Molten salts have a bright future in many technologically important areas such as nuclear energy, solar power plants and production of new materials. However, there are challenges towards structural material performance in these media, and this requires development of alloys or coatings with better corrosion resistance. We have used electrochemical techniques to study the corrosion mechanisms of alloys in model Li-K-Mg eutectic chloride environments that broadly simulate the heat transfer circuits of molten salt reactors. Moisture must be removed, otherwise it will dominate the corrosion response; we have developed a multi-step process to remove water, with in situ electrochemical monitoring of the residual amount. When the water content is low enough, magnesium can be used as a reference electrode. We have studied fundamentals of alloying effects, specifically critical alloy compositions for dealloying in model Fe-(Cr)-Ni alloys, as well as industrial alloy behaviour. Up to a certain temperature, a microporous Ni layer is formed upon dealloying of Fe and Cr. Compared with aqueous solutions, the parting limit for electrolyte dissolution of these less-noble elements decreases by several percent; this is due to very fast surface diffusion of elemental Ni in the molten chloride salt. We also found that at some electrochemical conditions, superficial dealloying could be detected at even higher Ni contents. The high surface mobility of Ni blurs the ordinary parting limit concept, but such limits are still present. The operative mass transport mechanism up to a certain homologous temperature is mediated by surface diffusion; we will also talk about the transition between this "percolation dealloying" and a mechanism controlled by lattice vacancy diffusion, where the porosity resembles negative dendrites. The relevance to stress corrosion cracking will be discussed briefly.

Corrosion mechanisms of alloys in molten chloride salts, including changing criteria for dealloying at different homologous temperatures

ACS Fall 2021

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