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

Tuesday 8:00 AM
March 1, 2022
Room: 203B
Location: Anaheim Convention Center

Session Chair: Nathan Hoyt, Argonne National Laboratory

8:00 AM  
Electrochemical Solutions for Corrosion in Molten Chloride Salt Systems: Kerry Rippy1; Judith Vidal1; Liam Witteman1; 1National Renewable Energy Laboratory
    Great progress has recently been made toward advancing molten chloride salt systems, particularly for concentrated solar power (CSP) and nuclear applications. However, significant concerns regarding the corrosive nature of molten chloride salts remain. Our team has been working to understand and mitigate corrosion of high-performance alloys in molten chloride salt systems, with a focus on electrochemical solutions. Primarily, we have focused on two aspects of corrosion: 1) Electrochemical mitigation of corrosion caused by the species MgOHCl, which forms in chloride salts when exposed to moisture, and 2) Galvanic corrosion, which alloys are vulnerable to when in electrical contact with both a more noble alloy and the salt, which acts as an electrolyte.

8:20 AM  
Development of an Electrochemical Model for Corrosion of 316H Stainless Steel by Molten FLiNaK Using Meso-scale YellowJacket: Chaitanya Bhave1; Michael Tonks1; Kumar Sridharan2; Guiqui Zheng3; 1University of Florida; 2University of Wisconsin-Madison; 3Massachusetts Institute of Technology
    The life-time of molten- salt- facing components in advanced reactor designs is directly impacted by corrosion. Corrosion damage in alloys is strongly concentrated along grain boundaries and results in microstructural evolution. In this work, we have developed an electrochemical phase-field model for the corrosion of 316H stainless steel by molten FLiNaK salt. The Cr depletion and microstructure evolution are driven by free energy functionals obtained from literature thermodynamic assessments. The model is validated against literature experimental corrosion mass loss measurements carried out at the University of Wisconsin-Madison. This model was implemented under the meso-scale YellowJacket project using the Multiphysics Object-Oriented Simulation Environment (MOOSE), an open-source finite-element framework.

8:40 AM  
UF₄/UF₃ Redox Control in NaF-KF-UF₄ Systems: Amanda Leong1; Jinsuo Zhang1; 1Virginia Polytechnic Institute
    Redox control is key to mitigating corrosion of structural materials in molten salt reactors (MSR). This work discusses the use of UF₄/UF₃ couple to control corrosion in MSR, where UF₃ is used to consume oxidant, thus inhibiting the dissolution of structural materials such as chromium. However, elevated UF₃ levels cause the formation of uranium carbides. Therefore, this work presents the upper and lower limits of UF₄/UF₃ in NaF-KF-UF₄ systems, dictated by the dissolution of chromium and the formation of uranium carbides. The UF₄/UF₃ concentrations are determined through a series of equilibrium tests at 600 - 750C. The upper limit is determined based on the equilibrium quotient (Keq) with CrF₂, while the formation of UC₂ determines the lower limit.

9:00 AM  
To Galvanically Corrode or Not Galvanically Corrode: Cody Falconer1; Yafei Wang1; William Doniger1; Matthew Weinstein1; Mohamed Elbakhshwan1; Kumar Sridharan1; Adrien Couet1; 1University of Wisconsin Madison
    Molten salts continue to be a complex and corrosive medium for materials in molten salt reactor designs. Protecting materials from degradation may require multiple material flavors. Mixing materials in a corrosive environment creates an even more complex environment with multiple corrosion mechanisms involved. Two corrosion mechanisms are hypothesized to affect dissimilar materials in a salt medium: galvanic coupling and activity gradient mass transport. To understand which of these mechanisms dominates, static isothermal corrosion tests and electrochemical techniques were used to study the corrosion mechanisms of dissimilar metal couples of 316L/Ni-201 and 316L/C-110 subjected to the same salt environment. Open circuit potential measurements were performed on each couple and showed similar electric potential differences while zero resistance ammeter measurements confirmed similar galvanic currents. However, isothermal corrosion testing showed galvanic current measurements did not accurately reflect the true corrosion of each couple and activity gradient mass transport dominates the behavior.

9:20 AM Break

9:40 AM  
A Novel Cyclic Voltammetry-based Automated Mini-probe for In-situ Corrosion Product Quantification during High-throughput HEA Corrosion Testing in Molten Salts: Bonita Goh1; Yafei Wang1; Will Doniger1; Phalgun Nelaturu1; Dan Thoma1; Kumar Sridharan1; Adrien Couet1; 1University of Wisconsin Madison
    The FeCrMnNi High Entropy Alloy systems promise to yield compositions possessing the set of desired properties for next-generation molten-salt-based energy systems. However, the combinatorial quarternary composition space is quasi-infinite. The well-established manufacture and corrosion-testing methodologies for identifying corrosion resistance of one single alloy are not adapted to the investigation of such compositional space. To accelerate alloy development, a high-throughput corrosion testing system is developed, from the additive manufacturing of alloys on a build plate, to the simultaneous corrosion-testing by melting individual salt droplets of ~0.3g on each alloy. A novel cyclic-voltammetry (CV)-based automated mini-probe is developed for the in-situ quantification of the dissolved corrosion product concentrations in each salt droplet. CV analysis is automated using a heuristic code to further accelerate data analysis. This integrated platform is anticipated to only take ~1 week to assess the corrosion resistance of 25 alloys with unprecedented monitoring and data acquisition capabilities.

10:00 AM  
Electrochemical Measurement of Uranium Concentration in FLiNaK: Matthew Newton1; Michael Simpson1; 1University of Utah
    Molten fluoride salt fueled reactors promise greater efficiency, improved safety, and less waste. Unfortunately, existing methods of composition monitoring are slow. Electrochemical sensors were identified as a method for monitoring real-time compositions that could improve safety and efficiency. In this study, molten salt mixtures of FLiNaK and 1-5 wt% UF4 were tested using cyclic voltammetry. The objective was to develop a method of measuring uranium concentrations in salts. This method could improve process controls and nuclear material safeguards. Experiments used tungsten electrodes and a platinum quasi-reference electrode. The working electrode’s surface area was controlled using a vertical translator. Solution resistance was measured using EIS and ICP-MS verified uranium concentrations. Cyclic voltammetry revealed reduction peaks at -0.5V and -0.75V representing the reduction of U4+ to U3+ and U3+ to U. ICP-MS was used to develop a correlation between the slope of these plots and uranium concentration. This correlation will be discussed

10:20 AM  
Redox Control as Corrosion Control Method in Molten FLiNaK: Krishna Moorthi Sankar1; Preet Singh1; 1Georgia Institute of Technology
    One of the major concerns for MSRs is the significant corrosion of structural alloys in FLiNaK. The corrosion in the form of selective dissolution of active alloying elements in these molten salts depends on the redox potential of the salt. This redox potential can be controlled by various chemical and electrochemical techniques and thereby corrosion of structural alloys in the salt can be controlled. In this study, corrosion behaviour of selected Ni and Fe based alloys were studied in molten FLiNaK at 700 oC. The effect of various chemical and electrochemical redox control techniques such as addition of reducing agents to the salt, purification of the salt, and the use of applied potential were studied to understand their ability to mitigate corrosion of selected alloys in FLiNaK. This presentation will discuss our results on the efficacy of each tested redox control method in controlling the corrosion of candidate alloys.