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

Thursday 8:30 AM
February 27, 2020
Room: Theater A-6
Location: San Diego Convention Ctr

Session Chair: Kumar Sridharan, University of Wisconsin-Madison


8:30 AM  Invited
Materials and Chemistry Research for Fluoride Cooled High Temperature Reactors: Alan Kruizenga1; Micah Hackett1; George Young1; Michael Hanson1; Augustus Merwin1; Francesco Carotti1; 1Kairos Power
    Kairos Power is developing the Fluoride-cooled High Temperature Reactor (FHR). FHR system development requires a comprehensive test and qualification program covering fuel, materials, coolant, and components. The structural material for use in the FHR is 316H, which has demonstrated low corrosion rates in the LiF-BeF2 (Flibe) primary coolant salt, provided chemistry of the salt is maintained during in-service use to reduce free fluorine. This presentation will provide an overview of the current ongoing technology development at Kairos Power and cover topics related to materials corrosion and mechanical properties tests in molten salt and analysis techniques Kairos Power materials and salt, and relevant interactions between molten salt and KP-FHR materials. The molten salt environments include both Flibe and nitrate (solar thermal) salt. Relevant test methods to be discussed include crack growth rate measurements using potential drop methods and susceptibility to stress corrosion cracking as well as corrosion behavior and the effects of chemistry control methods.

9:00 AM  
Effect of Europium Fission Product on Chromium Solubility in Molten KCl-MgCl2 Salt: Dino Sulejmanovic1; Stephen Raiman1; James Kurley1; 1Oak Ridge National Laboratory
    Molten chloride salts are potential candidates for use in advanced nuclear reactors (ANRs) as coolants and heat storage media. Compatibility of structural alloys with chloride salts is currently being investigated. Previous studies have shown that the depletion of the least noble alloy constituent (e.g. chromium) from common alloys such as SS316H is the main corrosion pathway in chlorides. Effect of fission product additives on chromium depletion has not been studied before. This presentation will highlight the effect of europium, one of the most abundant fission products in ANRs, on chromium depletion. Pure chromium specimens were exposed in purified KCl-MgCl2 salt at 700 and 800 °C in molybdenum capsules. Separately, 0.5 mol % of Eu and EuCl3 were added to the purified salt to serve as simulated source of fission product. Mass change data and the depth of chromium depletion will be evaluated with respect to the source of fission product.

9:20 AM  Student
Non-galvanic Mass Transport in Molten Fluoride Salt Isothermal Corrosion Cells: Cody Falconer1; William Doniger1; Evan Buxton1; Mohamed Elbakshwan1; Kumar Sridharan1; Adrien Couet1; 1University of Wisconsin - Madison
    Evaluation of the corrosion performance of potential structural alloys for use in the molten fluoride salt reactors has recently been performed using isothermal static corrosion environments. Simultaneous exposure of dissimilar materials (i.e. corrosion capsule differing from the corrosion sample) to molten salt has been shown to influence the corrosion behavior of both materials. Typically, preferential corrosion of a less noble material is achieved through a galvanic couple when two dissimilar materials are in direct electrical contact with one another. However, previous work shows that direct electrical contact may not be required for dissimilar materials to influence one another in a molten salt medium. The work presented here investigates the possible mechanism that drives non-galvanic mass transport in molten fluoride salts and how the mechanism can be slowed or perhaps eliminated in isothermal static corrosion environments. Corrosion exposures were performed for 1000 hours at 700 °C in molten FLiNaK.

9:40 AM  Student
First Principles Investigation of Surface Behavior in Ni-Cr Alloy in Molten Salt Systems: Jacob Startt1; Stephen Raiman2; Chaitanya Deo1; 1Georgia Insitiute Of Technology; 2Oak Ridge National Laboratory
    The segregation and eventual depletion behavior of Cr atoms in a molten salt contacting Ni-Cr alloy stands as one of the most prominent forms of corrosion observed in molten salt systems. The underlying mechanisms and driving forces behind this phenomenon are difficult to study experimentally and remain not well understood. In this work, first-principles methods are used to model the surface segregation behavior of Cr in an fcc Ni (100) surface. Segregation energies are calculated for a migrating Cr atom in each of the top five surface layers for a clean surface, and then for surfaces with a salt specie or impurity adsorbed above the Cr atom (Cl, F, O, H-Cl, H-F, H-O). An in-depth analysis of role of charge transfer in these processes will be discussed. This talk will present new insights into the complex relationship between the near-surface Cr atoms and salts components in molten salt reactor systems.

10:00 AM  
Advanced Characterization of Corrosion Behavior of Metals in Molten Chloride Salts: Lingfeng He1; Arthur Ronne2; Simerjeet Gill3; Kotaro Sasaki3; Yi Xie1; Yachun Wang1; Phillip Halstenberg4; Dmitriy Dolzhnikov4; Yu-chen Karen Chen-Wiegart2; Shannon Mahurin4; 1Idaho National Laboratory; 2Stony Brook University; 3Brookhaven National Laboratory; 4Oak Ridge National Laboratory
     Molten Salt Reactors (MSRs) are leading candidates for next-generation nuclear reactors to contribute to the U.S. energy supply. One of the most critical challenges in the fruition of MSRs is the corrosion of structural materials. To comprehend the corrosion behavior of commercial alloys and develop a corrosion mitigation strategy in the MSR environment, it is of significance to understand the basic corrosion behavior of model metals and alloys in molten salts. Scanning transmission electron microscopy equipped with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy systems and synchrotron-based techniques including transmission X-ray microscopy, extended X-ray absorption fine structure and X-ray absorption near edge structure are used to characterize the microstructure of Ni and Ni-20Cr model alloys corroded in molten chloride salts. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science.

10:20 AM Break

10:40 AM  Student
Improving the Corrosion Resistance of Ferritic-martensitic Steels in Molten Nitrate Salt via Diffusion Coatings for Concentrated Solar Power Applications: Ceyhun Oskay1; Tobias Meißner1; Benjamin Grégoire1; Alexander Bonk2; Mathias Galetz1; 1DECHEMA-Forschungsinstitut; 2German Aerospace Center (DLR)
     Concentrated solar power (CSP) systems are qualified to be one of the most promising renewable energy technologies due to the possibility of energy storage by heat transfer fluids (HTF). Owing to their beneficial thermophysical properties, molten nitrate salts are employed as the state-of-the-art HTF in CSP systems. The utilization of molten nitrate salts is accompanied by a higher corrosion rate and thus limits the material selection to Ni-based alloys due to their high corrosion resistance. In the interest of increasing the cost-efficiency of CSP systems, diffusion coatings can be applied on ferritic-martensitic steels to improve their corrosion resistance and thereby enable their employment.In this study, different types of diffusion coatings such as aluminizing and chromizing were applied on ferritic-martensitic steels e.g. P91 using pack-cementation method. Subsequently, their corrosion behavior in molten solar salt was investigated at temperatures within the range of 560°C-600°C and compared to that of Ni-based alloys.

11:00 AM  
First Steps Towards Predicting the Corrosion Rates of Structural Materials in Molten Salts: Rishi Pillai1; Stephen Raiman1; Bruce Pint1; 1Oak Ridge National Laboratory
     The selection of optimum containment/non-containment materials for molten salt systems is largely based on extensive and costly experimental evaluation with no gain in mechanistic understanding of the corrosion processes. The latter consequently means that the salt/material compatibility can only be verified for the investigated salt chemistries /material combinations and test conditions. There is an imminent need for physics-based models to predict corrosion behavior of materials as a function of salt chemistry, time and temperature.The aim of the present work was to evaluate the applicability of coupled thermodynamic-kinetic modelling to predict corrosion rates of commonly employed structural materials (ferritic, austenitic steels and Ni-base alloys). Element concentrations and phase distributions were obtained by scanning electron microscopy (SEM/EPMA). Phases were identified by energy/wavelength dispersive X-ray spectroscopy (EDX/WDX) and electron backscatter diffraction (EBSD). The modelling results were validated with experimental data in terms of depth of corrosive attack, phase transformations and Cr-depletion.

11:20 AM  
Mechanistic Understanding of Molten Salt Corrosion Using Electron Paramagnetic Resonance: James Kurley1; Juho Lehmusto2; Dino Sulejmanovic1; James Keiser1; Stephen Raiman1; 1Oak Ridge National Laboratory; 2Abo Akademi University
    Mechanistic understanding of molten salt corrosion has focused on characterizing the alloy post-corrosion with minimal characterization of the salt. Previous studies quantified Cr concentration in the salt using optical emission spectroscopy or mass spectrometry, but those techniques only measure concentrations without giving any information on structure. X-ray absorption techniques can reveal structure but require the high-flux capabilities of a synchrotron. The present study seeks to complement previous research by characterizing rapidly quenched salt with electron paramagnetic resonance (EPR) to elucidate corrosion product structure within the salt. Chromium speciation was studied by dissolving CrOx, CrCly, and Cr metal into NaCl:MgCl2 (58:42) and KCl:MgCl2 (68:32) to determine baseline EPR spectra of a variety oxidation states and coordination environments. The EPR spectra were measured between 150 and 350K to reveal subtle differences between structures. The baseline spectra were compared to salt samples after corrosion experiments of both commercial and model alloys.