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Insight into the behavior of structural materials in a molten salt environment

Mia Maric (left) and Dr Ondrej Muransky. Credit: Australian Nuclear Science and Technology Organization (ANSTO)An international research group led by…

Mia Maric (left) and Dr Ondrej Muransky. Credit: Australian Nuclear Science and Technology Organization (ANSTO)

An international research group led by ANSTO has found that cold rolling increases the sensitivity of the material to molten salt corrosion through increased grain length and other microstructural defects, which usually contribute to material strengthening.

Dr. Ondrej Muránsky, lead, high temperature and molten salt corrosion performance of advanced materials, nuclear fuel cycle at ANSTO and Mia Maric (both images above) said that this research is relevant to future molten salt reactor (MSR), concentrated solar heat (CST) and thermal energy storage (TES) currently under development.

The study, published in Corrosion Science was made of 31

6L stainless steel used in current nuclear reactors and is also considered as the structural material for future nuclear and non-nuclear power generation (MSR, CST) systems and systems for energy storage (TES).

Using electron diffraction and neutron diffraction techniques at ANSTO, investigators found that cold rolling leads to an introduction of microstructural defects that make the material stronger but also more susceptible to molten salt corrosion.

The use of high resolution neutron diffraction (HR ND) on the Echidna instrument revealed an increase in the amount of dislocations, whereas the electro-back scatter diffraction technique (EBSD) showed a significant increase in grain boundary in cold-rolled state compared with the as-annealed state.

“HRND and EBSD measurements give us information about the effect of transmitted plastic deformation on the alloy’s microstructure,” Muránsky says.

Credit: Australia’s Nuclear Science and Technology Organization (ANSTO)

Corrosion test was performed at ANSTO in a dedicated molten salt rig in FLiNaK melted salt at 600 ° C for 300 hours.

The corrosion resistance of tested alloy conditions (0%, 20% and 30% cold rolling) was evaluated from the material mass loss as a result of the exposure to the molten salt.

It was found that the amount of cold rolling has a strong relationship with the material mass in the molten salt.

Higher amounts of cold rolling resulted in increases in material mass loss.

“This means that there is an increased susceptibility to the material to react with the molten salt and thereby undergo accelerated corrosion,” Muránsky says. 19659005] An electron probe microanalyser (EPMA) at the University of New South Wales (UNSW) revealed that the bulk material maintained an even distribution of iron (Fe) and nickel (Ni), while there was increased diffusion of chromium (Cr) and molybdenum (Mo) to the grain boundaries.

“Here they could easily react with the molten salt that forms chromium and molybdenum-rich corrosion products,” Muránsky says.

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More information:
M. Maric et al. The effect of cold rolling on the microstructure and corrosion behavior of 316L alloy in FLiNaK melted salt, Corrosion Science (2018). DOI: 10,1016 / j.corsci.2018.07.006

Provided by:
Australian Nuclear Science and Technology Organization (ANSTO)

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