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

Abstract

The paper revisits the corrosion behavior of pure Cr in molten FLiNaK salt at 600°C from the perspective of corrosion electrochemistry. In this work, the potential-dependent, rate-limiting charge-transfer, and salt film-mediated mass-transport controlled regimes of Cr corrosion in FLiNaK at 600°C are investigated. The kinetic and thermodynamic parameters that limit electrodissolution and the consideration of grain orientation on these regimes are elucidated. At low Cr(III) concentrations, the corrosion process is governed by charge transfer control at low overpotentials and is crystal orientation dependent. However, when Cr(III) concentrations are high or when there is a high overpotential, the formation of a metal fluoride salt film on the Cr surface shifts the kinetic behavior to be governed by mass transport control at all anodic potentials with a surface morphology controlled by salt film deposition location and identity. Evan's diagrams were developed to consolidate and elucidate these observations. These findings were supported by an examination of the post-corrosion microstructure, X-ray diffraction of solidified salts, and thermo-kinetics analysis in each corrosion regime.

Morphological Evolution and Dealloying During Corrosion of Ni20Cr (wt.%) in Molten FLiNaK Salts

“…Thus, the salt may be considered as unpurified. Nevertheless, our recent work focusing on the validation of molten salt electrochemistry 25 reveals that during anodic polarization on the working electrode (the main focus of this study), impurities such as metallic cations, moisture, and molten K + ions can be consumed by reduction at the counter electrode to support the ionic current flow. This phenomenon merely suggests that the driving force of Cr/Cr (III) and/or Ni/Ni(II) reactions responsible for Ni20Cr dealloying is supported by the potentiostat instead of relying on the oxidizing impurities (e.g.…”

“…S2). 6,7,25,41,42 It is noted, however, that the K + /K electron transfer kinetics are significantly faster than the typical range of scan rates utilized in CV experiments (up to 500 mV s −1 ), 6 and therefore the effect of scan rate on the equilibrium K + /K potential value is minimal (see Fig. S2).…”

“…The multiple solid-state diffusion processes include LN element outward bulk lattice diffusion, short-circuit diffusion via grain boundaries and dislocation pipes or accelerated by radiation-induced defects, 24 MN element surface diffusion which uncovers unreacted Cr in underlying layers, and long-range ionic diffusion of dissolved metal cations or oxidizers, influenced by the solubility and fluid dynamics of the molten salts. 16,[25][26][27] The bulk diffusion-assisted dealloyingis observed to reduce the parting limit of dealloying in binary NiCr and FeCr alloys to approximately 20 at% of the LN in molten salts, 16,[25][26][27] a sharp contrast to the 50-60 at% typical in RT aqueous solutions. 22 In contrast, at lower T H such as during Ag-Au dealloying at room temperature, the LN bulk diffusion (Ag in this case) becomes negligible relative to its rate of oxidation.…”

“…The implication of bulk diffusion-assisted dealloying is that the corrosion process is now governed by a myriad of factors at different length scales that introduces further variability to the corrosion morphology, 29 including grain size, point defectsolute interaction 30,31 and electrochemical potential. 25 For instance, if the grain boundary diffusion of LN element is much faster than bulk lattice diffusion (e.g. due to finer grain size 27 ), severe grain boundary corrosion can be anticipated.…”

“…There is a need for comprehensive studies correlating variations in applied electrochemical potential with the thermodynamic regions for stability of selected soluble metal salt species and the associated alloy dissolution kinetics. 6,9 Our previous work, which investigated the corrosion behavior of pure Cr in molten FLiNaK, 25 demonstrated that Cr can exhibit a diverse range of potential dependent corrosion morphologies-from micro-porosity to surface faceting to salt-film induced smoothing -depending on the applied potential. Extending this approach, the present study explores the morphological evolution of Ni20Cr in FLiNaK under various applied potentials that enable dealloying.…”