Potentiodynamic Polarization of Pure Metals and Alloys in Molten LiF-NaF-KF (FLiNaK) Using the K/K⁺ Dynamic Reference Electrode

Abstract

The potentiodynamic polarization method using the K/K+ dynamic reference electrode was used to understand the corrosion behavior of Ni, Fe, Cr, the Ni-20Cr binary alloy, and 316L stainless steel in molten 46.5LiF 11.5NaF-42KF (FLiNaK, composition in mol %) salt at 700˚C. Exposure of the materials without an applied potential revealed that Ni and Fe were stable in FLiNaK, but Cr was dissolved rapidly. The apparent redox potentials of each material with respect to the K/K+ redox couple are compared to gain insight into how the applied potential relates to the dominant corrosion mechanism for each alloy. The polarized pure metals experienced uniform surface recession while corrosion in the alloys resulted in preferential elemental dissolution depending on the applied potential with respect to the K/K+ reference reaction.

Corrosion Electrochemistry of Chromium in Molten FLiNaK Salt at 600 °C

“…This pseudo K + /K reference potential is often referred to as a dynamic reference potential and is often used to calibrate electrochemical measurements in molten salts. 10,16,[19][20][21] Firstly, a CV scan was performed on the Pt wire at a scan rate of 100 mV s −1 toward the negative/cathodic terminal until a sharp increase in cathodic current was observed. At this point, the potential was reversed to its initial starting potential.…”

“…Some of these have extended from exposure studies to those utilizing electrochemical methods. 6,[8][9][10][73][74][75] Electrochemical methods provide an in situ and diagnostical approach to characterize the corrosion behavior of candidate materials in real-time either in the case of open-circuit (static immersion) or under polarized conditions. 11 For example, potentiodynamic polarization (PD) or linear sweep voltammetry (LSV) can (i) provide a straightforward approach to rank the relative stability of alloys in the molten salts over a range of potentials often representative of open circuit potentials (OCP) with oxidants, 6 (ii) enable the determination of corrosion potential (E corr ) and corrosion current density (i corr ) for quantitative analysis, [4][5][6][7] and (iii) identify the rate-determining steps (RDS) that controls the materials dissolution process as a function of driving force (i.e., electrode potential) at the metal/electrolyte interface.…”

Application of the Solute-Solvent EMF Cell to Measure Activity of NiF2 in Molten FLiNaK

“…The corrosion behavior of alloys can be predicted by the redox behavior of each constituent alloy component in the FLiNaK melt 5,11 , which is typically determined by using fluorine gas evolution to define the reference potential. In practice, data are generated by using transient measurements that relate the measured electrochemical potentials to the anodic or cathodic decomposition of a constituent in the FLiNaK eutectic 12,13 or by using an external reference electrode, typically based on the Ni 2+ /Ni redox couple [14][15][16] . Use of a stable, well defined reference electrode is crucial for making accurate and reproducible electrochemical measurements.…”

Application of the Solute-Solvent EMF Cell to Measure Activity of NiF2 in Molten FLiNaK

“…[5][6][7][8][9] The corrosion behavior of alloys is influenced by the redox behavior of each constituent alloy component in the FLiNaK melt, 5,10 which is typically determined by using fluorine gas evolution to define the reference potential. In practice, data are generated by using transient measurements that relate the measured electrochemical potentials to the anodic or cathodic decomposition of a constituent in the FLiNaK eutectic 11,12 or by using an external reference electrode, typically based on the Ni 2+ /Ni redox couple. [13][14][15] Use of a stable, well defined reference electrode is crucial for making accurate and reproducible electrochemical measurements.…”

“…The DRE of FLiNaK is K + /K reduction, which has been used by Doniger, Couet, and Sridharan as a reference for electrochemical corrosion measurements of iron, chromium, Type 316 L stainless steel, and Ni-20Cr in FLiNaK. 11 Durán-Klie, Rodrigues, and Delpech used the FLiNaK DRE to measure the potential of the Ni 2+ /Ni redox couple at different concentrations of NiF 2 in molten FLiNaK by measuring the open circuit potential of a nickel wire against the FLiNaK DRE. 12 The DRE approach is useful for relating measured redox potentials to a single reference point within the system.…”