Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy

Unraveling Impurity-Dependent Morphological and Chemical Evolution of Ni-20Cr Alloy in Eutectic LiCl-KCl Molten Salt

Understanding the interfacial evolution of alloys in molten salt with different amounts of water (H2O) and oxygen (O2) impurities is significant for applications in many fields, including concentrated solar power, molten salt reactors, and applications in pyrochemical reprocessing and electrorefining. Additionally, the impurity-driven corrosion mechanisms that lead to various morphological and chemical evolution characteristics at the interfaces of structural alloys and molten salts are not fully understood. In the present work, the three-dimensional (3D) morphological evolution of Ni-20Cr microwires in LiCl-KCl was studied at 500 °C under different moisture and oxygen conditions using in situ synchrotron transmission X-ray microscopy (TXM) and scanning transmission electron microscopy (STEM) techniques. No significant morphological changes were observed in Ni-20Cr microwires under vacuum conditions. However, the wires exhibited distinct morphological evolutions when exposed to molten salt containing H2O alone, as well as when both H2O and O2 were present. Furthermore, Cr2O3 precipitates were observed in the molten salt during corrosion with only H2O present, while Cr6+ species were identified in the salt when O2 was added. These findings are crucial for understanding the corrosion mechanisms of molten salt with different amounts of H2O and O2 contamination, providing insights for developing corrosion mitigation methods and improving the stability of containment alloys in molten salt applications.

Evaluation of the Wear of Ni 200 Alloy After Long-Term Carbon Capture in Molten Salts Process

Reducing CO2 emissions is one of the major challenges facing the modern world. The overall goal is to limit global warming and prevent catastrophic climate change. One of the many methods for reducing carbon dioxide emissions involves capturing, utilizing, and storing it at the source. The Carbon Capture in Molten Salts (CCMS) technique is considered potentially attractive and promising, although it has so far only been tested at the laboratory scale. This study evaluates the wear of the main structural components of a prototype for CO2 capture in molten salts-a device designed and tested in the laboratories of AGH University of Kraków. The evaluation focused on a gas barbotage lance and a reactor chamber (made from Nickel 200 Alloy), which were in continuous, long-term (800 h) contact with molten salts CaCl2-CaF2-CaO-CaCO3 at temperatures of 700-940 °C in an atmosphere of N2-CO2. The research used light microscopy, SEM, X-ray, computed tomography (CT), and 3D scanning. The results indicate the greatest wear on the part of the lance submerged in the molten salts (3.9 mm/year). The most likely wear mechanism involves grain growth and intergranular corrosion. Nickel reactions with the aggressive salt environment and its components cannot be ruled out. Additionally, the applied research methods enabled the identification of material discontinuities in the reactor chamber (mainly in welded areas), pitting on its surface, and uneven wear in different zones.

Elucidating the Transition of 3D Morphological Evolution of Binary Alloys in Molten Salts with Metal Ion Additives

Molten salts serve as effective high-temperature heat transfer fluids and thermal storage media used in a wide range of energy generation and storage facilities, including concentrated solar power plants, molten salt reactors and high-temperature batteries. However, at the salt-metal interfaces, a complex interplay of charge-transfer reactions involving various metal ions, generated either as fission products or through corrosion of structural materials, takes place. Simultaneously, there is a mass transport of ions or atoms within the molten salt and the parent alloys. The precise physical and chemical mechanisms leading to the diverse morphological changes in these materials remain unclear. To address this knowledge gap, this work employed a combination of synchrotron X-ray nanotomography and electron microscopy to study the morphological and chemical evolution of Ni-20Cr in molten KCl-MgCl2, while considering the influence of metal ions (Ni2+, Ce3+, and Eu3+) and variations in salt composition. Our research suggests that the interplay between interfacial diffusivity and reactivity determines the morphological evolution. The summary of the associated mass transport and reaction processes presented in this work is a step forward toward achieving a fundamental comprehension of the interactions between molten salts and alloys. Overall, the findings offer valuable insights for predicting the diverse chemical and structural alterations experienced by alloys in molten salt environments, thus aiding in the development of protective strategies for future applications involving molten salts.

Exploring Cr and molten salt interfacial interactions for molten salt applications

Molten salts play an important role in various energy-related applications such as high-temperature heat transfer fluids and reaction media. However, the extreme molten salt environment causes the degradation of materials, raising safety and sustainability challenges. A fundamental understanding of material-molten salt interfacial evolution is needed. This work studies the transformation of metallic Cr in molten 50/50 mol% KCl-MgCl2via multi-modal in situ synchrotron X-ray nano-tomography, diffraction and spectroscopy combined with density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Notably, in addition to the dissolution of Cr in the molten salt to form porous structures, a δ-A15 Cr phase was found to gradually form as a result of the metal-salt interaction. This phase change of Cr is associated with a change in the coordination environment of Cr at the interface. DFT and AIMD simulations provide a basis for understanding the enhanced stability of δ-A15 Cr vs. bcc Cr, by revealing their competitive phase thermodynamics at elevated temperatures and probing the interfacial behavior of the molten salt at relevant facets. This study provides critical insights into the morphological and chemical evolution of metal-molten salt interfaces. The combination of multimodal synchrotron analysis and atomic simulation also offers an opportunity to explore a broader range of systems critical to energy applications.

Radionuclide tracing based in situ corrosion and mass transport monitoring of 316L stainless steel in a molten salt closed loop

In the study, we report an in situ corrosion and mass transport monitoring method developed using a radionuclide tracing technique for the corrosion study of 316L stainless steel (316L SS) in a NaCl-MgCl2 eutectic molten salt natural circulation loop. This method involves cyclotron irradiation of a small tube section with 16 MeV protons, later welds at the hot leg of the molten salt flow loop, generating radionuclides 51Cr, 52Mn, and 56Co at the salt-alloy interface. By measuring the activity variations of these radionuclides at different sections along the loop, both the in situ monitoring of the corrosion attack depth of 316L SS and corrosion product transport and its precipitation in flowing NaCl-MgCl2 molten salt are achieved. While 316L SS is the focus of this study, the technique reported herein can be extended to other structural materials being used in a wide range of industrial applications.

In-Situ Analysis of Corrosion Products in Molten Salt: X-ray Absorption Reveals Both Ionic and Metallic Species

Understanding and controlling the chemical processes between molten salts and alloys is vital for the safe operation of molten-salt nuclear reactors. Corrosion processes in molten salts are highly dependent on the redox potential of the solution that changes with the presence of fission and corrosion processes, and as such, reactor designers develop electrochemical methods to monitor the salt. However, electrochemical techniques rely on the deconvolution of broad peaks, a process that may be imprecise in the presence of multiple species that emerge during reactor operation. Here, we describe in situ measurements of the concentration and chemical state of corrosion products in molten FLiNaK (eutectic mixture of LiF-NaK-KF) by high-resolution X-ray absorption spectroscopy. We placed a NiCr foil in molten FLiNaK and found the presence of both Ni²⁺ ions and metallic Ni in the melt, which we attribute to the foil disintegration due to Cr dealloying.

Temperature-Dependent Morphological Evolution during Corrosion of the Ni-20Cr Alloy in Molten Salt Revealed by Multiscale Imaging

Understanding the mechanisms leading to the degradation of alloys in molten salts at elevated temperatures is significant for developing several key energy generation and storage technologies, including concentrated solar and next-generation nuclear power plants. Specifically, the fundamental mechanisms of different types of corrosion leading to various morphological evolution characteristics for changing reaction conditions between the molten salt and alloy remain unclear. In this work, the three-dimensional (3D) morphological evolution of Ni-20Cr in KCl-MgCl₂ is studied at 600 °C by combining in situ synchrotron X-ray and electron microscopy techniques. By further comparing different morphology evolution characteristics in the temperature range of 500-800 °C, the relative rates between diffusion and reaction at the salt-metal interface lead to different morphological evolution pathways, including intergranular corrosion and percolation dealloying. In this work, the temperature-dependent mechanisms of the interactions between metals and molten salts are discussed, providing insights for predicting molten salt corrosion in real-world applications.

Effect of Temperature and Impurity Content to Control Corrosion of 316 Stainless Steel in Molten KCl-MgCl2 Salt

The corrosion resistance of 316 stainless steel (316SS) in molten KCl-MgCl₂ salts was studied through static immersion corrosion at high temperatures. Below 600 °C, the corrosion rate of 316SS increased slowly with increasing temperature. When the salt temperature rises to 700 °C, the corrosion rate of 316SS increases dramatically. The corrosion of 316SS is mainly due to the selective dissolution of Cr and Fe at high temperatures. The impurities in molten KCl-MgCl₂ salts could accelerate the dissolution of Cr and Fe atoms in the grain boundary of 316SS, and purification treatment can reduce the corrosivity of KCl-MgCl₂ salts. Under the experimental conditions, the diffusion rate of Cr/Fe in 316SS changed more with temperature than the reaction rate of salt impurities with Cr/Fe.

One dimensional wormhole corrosion in metals

Corrosion is a ubiquitous failure mode of materials. Often, the progression of localized corrosion is accompanied by the evolution of porosity in materials previously reported to be either three-dimensional or two-dimensional. However, using new tools and analysis techniques, we have realized that a more localized form of corrosion, which we call 1D wormhole corrosion, has previously been miscategorized in some situations. Using electron tomography, we show multiple examples of this 1D and percolating morphology. To understand the origin of this mechanism in a Ni-Cr alloy corroded by molten salt, we combined energy-filtered four-dimensional scanning transmission electron microscopy and ab initio density functional theory calculations to develop a vacancy mapping method with nanometer-resolution, identifying a remarkably high vacancy concentration in the diffusion-induced grain boundary migration zone, up to 100 times the equilibrium value at the melting point. Deciphering the origins of 1D corrosion is an important step towards designing structural materials with enhanced corrosion resistance.

Evolution of micro-pores in Ni-Cr alloys via molten salt dealloying

Porous materials with high specific surface area, high porosity, and high electrical conductivity are promising materials for functional applications, including catalysis, sensing, and energy storage. Molten salt dealloying was recently demonstrated in microwires as an alternative method to fabricate porous structures. The method takes advantage of the selective dissolution process introduced by impurities often observed in molten salt corrosion. This work further investigates molten salt dealloying in bulk Ni-20Cr alloy in both KCl-MgCl2 and KCl-NaCl salts at 700 ℃, using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD), as well as synchrotron X-ray nano-tomography. Micro-sized pores with irregular shapes and sizes ranging from sub-micron to several microns and ligaments formed during the process, while the molten salt dealloying was found to progress several microns into the bulk materials within 1-16 h, a relatively short reaction time, enhancing the practicality of using the method for synthesis. The ligament size increased from ~ 0.7 μm to ~ 1.3 μm in KCl-MgCl2 from 1 to 16 h due to coarsening, while remaining ~ 0.4 μm in KCl-NaCl during 16 h of exposure. The XRD analysis shows that the corrosion occurred primarily near the surface of the bulk sample, and Cr2O3 was identified as a corrosion product when the reaction was conducted in an air environment (controlled amount sealed in capillaries); thus surface oxides are likely to slow the morphological coarsening rate by hindering the surface diffusion in the dealloyed structure. 3D-connected pores and grain boundary corrosion were visualized by synchrotron X-ray nano-tomography. This study provides insights into the morphological and chemical evolution of molten salt dealloying in bulk materials, with a connection to molten salt corrosion concerns in the design of next-generation nuclear and solar energy power plants.

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids

High-temperature molten salt research is undergoing somewhat of a renaissance these days due to the apparent advantage of these systems in areas related to clean and sustainable energy harvesting and transfer. In many ways, this is a mature field with decades if not already a century of outstanding work devoted to it. Yet, much of this work was done with pioneering experimental and computational setups that lack the current day capabilities of synchrotrons and high-performance-computing systems resulting in deeply entrenched results in the literature that when carefully inspected may require revision. Yet, in other cases, access to isotopically substituted ions make those pioneering studies very unique and prohibitively expensive to carry out nowadays. There are many review articles on molten salts, some of them cited in this perspective, that are simply outstanding and we dare not try to outdo those. Instead, having worked for almost a couple of decades already on their low-temperature relatives, the ionic liquids, this is the perspective article that some of the authors would have wanted to read when embarking on their research journey on high-temperature molten salts. We hope that this will serve as a simple guide to those expanding from research on ionic liquids to molten salts and vice versa, particularly, when looking into their bulk structural features. The article does not aim at being comprehensive but instead focuses on selected topics such as short- and intermediate-range order, the constraints on force field requirements, and other details that make the high- and low-temperature ionic melts in some ways similar but in others diametrically opposite.

Formation of three-dimensional bicontinuous structures via molten salt dealloying studied in real-time by in situ synchrotron X-ray nano-tomography

Three-dimensional bicontinuous porous materials formed by dealloying contribute significantly to various applications including catalysis, sensor development and energy storage. This work studies a method of molten salt dealloying via real-time in situ synchrotron three-dimensional X-ray nano-tomography. Quantification of morphological parameters determined that long-range diffusion is the rate-determining step for the dealloying process. The subsequent coarsening rate was primarily surface diffusion controlled, with Rayleigh instability leading to ligament pinch-off and creating isolated bubbles in ligaments, while bulk diffusion leads to a slight densification. Chemical environments characterized by X-ray absorption near edge structure spectroscopic imaging show that molten salt dealloying prevents surface oxidation of the metal. In this work, gaining a fundamental mechanistic understanding of the molten salt dealloying process in forming porous structures provides a nontoxic, tunable dealloying technique and has important implications for molten salt corrosion processes, which is one of the major challenges in molten salt reactors and concentrated solar power plants.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium

Enhancing the solar energy storage and power delivery afforded by emerging molten salt-based technologies requires a fundamental understanding of the complex interplay between structure and dynamics of the ions in the high-temperature media. Here we report results from a comprehensive study integrating synchrotron X-ray scattering experiments, ab initio molecular dynamics simulations and rate theory concepts to investigate the behavior of dilute Cr3+ metal ions in a molten KCl-MgCl2 salt. Our analysis of experimental results assisted by a hybrid transition state-Marcus theory model reveals unexpected clustering of chromium species leading to the formation of persistent octahedral Cr-Cr dimers in the high-temperature low Cr3+ concentration melt. Furthermore, our integrated approach shows that dynamical processes in the molten salt system are primarily governed by the charge density of the constituent ions, with Cr3+ exhibiting the slowest short-time dynamics. These findings challenge several assumptions regarding specific ionic interactions and transport in molten salts, where aggregation of dilute species is not statistically expected, particularly at high temperature.