Insights on the corrosion thermodynamics of chromium in molten LiF-NaF-KF eutectic salts

Abstract

The thermodynamic stability of chromium in the Cr0, Cr2+, and Cr3+ oxidation states considering various F− ion coordination compounds was investigated in molten LiF–NaF-KF (FLiNaK) eutectic salt. Potential fluoride ion activity (F− and CrF3−) diagrams were constructed to predict the most stable Cr oxidation states as a function of anion activity, solvation state of chromium ions and potential at 600 °C. The Gibbs free energies of these compounds were estimated by utilizing a combined cyclic voltammetry – Nernst theory analysis approach in FLiNaK salt. To verify the constructed diagrams, X-ray diffraction was utilized after exposure of Cr at various applied potentials to determine whether compounds detected in solidified FLiNaK salts agreed with thermodynamic calculations. This work aims to identify key thermodynamic factors that are significant to chromium corrosion for molten salt nuclear reactor applications. The F− stability region spans the entire region where spontaneous corrosion of Cr occurs. Cr may be oxidized to Cr2+ and Cr3+ in the presence HF (due to moisture as an impurity) spontaneously except some conditions of p1/2H2/aHF etc. This situation is not changed qualitatively for various states of solvation between oxidized Cr solute in F− solvent and this aspect is substantially similar for the two cases (pair 1: Cr0/CrF3−/ CrF63−; pair 2: Cr0/CrF42−/CrF52−) considered in this work.

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.…”

“…This connection is a foundational concept necessary to describe corrosion behavior. Recent work 16 developed potential-activity diagrams (analogs to Pourbaix diagrams) to describe the electrochemical thermodynamics of Cr redox reactions in FLiNaK. These diagrams provided a map of phase stability regions of Cr, its corrosion products (such as Cr(III) in the form of CrF 6 3− ), and included oxidants as a function of potential and ion activities, providing predominant anodic/cathodic reaction identification as a function of potential and activity.…”

Analysis of Microwave Resonant Cavity Transducer Degradation Mechanisms

“…Computer simulations can offer a valuable insight in estimating material degradation and its impact on sensor deterioration. We have focused on modeling the effects of creep [12][13][14][15][16] and corrosion [17][18][19][20][21] on degradation of the sensor's stainless steel 316 membrane in FLiBe. COMSOL multiphysics software was used to model the dynamic pressure on the flow sensor, the effects of creep, and the extent of chromium depletion as a proxy for the amount of corrosion.…”

“…In future development, more work needs to be done in calculating material and reaction properties to obtain a more rigorous model. We have taken the molten salt to be stagnant, but experimental data has shown a significant increase in chromium depletion in systems with flow [19]. Additionally, the dynamic pressure of the flowing sodium could increase the rate of corrosion [20], which needs to be accounted for in the model.…”

Second Annual Report on Development of Microwave Resonant Cavity Transducer for Fluid Flow Sensing

“…Computer simulations can offer a valuable insight in estimating material degradation and its impact on sensor deterioration. We have focused on modeling the effects of creep [15][16][17][18][19][20] and corrosion [15,[21][22][23][24][25] on degradation of the sensor's stainless steel 316 membrane in FLiBe (mixture of lithium fluoride and beryllium fluoride) salt. COMSOL multiphysics software was used to model the dynamic pressure on the flow sensor, the effects of creep, and the extent of chromium depletion as a proxy for the amount of corrosion.…”