Materials and Chemistry for Molten Salt Systems: Corrosion I
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

Wednesday 8:30 AM
February 26, 2020
Room: Theater A-6
Location: San Diego Convention Ctr

Session Chair: Michael Short, Massachusetts Institute of Technology


8:30 AM  Invited
The Department of Energy Molten Salt Reactor Development Overview: Lou Qualls1; 1Oak Ridge National Laboratory
    Molten salt reactors (MSR) have the potential to provide affordable, clean power, with passive safety and play a significant role in the closing of the nuclear fuel cycle. A resurgence in interest in MSRs is now being led by a number of private industry developers. A Molten Salt Reactor was successfully built and operated at Oak Ridge National Laboratory in the 1960s, demonstrating much of the needed technology. However, many technical needs required for commercial deployment remain undemonstrated. Specifically, the molten salt environment is challenging to materials designers and reactor chemists and due to the diversity of molten-salt reactor concepts being proposed, there are many salt and material combinations that require study. The Department of Energy Molten Salt Reactor Campaign is working with reactor developers to bring a molten salt reactors to market by conducting fundamental and applied research. This talk will present an overview of MSR development challenges and activities. The current state of materials and chemistry research for MSRs will be discussed and a window into future developments will be offered.

9:00 AM  Student
Investigation of the Mechanism Behind Proton Irradiation Decelerated Corrosion in Molten Fluorides: Weiyue Zhou1; Yang Yang2; Lingfeng He3; Andrew Minor2; Michael Short1; 1Massachusetts Institute of Technology; 2Lawrence Berkeley National Laboratory; 3Idaho National Laboratory
    Corrosion in molten fluoride salts proceeds mainly via selective dealloying of Cr from the alloys into the salts. For MSR application, it is important to understand how radiation damage influences this process. To accomplish this, a simultaneous proton irradiation/corrosion facility is constructed at MIT. By firing a high energy proton beam at the center of the sample, direct comparisons between corrosion with and without radiation can be made on one sample, with the proton irradiation as the only difference. Results of Ni-20Cr model alloys in molten fluorides show a deceleration of intergranular corrosion under the proton irradiation. Quantitative representation of the SEM cross-section results are performed to support the qualitative-evident conclusion and provides a deceleration factor related to the dose rate. The possible mechanism behind the observed phenomenon will be illustrated, focusing on the altered diffusion kinetics under the irradiation with TEM results supporting it.

9:20 AM  
Effect of Salt Additives and Impurities on Flowing Commercial Chloride Salt Compatibility: Bruce Pint1; Dino Sulejmanovic1; J Kurley1; 1Oak Ridge National Laboratory
    The next generation of concentrating solar power (CSP) plants at >700°C will require thermal storage such as chloride salts. Once reasonable compatibility is demonstrated in isothermal conditions, flowing tests in a temperature gradient are needed to truly demonstrate that mass transfer is not a concern. Previously, highly purified, commercial MgCl2-KCl-NaCl+0.04%Mg was tested in an alloy 600 (Ni-16Cr-9Fe) thermal convection loop (TCL) with peak temperatures of 700°C for 1000h and a second experiment at 750°C for 110h, both with a temperature gradient of >100°C. Acceptable corrosion rates were measured for alloy 600 and C276 (Ni-16Cr-15Mo) specimens. The next phase of the work will conduct a similar alloy 600 TCL experiment with heat treated commercial salt, perhaps with a higher Mg content, to understand how impurities and additives may affect compatibility. This work was funded by the U.S. Department of Energy Solar Energy Technologies Office under Award Number 33873.

9:40 AM  
Effects of Ionizing Radiation on Molten Chloride Salts: Ruchi Gakhar1; Gregory Horne1; Jay LaVerne2; James Wishart3; Simon Pimblott1; 1Idaho National Laboratory; 2University of Notre Dame; 3Brookhaven National Laboratory
    Molten salts, like other matter, are ionized upon irradiation and transient reactive species are generated. Steady-state and pulse radiolysis techniques have been to understand the radiation-induced chemistry of molten chloride salt systems and develop a predictive radiation chemical kinetics model. The LEAF picosecond pulse radiolysis facility has been used to observe the polyhalide ions, Cl2- and Cl3-, in molten ZnCl2. The transients are long-lived and likely to react even with dilute solutes. A series of studies of increasing complexity have been initiated to understand the primary species produced in the radiolysis of pure molten chloride salts and then measure their reactivity with the most important solutes expected in molten salt reactors. These experiments will be complemented by steady-state gamma and He ion radiolysis. Coupling steady-state measurements and time-resolved reaction kinetics will allow for the deployment of multi-scale modeling methodologies for these systems, providing quantitative mechanistic insight and predictive capabilities.

10:00 AM  
Europium Induced Alloy Corrosion and Cracking in Molten Chloride Salt: Shaoqiang Guo1; Jinsuo Zhang1; 1Virginia Polytechnic Institute and State University
    Molten chloride salts can be corrosive to the structural alloys used in nuclear applications where they have been considered as, for example, advanced coolants for molten salt fast reactors, and the electrolyte for reprocessing of spent nuclear fuels. This paper investigated the corrosion of Alloy 709 and Inconel 718 in KCl-LiCl-EuCl3 molten salt under Ar atmosphere at 500°C. Fission product europium was discovered to be very corrosive in molten chlorides. Cyclic voltammetry (CV) and inductively coupled plasma mass spectrometry (ICP-MS) analysis proved significant release of ionic nickel and iron products in the melt. Solid corrosion product powders that consists of metallic nickel and oxides of Cr2O3, FeCr2O4, and NiO were found in the salt solution. Scanning electron microscope (SEM) observed a deposited Ni layer on the alloy surface due to the reduction of Ni(II) in the later stage when Eu(III) was depleted.

10:20 AM Break

10:40 AM  
Salt Processing for Corrosion Mitigation in Concentrated Solar Power Systems: Jicheng Guo1; Nathaniel Hoyt1; Mark Williamson1; 1Argonne National Laboratory
    Molten MgCl2-KCl-NaCl mixtures are promising heat transfer fluid (HTF) candidates for next generation concentrating solar power systems (Gen3 CSP), because of their low cost, high decomposition temperature, and favorable heat transfer characteristics. However, if not properly processed, the impurities in molten chloride salts can corrode high-temperature metal alloys proposed for use in CSP systems. Here we demonstrate salt processing and purification techniques to eliminate the impurities and control the molten salt redox potential. During the purification process, the presence of corrosive impurities as well as the redox potential of molten MgCl2-KCl-NaCl mixtures is continuously monitored using a multifunctional voltammetry sensor that provides endpoint determination for the process. Alloy samples such as Inconel exposed to the purified salts are compared with those exposed to the unprocessed salts. The results show that the corrosion of the metal alloy by the molten chloride is effectively mitigated using the developed salt processing procedure.

11:00 AM  
Performance of Corrosion Resistant Claddings on 316H Stainless Steel in Molten Fluoride Salt: Adrien Couet1; William Doniger1; Cody Falconer1; Evan Buxton1; Mohamed Elbakhshwan1; Chuan Zhang2; Cem Topbasi3; Kumar Sridharan1; 1University of Wisconsin-Madison; 2Computherm, LLC; 3Electric Power Research Institute
    In order to facilitate the short-term design and construction of Molten Salt Reactors, cladding systems have been proposed for codified alloys as a methodology to mitigate the effects of corrosion and extend the lifetime of structural materials. This work focuses on the corrosion performance in molten fluoride salt of cladding systems on 316H stainless steel in a static isothermal corrosion cells. These systems include nickel-weld overlay, nickel electroplating, and molybdenum hot isostatic press and, diode laser cladding diode laser cladding processes. Corrosion was performed for 1000 hours at 700°C in purified molten FLiNaK salt. In addition, thermal aging tests were performed in parallel in an inert atmosphere. The pre- and post- corrosion cladding and cladding/substrate interface were characterized using SEM, EDS, XRD, and micro hardness techniques. Experimental results of interface diffusion were compared to the ones obtained from CALPHAD simulations in order to predict cladding degradation at longer exposure times.

11:20 AM  Student
Molten Chloride Salt Corrosion in Ni based Vs Fe based Alloys: Brendan Dsouza1; Weiqian Zhuo1; Jinsuo Zhang1; 1Virginia Polytechnic Institute and State University
    Molten chloride salt is considered as a potential candidate for the next generation Molten Salt Reactors (MSR), however, they pose grave challenges due to their corrosive nature at higher temperatures (+600°C) where the mechanism is an impurity driven process. In this study, appropriate thermal and chemical purification processes have been developed for the NaCl-KCl- MgCl2 (5.1-48.1-46.8 mol%) salt to remove the oxygen and moisture bearing impurities. Static corrosion tests were conducted for Ni based (H-230 and C-276) and Fe based (709-4B2 and SS316/L) alloys using the purified salt at 800°C for 100 hours in an inert Ar gas environment. The alloys exhibited extensive pitting with severe Cr depletion near the surface. The corrosion rates were found to be in the increasing order of SS316/L > 709-4B2 > H-230 > C276. Higher Mo content in the matrix has shown considerable increase in resistance of the alloy against chloride salt attack.

11:40 AM  
Controlling Alloy Corrosion in Molten Chloride Salts: What Matters and What Doesn’t?: Stephen Raiman1; Dino Sulejmanovic1; Bruce Pint1; Jake McMurray1; Richard Mayes1; Kristian Myhre1; Matt Kurley1; William Ponder2; Jacob Startt3; 1Oak Ridge National Laboratory; 2University of Tennessee; 3Georgia Institute of Technology
     To minimize degradation of salt-facing components, it is important to understand the material and chemical factors that affect corrosion. Dating back to the MSRE, efforts toward controlling corrosion in molten fluoride salts focused on careful alloy selection, thorough salt purification, and additives for redox control. Recent work in chloride salts has revealed new insights into the behavior of chloride-salt-facing alloys. This work highlights recent findings showing how controlling salt chemistry affects corrosion in chloride salts. New developments in salt purification and redox control additives are shown to have direct and measurable effects on corrosion. Model alloys are used to isolate the effects of alloy composition and microstructure. Spectroscopic techniques are used in conjunction with traditional characterization to measure new salt properties.This work presents new progress toward understanding what matters most when controlling corrosion in nuclear and concentrating solar systems that use chloride salts coolants.