(General Student Poster Award Winner, 3rd Place) Designing and Evaluating Microelectrodes for Molten Salt Corrosion Electrochemistry
Authors: Chan Ho Lun , Singh Harjot , Romanovski Valentin , Romanovskaia Elena , Giordano Joseph , Scully John R. 
Published: December 2024

Due to the coupling of high temperature operation (>500°C) and propensity to contain oxidizing impurities (e.g., moisture, metallic cations), corrosion of structural alloys in molten chloride and fluoride salts is a major concern among developers of Generation IV nuclear reactors. To mitigate corrosion, it is essential to understand the corrosion mechanism and accurately determine its rate. Prior studies employ a forensic approach for corrosion testing, which does not provide insights on instantaneous rate of corrosion [1]. The use of electrochemical impedance spectroscopy (EIS) can provide in-operando method to interrogate dissolution processes. However, due to the high electrical conductivity and double-layer capacitance of metal salt interface [2], low frequency regimes normally dominated by polarization resistance (Rp) in aqueous systems require data acquisition at low frequencies. (<10-3 Hz). This renders access to charge-transfer resistance (Rct) and diffusional impedance effects difficult. This difficulty is compounded by issues such as signal noise associated with the use of long cables in conventional molten salt corrosion cells [2].
To overcome this challenge, a working electrode may be designed as a 2D microdisk electrode to was used to interrogate the low-frequency impedance regimes. This disk has a non-uniform current distribution at such low Wagner number but it is easier to access the charge transfer and/or diffusional impedance regime, otherwise unattainable with the conventionally used partially immersed 3D macro-electrodes utilized in most studies [3]. In this work, a flush mounted microdisk electrode, compatible for use in molten LiF-NaF-KF (i.e., FLiNaK) salts at 600°C was designed and evaluated for its effectiveness towards accurately determining Rp through the EIS method. Pure Cr was selected as the model working electrode since its corrosion behavior in FLiNaK at 600°C has been thoroughly investigated [4,5,6],. Cr microdisks, with areas ranging from 100 to 10-2 cm2, were fabricated and subsequently exposed in FLiNaK containing 1 wt.% of EuF3 at 600°C for 24 hrs. EIS measurements were carried out over the commonly utilized frequency range from 105 to 10-2 Hz. Moreover, the error introduced by implementing various low “cut off” frequencies was examined.

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). This work was performed in the Department of Materials Science and Engineering (DMSE) in the Center for Electrochemical Science and Engineering (CESE) at the University of Virginia. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility (NMCF) with National Science Foundation (NSF) under award CHE-2102156. H.C. acknowledges the National Science Foundation Graduate Research Fellowship (GRFP), GRANT#: 1842490.

References:
[1] S. Guo, J. Zhang, W. Wu, W. Zhou, Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications, Progress in Materials Science 97 (2018) 448–487.
[2] G.L. Fredrickson, G. Cao, P.K. Tripathy, M.R. Shaltry, S.D. Herrmann, T.-S. Yoo, T.Y. Karlsson, D.C. Horvath, R. Gakhar, A.N. Williams, R.O. Hoover, W.C. Phillips, K.C. Marsden, Review—Electrochemical Measurements in Molten Salt Systems: A Guide and Perspective, J. Electrochem. Soc. 166 (2019) D645–D659.
[3] H.L. Chan, E. Romanovskaia, J. Qiu, P. Hosemann, J.R. Scully, Insights on the corrosion thermodynamics of chromium in molten LiF-NaF-KF eutectic salts, Npj Mater Degrad 6 (2022) 46.
[4] E. Romanovskaia, H.L. Chan, V. Romanovski, F. Garfias, M. Hong, S. Mastromarino, P. Hosemann, R. Scarlat, J.R. Scully, An In Situ, multi-electrode electrochemical method to assess the open circuit potential corrosion of Cr in unpurified molten FLiNaK, Corrosion Science 222 (2023) 111389.
[5] H.L. Chan, E. Romanovskaia, V. Romanovski, D. Sur, M. Hong, P. Hosemann, J.R. Scully, Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C, J. Electrochem. Soc. 170 (2023) 081502
[6] H.L. Chan, J.R. Scully, Deciphering when Metal Corrosion is Spontaneous in Molten Fluorides Using Potential-Activity Diagrams, CORROSION 79 (2023): 1236–1240.