Materials and Chemistry for Molten Salt Systems: Electrochemistry
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Stephen Raiman, University Of Michigan; Kumar Sridharan, University of Wisconsin-Madison; Nathaniel Hoyt, Argonne National Laboratory; Jinsuo Zhang, Virginia Polytechnic Institute and State University; Michael Short, Massachusetts Institute of Technology; Raluca Scarlat, University of California, Berkeley

Tuesday 2:00 PM
March 16, 2021
Room: RM 49
Location: TMS2021 Virtual

Session Chair: Nathaniel Hoyt, Argonne National Laboratory


2:00 PM  
Research Paths on Spectroelectrochemistry for Molten Salt Chemistry in the U.S with Respect to Other Nations: Supathorn Phongikaroon1; 1Virginia Commonwealth University
    Molten salts have been a critical component in industrial metallurgical processes. Since the 1950s, special attention has been focused on the use of molten salts in other applications such as batteries, solar energy, and chemical processing. Beneficial features of molten salts, including a high thermal capacity, good heat transfer characteristics, and strong resistance to effects of radiation in high-enriched and high-burnup nuclear fuels, have led to use of molten salts in nuclear reactor designs and other nuclear-related applications. This work presents progress and statistics on spectroelectrochemistry studies for molten salt chemistry (e.g., speciations, corrosions, thermophysical properties, etc.) of different countries within recent years. The data sets were obtained from available open public sources and web-based domains. The results will be presented and discussed on the U.S. R&D through collaborative efforts between universities and national laboratories with respect to other nations such as France, South Korea, Japan, Russia, and China.

2:30 PM  
Materials Compatibility and Potential Stability of Reference Electrodes for Molten FLiBe: Francesco Carotti1; Raluca Scarlat2; 1University of Wisconsin, Madison; 2University of California, Berkeley
    FLiBe (2LiF-BeF2) molten salt is considered as heat transfer fluid for advanced nuclear energy systems, including fluoride salt-cooled high-temperature reactors (FHRs). Electrochemical techniques play a key role in FLiBe technology development as they enable monitoring of FLiBe redox potential and characterization of salt chemical and transport phenomena. Any electrochemical setup requires a reference electrode (RE) that provides an in-situ potential reference. This talk will present and compare the performances of the REs most commonly adopted in FLiBe electrochemical studies: Ni/Ni(II) thermodynamic RE, dynamic RE and Pt quasi-RE. The results in terms of potential stability, thermodynamics and background noise for each electrode type are discussed, providing a metric for RE selection in laboratory and industrial application. In addition, the results of a full material characterization on the Ni/Ni(II) RE are presented, pointing at the suitability of the RE materials for long term operations in FLiBe.

2:50 PM  
Redox Measurement and Control in Molten Chloride Fast Reactor Fuel Salt: Matthew Newton1; D. Hamilton1; Michael Simpson1; 1Univ of Utah
    One problematic issue with molten salt reactors (MSRs) is the tendency for redox potentials of salts to increase with irradiation, causing greater salt corrosivity. Na, Ca, U, and Zr were identified as redox buffer candidates in a NaCl-CaCl2-UCl3 salt. Redox potentials—represented by open circuit potential (OCP)—were measured at a tungsten working electrode vs. a Ag/AgCl reference electrode housed in a mullite tube. Continuous OCP measurements were taken at 600°C while NiCl2 was added to the molten salt to replicate irradiation effects on redox potential. OCP rose by 0.23 V with the addition of NiCl2. A zirconium rod was submerged in the salt, causing an OCP drop of -0.63 V over 100 minutes. Results for U, Na, or Ca redox buffers will also be presented. Uranium and zirconium are promising redox buffers since they neither dissolve in the salt nor reduce the potential below sustainable operating conditions for MSRs.

3:10 PM  
Oxidation Potential of Molten Halide Salts: A First Principles Study: Jianguo Yu1; Guoping Cao1; 1Idaho National Lab
    Chloride salts are promising candidate fuel and coolant for some molten salt reactor (MSR) concepts as they offer potential for sustained operation in a fast neutron spectrum with minimal salt processing, passive safety, and a high-power density. However, molten chlorides present significant corrosion challenges to structural materials. Currently, the corrosion data for relevant metal alloys in chloride salts are scarce, presenting technical risks for MSR design and operation. In this work, modeling of salt chemistry and corrosion is performed through first-principle density functional theory (DFT)-based multi-scale modeling. By comparing chloride salt UCl3 and fluoride UF3 counterpart, it was found that chloride salt UCl3 is much more stable than fluoride salt UF3. It also shows that the modeling in salt chemistry and corrosion control is very promising.

3:30 PM  
High-throughput Electrochemical Characterization and Screening of CSP-relevant Alloys: Nathaniel Hoyt1; Jicheng Guo1; Mark Williamson1; 1Argonne National Laboratory
    The use of electrochemistry offers enormous capabilities to determine the material properties of alloys immersed in high-temperature molten salts. Electrochemical approaches typically involve coupled salt/alloy interactions, but if salt properties are well-known and maintained in a consistent state, fundamental characterization of the alloy alone can be achieved. Knowledge of these fundamental properties, such as the activity of alloy constituents, is crucial to predict the corrosion behavior of structural metals when exposed to challenging molten salt conditions. Toward that end, Argonne National Laboratory has been conducting an extensive campaign examining fundamental properties and providing rapid screening of key CSP-relevant alloys. Results from testing in isothermal, non-isothermal, and high-throughput corrosion cell configurations will be discussed.

3:50 PM  
Corrosion of Zircaloy-2 in Molten LiCl-xLi2O at 650 °C: Vineeth Kumar Gattu1; Evan Wu1; William Ebert1; 1Argonne National Laboratory
    Coupon and electrochemical corrosion tests were conducted to study the corrosion behavior of Zircaloy-2 in molten LiCl salt with small amounts of added Li2O at 650 °C. Small sections of Zircaloy-2 cladding were used as working electrodes in a standard three-electrode cell with a Ni counter electrode and Ni|NiO reference electrode. Potentiodynamic polarization (PD) scans and potentiostatic (PS) tests were conducted in salts with Li2O contents between 1-10 wt%. Series of PS tests were performed at voltages between -1.6 and -0.6 V vs Ni|NiO with periodic electrochemical impedance spectroscopy analyses to measure Zircaloy corrosion rates under different redox conditions and detect the formation of oxide layers, secondary phases, and the possible reduction of salt constituents. The electrochemical results and microscopic analyses of the corroded surface and corrosion products will be discussed in terms of the effects of salt composition on the corrosion processes and kinetics.

4:10 PM  
Development of an Electrochemical Phase Field Model for the Corrosion of Ni-Cr Alloys by Molten Fluoride Salts: Chaitanya Bhave1; Michael Tonks1; David Andersson2; Jake McMurray3; 1University of Florida; 2Los Alamos National Laboratory; 3Oak Ridge National Lab
    Corrosion of Ni-Cr alloys used for containment of molten salt is a major challenge in the commercial utilization of molten salt reactor (MSR) technology. In this work, an electrochemical phase-field model has been developed for predicting corrosion mass-loss rates and microstructure evolution of Ni-Cr alloys in molten salts. An electro-diffusion component was added to the Cahn-Hilliard equation to calculate component evolution, and a conserved current condition is solved to ensure electroneutrality in the system. Literature thermodynamic assessments and voltammetry data are used to inform the model thermodynamics and kinetics. Model verification has been performed to confirm that the reaction rates follow Butler-Volmer kinetics. Simulations of Ni-Cr alloy corrosion in molten LiF-BeF2 (FLiBe) are validated against experimental corrosion data. This work was performed using the Multiphysics Object-Oriented Simulation Environment (MOOSE), an open-source finite-element framework. The corrosion model developed in this work will be used to inform engineering-scale reactor models.