University Scientists Create 3D Model to Determine Mechanism of Corrosion

Molten salt corroded a metal barrier, appearing disconnected on a slice view of the damage (right). Researchers imaged the corrosion in 3D and reconstructed the path the salt took through metal (left). Image courtesy of Yang Yang/Penn State.

A multi-institutional research team, co led by Penn State University (University Park, Pennsylvania, USA), released the results of a study to understand the behavior of molten salt, a proposed coolant for next-generation nuclear reactors and fusion power. The results of their study, published in the latest issue ofNature Communications, have significant implications for advanced energy production. 

The problem the research team hoped to solve: how did molten salt breach its metal container? In examining this problem, the researchers initially imaged a cross-section of the sealed container, finding no clear pathway for the salt appearing on the outside. They then used electron tomography, a 3D imaging technique, to reveal the tiniest of connected passages linking two sides of the solid container. That finding only led to more questions for the team researching the strange phenomenon. 

“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,” says co-corresponding author Yang Yang, assistant professor of engineering science and mechanics and of nuclear engineering at Penn state. “We found that this penetrating corrosion was so localized, it only existed in one dimension—like a wormhole.” 

Typically, wormholes on Earth are made by insects who dig into the ground and create one hole, only to return to the surface through a new hole. This creates the impression that the burrowing insect disappears at one point in space and time and reappears in another. Electron tomography could reveal on a microscopic scale the hidden tunnels of the molten salt’s route, whose morphology looks similar to that of wormholes. 

To interrogate how the molten salt “digs” through metal, Yang and the team developed new tolls and analysis approaches. According to Yang, their findings not only uncover a new mechanism of corrosion morphology, but also point to the potential of intentionally designing such structures to enable more advanced materials. 

“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,” says co-corresponding author Andrew M. Minor, professor of materials science and engineering at the University of California Berkeley. 

The team 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 sale. The resulting resolution is 10,00 times higher than conventional detection materials, Yang says. 

“Materials are not perfect,” says co-corresponding author Michael Short, associate professor of nuclear science and engineering at the Massachusetts Institute of Technology (MIT). “They have vacancies, and the vacancy concentration increases as the material is heated, is irradiated or, in our case, undergoes corrosion. 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, a recycling solvent, and a nuclear reactor coolant, selectively removes atoms from the material during corrosion. This results in the formation of 1D wormholes along 2D defects, called grain boundaries, in the metal. The researchers found that molten salt filled the voids of various metals in unique ways. 

“Only after we know how the salt infiltrates can we intentionally control or use it,” says co-first author Weiyue Zhou, postdoctoral associate at MIT. “This is crucial for the safety of many advanced engineering systems.” 

Now that the researchers better understand how molten salt traverses specific metals—and how it changes depending on the salt and metal types—they said 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 howe can capture the phenomenon with simulation to help understand the mechanisms,” says co-author Mia Jin, assistant professor of nuclear engineering at Penn State. “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.” 

Source: Penn State,