Electrochemical Analysis of Metal Stability in Candidate Reference Electrode Fluoride Salts (FLiNaK + 1 wt% AgF, MnF2, or NiF2)

Abstract

This paper investigates the long-term stability of Ag/AgF, Mn/MnF2, and Ni/NiF2 redox couples for use in reference electrodes (REs) for molten fluoride salt systems such as molten salt reactors. A quasi-RE system was used with FLiNaK + 1 wt% AgF, MnF2, or NiF2, to evaluate the stability of various metals that may be used as the reference electrode body, reference internal element, or membrane for a molten fluoride salt RE design. Cyclic voltammetry was used to measure changes in the electrochemical response of different metal wires in the melts tested. Salt samples were analyzed for dissolved metal concentrations and oxide concentrations via inductively coupled plasma mass spectroscopy and combustion analysis, respectively. To observe the surface morphology of the electrodes used in the Ag/AgF test, cross-sectional imaging of the electrodes was provided via scanning electron microscopy (SEM). The resulting electrochemical measurements show that a RE constructed using a Mn/MnF2 or Ni/NiF2 redox couple should perform with greater potential stability compared to a Ag/AgF redox couple. The instability of the Ag/AgF redox couple is due to the strong oxidizing property of the AgF salt with metals compared to the MnF2 and NiF2 salts. The SEM results from the Ag/AgF test indicate that Ag metal is corrosion resistant, but having AgF in the salt causes it to be corrosive towards other metals (Ni and W) that are considered relatively corrosion resistant in molten salts.