Materials and Chemistry for Molten Salt Systems: Mechanisms of Corrosion in Molten Salt
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; Michael Short, Massachusetts Institute of Technology; Kumar Sridharan, University of Wisconsin-Madison; Nathaniel Hoyt, Argonne National Laboratory
Tuesday 2:30 PM
March 21, 2023
Room: 27A
Location: SDCC
Session Chair: Raluca Scarlat, University of California, Berkeley
2:30 PM
Ab-initio Molecular Dynamics Examination of Critical Steps for Molten Salt Corrosion of Stainless Steels: Anton Schneider1; Lin-Chieh Yu1; Yongfeng Zhang1; 1University of Wisconsin Madison
Molten salts open new horizons for future nuclear technologies but also bring new challenges by corroding structural alloys. The corrosion is induced by impurities such as oxygen or moisture. The proposed mechanism for moisture induced corrosion involves dissociation of water molecules and formation of hydrogen chloride, which then oxidize metal atoms. Using ab-initio molecular dynamics (AIMD), we examined several critical steps in the proposed mechanism, including water dissociation and diffusion, and its reaction with metal atoms in FeCr facing a NaCl salt. The results showed that a water molecule did not dissociate from 1100K to 1600K, rather it diffused to the salt-metal interface, leading to direct formation of chromium oxide. In contrast, separately introduced OH- and H+ ions immediately recombined into a water molecule. Albeit the short AIMD simulation time, the results indicate that early-stage moisture induced molten salt corrosion may involve direct oxidation of Cr by water molecules.
2:50 PM
Atomistic Kinetics Monte Carlo Simulations of Molten Salt Corrosion of Ni-Cr Alloys: Lin-Chieh Yu1; Anton Schneider1; Yongfeng Zhang1; 1University of Wisconsin-Madison
Molten salt corrosion is a critical material challenge hindering the deployment of molten salt reactors (MSRs). The corrosion process, i.e., selective dissolution of alloying elements such as Cr, involves multiple kinetic processes including diffusion of oxidizing impurities, e.g., water and/or oxygen, electrochemical surface reaction, and surface and bulk diffusion in alloys. Such a complex dependence on various factors leads to scattered experimental data with ambiguous governing mechanisms. Here, we present an atomic kinetic Monte Carlo model for molten salt corrosion considering the above processes. By applying the model to Ni-Cr alloys, the variation in corrosion rate and morphology, including 3D nanoporous structure in purposeful dealloying and surface dissolution in degrading corrosion, is explained by the interplay between diffusion processes and electrochemical reaction and the effect of composition. Such a model is powerful for identifying the rate-limiting step in various conditions and aiding the design of advanced alloys for MSRs.
3:10 PM
Solid-Solution Alloying Effects in Molten Chloride Salt Corrosion: Touraj Ghaznavi1; Suraj Persaud2; Roger Newman1; 1University of Toronto; 2Queen's University
A comprehensive study of solid-solution alloying effects in any kind of corrosion requires the use of model alloys prepared in a high-throughput melter. We used an Arcast induction melter to prepare Ni-Fe(Cr) alloys for corrosion studies in a molten K-Li-Mg eutectic. After annealing, they were subjected to a variety of electrochemical corrosion procedures, using Mg as reference electrode. The new findings include a lower than usual parting limit for dealloying, due to the very high surface mobility of the most noble element (Ni). This limit was decreased still further when the temperature was increased to the point that lattice diffusion started to contribute to the transport of alloying elements. At lower temperatures such as 350 °C, the microporous dealloying morphology resembled that seen in aqueous systems (but much coarser), while at temperatures such as 600 °C, a more “1-D” type of corrosion occurred, but still respected a parting limit.
3:30 PM
Corrosion and Dealloying Phenomena of Binary Ni-Cr Alloys in Molten FLiNaK Salts: Ho Lun Chan1; Elena Romanovskaia1; Valentin Romanovski1; Minsung Hong2; Peter Hosemann2; John Scully1; 1University of Virginia; 2University of California Berkeley
In molten fluoride salts, the preferential leaching of chromium or dealloying through grain boundary migration is a prevalent phenomenon that has been observed in many materials system including Ni-Cr alloys. Dealloying was primarily discussed in the context of microstructural attributes; and post-mortem characterization analyses. For instances, the selective leaching of alloying elements, corrosion regime, surface and bulk diffusion kinetics and coarsening of dealloyed layers are all dependent upon electrochemical potential, physical conditions and the environment which regulate these diverse processes such as element oxidation relative to Nernst potentials, surface diffusion and the resulting corrosion morphology. In this work, electrochemical dealloying of binary Ni-Cr alloys (Ni5Cr, Ni10Cr, Ni20Cr, wt% or at%) was studied in molten LiF-NaF-KF eutectic salt at 600°C. A series of electrochemical methods, including linear sweep voltammetry and chronopotentiometry , were performed on these materials in FLiNaK salts with and without the controlled additions of NiF2 and CrF3 salts.
3:50 PM
4D-STEM/EDS Characterization of Molten Salt Corrosion in NiCr Alloy: Sean Mills1; Ryan Hayes1; Steven Zeltmann1; Raluca Scarlat1; Andrew Minor1; 1University of California-Berkeley
Structural metals used in nuclear reactor environments are exposed to coupled extremes such as irradiation, high temperature, and corrosion which act synergistically to degrade material performance. These processes are directly limited or accelerated by nanoscale point defects such as vacancies, interstitials, and voids. Quantifiable methods of elucidating formation of point defects and their role in diffusion kinetics and material failure, have been limited to bulk or ex-situ measurements. Development of 4D-STEM via high-speed direct electron detectors provides opportunities to map distributions of nanoscale features. Here, we examine the exposure of NiCr alloy to LiF-NaF-KF eutectic salts (FLiNaK) by measuring point defects (with 4D-STEM) and Cr migration (with EDS-STEM) at the metal-salt interface. Measurements of corrosion defects at high spatial resolution advances our understanding of complex corrosion environments, thereby providing a pathway for engineering materials designed in future nuclear energy systems.
4:10 PM Break
4:25 PM
Understanding the Mechanisms of Corrosion in 316 Stainless Steels and Advanced Ni-based Alloys in Molten FLiNaK at 700oC: William Doniger1; Govindarajan Muralidharan2; Adrien Couet1; Kumar Sridharan1; 1University of Wisconsin-Madison; 2Oak Ridge National Laboratory
The corrosion behavior of 316H and 316L stainless steels and computationally designed advanced Ni-based alloys with creep strength superior to Hastelloy-N has been investigated in FLiNaK at 700oC. The higher degree of sensitization in 316H compared to 316L on account of higher carbon content was beneficial for corrosion resistance. In both types of stainless steels, surface recession and preferential grain boundary attack were observed. For the Ni-based alloys surface recession was minimal and corrosion and depletion of alloying elements occurred from within the grains, and weight loss was notably lower than for the stainless steels. Analysis of salt composition before and after corrosion tests, microstructural and compositional analysis of corrosion effected near-surface region, and weight change measurements provided a comprehensive understanding of corrosion mechanisms in these candidate structural alloys. Electrochemical polarization tests provided a scientific basis for understanding the mechanisms for preferential grain boundary attack versus uniform grain corrosion.
4:45 PM
Electrochemical Corrosion Testing in Molten Fluoride (FLiNaK) Salts: Matthew Lawson1; Drew Glenna1; Haiyan Zhao1; 1University of Idaho
As the demand for electrical power constantly rises, which is further exacerbated by the need to reduce the carbon footprint of current technologies, nuclear power is a proven solution to the problem. Although current nuclear power technology currently leaves behind legacy waste, next generation reactors (Gen-IV) utilize molten salts as reactor coolant to provide a much cleaner energy source. Although the higher operating temperature of the molten salt reactors (MSR) allows for a higher output of electrical energy, one issue stands in the way of moving forward with MSR’s: corrosion. Since molten salts are highly corrosive to the commonly utilized iron-based structural materials, new structural materials are being investigated. Nickel based alloys show significantly higher corrosion resistance to molten salts, so this paper investigates the corrosion rates of Inconel 600 and compares to pure nickel using Linear Polarization Resistance, Tafel, and Electrochemical Impedance Spectroscopy.
5:05 PM
Accelerated Corrosion of Nickel-Chromium by Europium Trifluoride in FLiNaK: Ryan Hayes1; Sean Mills1; Andrew Minor1; Raluca Scarlat1; 1University of California Berkeley
Nickel based alloys have been proposed as strong candidate materials for generation IV reactor concepts. The Molten Salt Reactor Experiment demonstrated that their high-temperature stability and corrosion resistance are well suited for the high-neutron flux environment within a salt-cooled nuclear reactor. While these nickel alloys have shown resistance to corrosion in fluoride salts compared to stainless steels, the mechanism by which they are corroded is not well understood. Elucidating this mechanism is made difficult by the many alloying elements often contained in these materials. As such, two samples of a simpler binary alloy of nickel and chromium (80:20 wt%) were exposed to FLiNaK loaded with EuF3 for 4 hours at 600 °C. In varying the amount of oxidizing europium fluoride in the melt, a comparison of corrosion depth between samples allows for an insight on the roles of diffusion and kinetics in corrosion of more complex nickel alloys.
5:25 PM
Systematic Corrosion Model for Non-isothermal Molten Salt Loop: Jinsuo Zhang1; 1Virginia Polytechnic Institute and State University
Experiments have shown that corrosion and precipitation can occur in non-isothermal molten salt loops. However, there is no corrosion models to predict the corrosion and precipitation rate and the relationship between corrosion and precipitation. In the present study, we will develop a systematic model that will take into the systematic parameters such as the salt flow rate and the loop temperature profile. The model has the capability to predict corrosion and precipitation distribution along the loop as well as their rates. Considering that local parameters such as local electrochemical reactions and temperature are also important, the systematic model will be integrated with the local corrosion model. The model will also consider redox potential control and its influence on the corrosion/precipitation profiles along the non-isothermal loop.
5:45 PM
Assessing Environmentally-Assisted Cracking of 316L in Molten FLiNaK: Xavier Quintana1; Dustin Mangus1; Jake Quincey1; Julie Tucker1; Samuel Briggs1; 1Oregon State University
Fluoride molten salts have been targeted as a promising coolant for next generation reactors due to their favorable thermophysical properties, but present unique materials challenges primarily driven by the aggressive corrosion attack due to the reducing environment. Stress present in structural materials exacerbate degradation. Challenges with in-situ data collection mean data for this system is sparse and the degree of which stress accelerates degradation is not fully realized. Oregon State University’s Molten Salt Load Frame (MSLF) enables environmentally assisted cracking (EAC) tests to be conducted via slow strain rate and fatigue crack growth tests. Tests are conducted with 316L stainless steel. Samples are immersed in flowing, convection driven LiF-NaF-KF eutectic (FLiNaK) simulating MSR-like conditions and are subject to variable fatigue or constant strain rate until failure. Pre- and post-immersion characterization and fractography is conducted to identify failure modes and evaluate the effects of molten salt on 316L.