Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy

Abstract

Growing interest in molten salts as effective high-temperature heat-transfer fluids for sustainable energy systems drives a critical need to fundamentally understand the interactions between metals and molten salts. This work utilizes the multimodal microscopy methods of synchrotron X-ray nanotomography and electron microscopy to investigate the 3D morphological and chemical evolution of two-model systems, pure nickel metal and Ni-20Cr binary alloy, in a representative molten salt (KCl-MgCl 2 50−50 mol %, 800 °C). In both systems, unexpected shell-like structures formed because of the presence of more noble tungsten, suggesting a potential route of using Ni−W alloys for enhanced molten-salt corrosion resistance. The binary alloy Ni-20Cr developed a bicontinuous porous structure, reassembling functional porous metals manufactured by dealloying. This work elucidates better mechanistic understanding of corrosion in molten salts, which can contribute to the design of more reliable alloys for molten salt applications including next-generation nuclear and solar power plants and opens the possibility of using molten salts to fabricate functional porous materials.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium

“…However, they can be very corrosive to metal alloy materials under the extreme, real-world operating conditions (873-1073 K) of CSP and MSR systems. [10][11][12] A major issue is related to chromium being primarily depleted from the alloys upon contact with the molten KCl-MgCl 2 , leading to the formation of dilute CrCl 3 in the melt. 10 Despite extensive investigations of molten salts in the past, the only insights regarding how low-concentration Cr 3+ metal ions behave in the high-temperature ionic medium have been achieved through optical spectroscopy.…”

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids

“…There has been a constant stream of articles − including salts in their molten or glass states since the early 1900s; see for example, the works of Zachariasen and Rosenhain . Yet, through the years, molten salts continue to be rediscovered for applications in energy technologies including nuclear energy, − solar energy harvesting, − batteries, − and separations to mention just a few.…”

“…Recently, a faster Drude-type model that includes some of the same ingredients as in these other more expensive models has also been successfully developed and applied to the case of MgCl 2 and its mixtures with KCl . A variety of articles based on machine-learning simulation schemes for molten salts have also recently appeared in the literature. − Advances in computational models have come hand-in-hand with new experimental studies, ,,,− ,− and in some cases, the combination of these challenges strongly entrenched assumptions, for example the octahedral coordination of U 3+ , the tetrahedrality in coordination for Mg 2+ , , and other properties that collectively have influenced the literature for 30 years or more. ,− The works cited thus far are meant to provide some context and are not exhaustive or cover many of the most studied salts and their mixtures of which the eutectics are of special relevance.…”

In-situ analysis of corrosion products in molten salt: concurrent X-ray absorption and electrochemistry reveal both ionic and metallic species

“…Therefore, we propose an alternative explanation that small Nirich grains were liberated from the foil due to the dissolution of Cr. Previous X-ray tomography and quasi in situ TEM observations of NiCr wires in molten KCl -MgCl 2 showed dealloying, grain boundary attacks, and salt penetration in cracks and pores [26][27][28]. Such dealloying leaves behind a sponge-like structure of Ni-rich grains that would easily detach into the solution.…”