearly shown in red; the dark blue color in (a) to (c) indicates free space such as cracks, voids, crevices etc., while the red color in (d) indicates wormholes filled with molten salt; (e) a representative false-colored SEM image showing the cross-section of Ni-20Cr after corrosion; (f) FIB-SEM 3D reconstruction of the volume shown in (e). Image credit: Yang et al., doi: 10.1038/s41467-023-36588-9.
“Corrosion, a ubiquitous failure mode of materials, is traditionally measured in three dimensions or two dimensions, but those theories were not sufficient to explain the phenomenon in this case,” said Dr. Yang.
Yang is a researcher with the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory and the Materials Research Institute at the Pennsylvania State University.
“We found that this penetrating corrosion was so localized, it only existed in one dimension — like a wormhole.”
“Corrosion is often accelerated at specific sites due to various material defects and distinct local environments, but the detection, prediction and understanding of localized corrosion is extremely challenging,” added Professor Andrew Minor, a researcher at Lawrence Berkeley National Laboratory and the University of California Berkeley.
The researchers hypothesized that wormhole formation is linked to the exceptional concentration of vacancies — the empty sites that result from removing atoms — in the material.
To prove this, they combined 4D scanning transmission electron microscopy with theoretical calculations to identify the vacancies in the material.
Together, this allowed the researchers to map vacancies in the atomic arrangement of the material at the nanometer scale.
The resulting resolution is 10,000 times higher than conventional detection methods.
“Materials are not perfect.
They have vacancies, and the vacancy concentration increases as the material is heated, is irradiated or, in our case, undergoes corrosion,” said Dr. Michael Short, a researcher at MIT.
“Typical vacancy concentrations are much less than the one caused by molten salt, which aggregates and serve as the precursor of the wormhole.”
Molten salt, which can be used as a reaction medium for materials synthesis, recycling solvent and more in addition to a nuclear reactor coolant, selectively removes atoms from the material during corrosion, forming the 1D wormholes along 2D defects, called grain boundaries, in the metal.
The scientists found that molten salt filled the voids of various metal alloys in unique ways.
“Only after we know how the salt infiltrates can we intentionally control or use it,” said Dr. Weiyue Zhou, a postdoctoral researcher at MIT.
“This is crucial for the safety of many advanced engineering systems.”
Now that the authors better understand how the molten salt traverses specific metals — and how it changes depending on the salt and metal types — they hope to apply that physics to better predict the failure of materials and design more resistant materials.
“As a next step, we want to understand how this process evolves as a function of time and how we can capture the phenomenon with simulation to help understand the mechanisms,” said Dr. Mia Jin, a researcher at the Pennsylvania State University.
“Once modeling and experiments can go hand-in-hand, it can be more efficient to learn how to make new materials to suppress this phenomenon when undesired and utilize it otherwise.”
Fascinating Ja? I myself am well fascinated by this shit as I have been looking for examples of wormhole like behavior in crystalline constructs for many years.
It was when I went down the synthetic engineering rabbit hole care of Polymer Engineering that my mind was taken hostage to science and impossible causes.
Finally a key to help unravel it all pops out into this reality on demand!
What an amazing coincidence, Ja?!
The findings were published in the journal Nature Communications.
Y. Yang et al. 2023. One dimensional wormhole corrosion in metals. Nat Commun 14, 988; doi: 10.1038/s41467-023-36588-9