Corrosion Behavior of Binary Ni-Cr Alloys in Molten FLiNaK Salts at Different Electrochemical Potentials
Authors: Chan Ho Lun , Romanovskaia Elena , Mills Sean , Hong Minsung , Romanovski Valentin , Kuster Benjamin , Peddeti Chaitanya , Bieberdorf Nathan , Minor Andy , Hosemann Peter , Asta Mark , Scully John R. 
Published: February 2024

The development of molten salt reactors (MSRs) is a collaborative effort among academia, government laboratories, and industry, with the aim of realizing the potential of clean nuclear energy. However, the corrosion of structural alloys presents a significant engineering challenge in this application. Addressing this issue requires investment not only in testing and characterizing candidate alloys, but also in developing a scientific framework to understand the fundamental mechanisms governing the corrosion of metallic materials in molten fluorides which can then guide alloy design [1].
In this work, the corrosion behavior of binary Ni-Cr alloys (5-20 wt.% Cr) in molten LiF-NaF-KF (or FLiNaK) salts between 550oC and 750oC was interrogated using electrochemical methods coupled with thermo-kinetic analysis. Electrochemical techniques, including linear sweep voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry, were utilized to elucidate the rate-limiting kinetic factors and to identify the dependence of corrosion morphology on applied potential. Additionally, Ni-Cr alloys were also immersed 99.9% pure FLiNaK salts with a known oxidizer concentration (e.g., 0.1-5 wt.% EuF3) coupled with open-circuit potential measurement to provide a time-based description of morphological evolution in the case of a dynamically changing electrode potential. These results are corroborated by examination of the post-corrosion morphology using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques as well as salt composition analysis using inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD) and in situ cyclic voltammetry.
The systematic choice of potential revealed multiple potential regimes in which Ni-Cr alloys exhibit distinctly different corrosion morphologies. At intermediate potentials, a porous bicontinuous structure is formed by dealloying of the less noble element (Cr) and percolation into the Ni-rich alloy within grains and along grain boundaries. At other potentials, charge transfer control dissolution of both Cr and Ni leads to the formation of a crystallographic faceted structure. The morphological differences suggest the presence of multiple corrosion processes in Ni-Cr in FLiNaK controlled by either transport or interfacial reaction. These findings can be explained through electrochemical processes at the alloy-salt interface, as well as the roles of surface, grain boundary and bulk diffusion kinetics involving the alloying constituents. Furthermore, the effect of radiation damage on Ni20Cr alloys on these rate-limiting corrosion regimes will be investigated.

Acknowledgment

Research is primarily supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES).
[1] Chan, H.L., et al. Npj Materials Degradation 6(1), 46 (2022).