Chloride Molten Salt Research Articles

Corrosion Behavior of 20G and TP347H in Molten LiCl-NaCl-KCl Salt.

The corrosion behavior of 20G and TP347H materials was investigated in molten LiCl-NaCl-KCl salt. The corrosion rates of these materials in molten chloride salt are high and are strongly affected by the alloying surface oxide formation. The 20G shows uniform surface corrosion with almost no protective oxide formation on the surface. In contrast, the austenitic steel TP347H exhibits better corrosion resistance in molten chloride salts due to its high Cr content. Owing to the highly corrosive nature of molten chloride salts, the Cl- in molten salt could react with oxides and alloy, inducing intergranular corrosion of austenitic steel in molten chloride salt environments.

Quaternary nitrate and chloride molten salts for the next concentrating solar power plants: Corrosion considerations for the use of AISI 304L steel

The study addresses the advancement in concentrating solar power (CSP) plants by transitioning from binary nitrate salts to nitrate and chloride quaternary salts, focusing on the corrosivity of four salts as evaluated by immersion testing of AISI 304L stainless steel. Four compositions of salts were evaluated, from solar salt to equimolar NaNO3‐KNO3‐NaCl‐KCl, up to 21 days (500 h) at 500 °C in an open atmosphere, determining the corrosion kinetics using gravimetry. The morphology, chemical composition, and microstructure of the corrosion products were characterized using XRD, FESEM-EDS, and GD-OES. Exposure to the molten salt with 0 mol% Cl‐ (Solar Salt) resulted in negligible corrosion kinetics, consistent with previous studies. The salt with 14 mol% Cl‐ caused a stable corrosion product with a corrosion rate 30 times higher than without chloride. All quaternary salts exhibited a multilayer structure of the top surface with selective chromium (Cr) removal. Specimens exposed to salts with more than 29 mol% Cl‐ displayed a similar structure with Cr and iron (Fe) removal, resulting in more brittle layers and corrosion rates 90 to 250 times higher than the salt without chloride. GD-OES analysis confirmed Cl diffusion into the oxide layer, highlighting the role of pCl2(g) and pO2(g) in driving corrosivity. Based on the results, using in salts with 29 mol% Cl‐ at 500 °C is discouraged, while using 304L with 14 mol% Cl‐ in a cold tank may be considered.

Optical Basicity for Understanding the Lewis Basicity of Chloride Molten Salt Mixtures.

Currently, there is a lack of reliable measurement techniques for understanding the basicity of molten chloride salts. Optical basicity, an ultraviolet-visible (UV-vis) spectroscopic method for measuring Lewis basicity, is explored for its applicability to molten chloride salts. Shifts in probe ion (Pb2+ and Bi3+) electronic transitions are observed that show that cations in chloride salts follow the same basicity series as in oxide melts, and an increase in basicity is observed with increasing temperature. Pb2+ and Bi3+ are validated as effective probe ions in alkali, alkaline earth, and aluminum-sodium chloride molten salts. However, the utility of Bi3+ is limited by the volatility of BiCl3 at high temperatures, posing a challenge for use in high-temperature molten salt applications. Since optical basicity measures the extent of electron donation, it may be a useful metric for electrochemical corrosion.

Enhancements in thermal properties of binary alkali chloride salt by Al2O3 nanoparticles for thermal energy storage

The cost-effective molten chloride salts are promising heat transfer fluids for next-generation concentrating solar plants. However, their limited specific heat capacities and thermal conductivities hinder their applications. In this paper, molten salt-based nanofluids based on a binary chloride (50 mol.% NaCl, 50 mol.% KCl) with Al2O3 nanoparticles were proposed to enhance the thermal properties. Firstly, molecular dynamics simulations were employed to investigate the thermal properties of these nanofluids, focusing on various nanoparticle volume fractions within 1100∼1600 K. The findings reveal that as the nanoparticle fraction increases from 0% to 7%, the specific heat capacity and dynamic viscosity improve by≈14.9% and 34.8%–38.6%, respectively. Concurrently, the thermal conductivity increases by 10.0%–16.5%. Then, microstructure evolution was analyzed to elucidate the enhancing mechanism of the thermal conductivity. Specifically, the negatively charged nanoparticle surface selectively attracts Na+ and K+, resulting in the formation of a compressed layer around the nanoparticle where the short-range order of ions was intensified. As a result, the compressed layer serves as a thermal conduit between the Al2O3 nanoparticle and the surrounding salt, resulting in the enhancement of the thermal conductivity. Furthermore, additional analyses of thermal diffusion properties and energy density distributions provide indirect evidence for the existence of the compressed layer where ionic motion is restricted, further validating the enhancing mechanism.

Communication—The Development of a Stable and Practical Saturated Reference Electrode for Molten Chloride Salt Systems

A simply constructed, stable, Ni/Ni2+ saturated reference electrode (SRE) has potential to measure thermodynamic behavior of molten chloride salts more reliably. Like the Ag/Ag+ reference electrode (RE), the Ni/Ni2+ SRE is made of commercially available materials. Initial experiments in molten CaCl2 and LiCl show the Ag/Ag+ RE potential drifting two times faster than the SRE. Furthermore, experiments show the replicability of SREs by comparing two Ni/Ni2+ SREs with different compositions of NiCl2 which is supportive of saturated phase behavior.

Corrosion behaviour of FeCoNiCrAl high-entropy alloy in molten NaCl–KCl–MgCl2 chloride salt

Molten NaCl–KCl–MgCl2 is becoming a potential heat transfer fluid (HTF) and thermal energy storage (TES) material in the third generation of concentrated solar energy power (CSP) stations due to its excellent heat storage-capacity and thermal stability. However, the strong corrosion of the molten chloride salt at high temperature limits its industrial application. In this study, the corrosion behaviour of FeCoNiCrAl HEA with BCC + FCC phase structure in molten NaCl–KCl–MgCl2 (24.5–20.55–54.95 wt-%) at 650 °C under argon was investigated by a combination of weight loss and electrochemical methods. The results show that, the corrosion of FeCoNiCrAl HEA in molten chloride salt mainly manifests as the selective dissolution of Fe and Cr. With the further increase of immersion time, oxides gradually crack and slightly flake, resulting in large weight loss of FeCoNiCrAl HEA.

Corrosion Behavior of Candidate Structural Materials for Molten Salt Reactors in Flowing NaCl-MgCl2

Abstracts. A high-temperature molten salt natural convection loop was designed and manufactured for chloride-based salts. NaCl-MgCl2 salt was prepared and injected into the loop. Corrosion coupons composed of SS304, SS316L, and high-Ni alloys were also prepared as candidate structural materials for molten chloride salt reactors. After a corrosion experiment was conducted for 500 h, the salt was drained from the loop to the drain tank; inductively coupled plasma optical emission spectroscopy was used to investigate metal dissolution from the materials into the salt. The corroded materials were analyzed using scanning electron microscopy and energy-dispersive X-ray spectroscopy to calculate the corrosion rate. Materials exposed to NaCl-MgCl2 molten salt showed varying levels of corrosion resistance. The high-Ni alloy demonstrated the highest resistance, followed by SS316L and SS304. Furthermore, corrosion products were observed to migrate along the molten salt through natural convection, eventually depositing onto the surface of the high-Ni alloy in the cold leg of the loop.

Heterogeneous Structure, Mechanisms of Counterion Exchange, and the Spacer Salt Effect in Complex Molten Salt Mixtures Including LaCl3.

Complex molten chloride salt mixtures of uranium, magnesium, and sodium are top candidates for promising nuclear energy technologies to produce electricity based on molten salt reactors. From a local structural perspective, LaCl3 is similar to UCl3 and hence a good proxy to study these complex salt mixtures. As fission products, lanthanide salts and their mixtures are also very important in their own right. This article describes from an experimental and theory perspective how very different the structural roles of MgCl2 and NaCl are in mixtures with LaCl3. We find that, whereas MgCl2 becomes an integral part of multivalent ionic networks, NaCl separates them. In a recent article (J. Am. Chem. Soc. 2022, 144, 21751-21762) we have called the disruptive behavior of NaCl "the spacer salt effect". Because of the heterogeneous nature of these salt mixtures, there are multiple structural motifs in the melt, each with its particular free energetics. Our work identifies and quantifies these; it also elucidates the mechanisms through which Cl- ions exchange between Mg2+-rich and La3+-rich environments.

Phosphate-Based Dechlorination of Electrorefiner Salt Waste using a Phosphoric Acid Precursor.

Electrochemical processing of spent nuclear fuel in molten chloride salts results in radioactive salt waste. Chlorine removal from the salt has been identified as an effective and efficient first step in the management of high-level waste. In this work, a simple salt was dechlorinated with a phosphoric acid phosphate precursor, resulting in a glassy dechlorinated product. The dechlorination efficacy was evaluated in air and argon environments. This work serves as an initial step to advance the Technological Readiness Level of H3PO4-based dechlorination step toward implementation of iron phosphate waste forms to immobilize electrochemical fuel reprocessing salt waste streams.

New frontiers in thermal energy storage: An experimental analysis of thermophysical properties and thermal stability of a novel ternary chloride molten salt

It has been always a daunting task to develop thermal systems with a wide range of thermochemical properties and high stability. It presents a significant challenge, especially when aiming to utilize them for heat storage and transfer applications. In this regard, the thermal characteristics and thermal stability behaviour of molten salt mixture as a heat transfer fluid (HTF) at high temperatures were explored. A novel ternary eutectic salt mixture (base mixture) made of cuprous chloride (CuCl), potassium chloride (KCl) and sodium chloride (NaCl) was investigated as HTF for thermal energy storage (TES) system with a range up to 653 °C in a concentrated solar power (CSP) plant. To extend the temperature span and stability, the effect of an additive was investigated. Different compositions were studied in detail concerning the properties and stability regarding the additive. The molten salt with 7 % CaCl2 additive improved thermal stability and operating temperature from 653 °C to 700 °C. The transport characteristics and thermal stability of the eutectic mixture with and without the additive were measured by high-temperature thermal analyzers. The results indicated that the melting temperature decreased and the average specific heat capacity considerably increased with the addition of CaCl2 additive compared to the ternary chloride base mixture. As the temperature increased, the eutectic salt mixture exhibited a reduction in viscosity and density. The viscosity and density of the salt mixture with and without the additive were almost uniform at 700 °C. The average thermal conductivity of the ternary salt mixture with and without additives was 0.72 W/(m°C) which is much more than that of Solar and Hitec salts. In a 100 h experiment of long-period isothermal stability, the eutectic salt mixture showed less than 4 % weight loss at 700 °C. It was defined as the highest operating temperature at which the eutectic salt mixture remained stable. The stability and recyclability of the salt mixture were confirmed through the short-period stability test. The effect of the CaCl2 additive on thermal stability of base salt mixture was analyzed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) techniques. According to the results, this novel eutectic salt mixture had outstanding thermal stability up to 700 °C and could be employed as a viable TES material in CSP plants.

Corrosion behavior of Hastelloy alloys and stainless steel in the molten chloride salt NaCl-CaCl2-CeCl3

ABSTRACT Compatibility tests are carried out on Ni-based Hastelloy alloys and stainless steel under the influence of the molten chloride salt NaCl-CaCl2-CeCl3 to investigate the corrosion behavior of molten salts with structural materials. Weight change measurements before and after the reaction tests show that the Hastelloy alloys have better corrosion resistance than stainless steel. The effects of molten salt purification on the corrosion behavior in the Hastelloy alloys are explored. The composition of the reacting phases is identified from transmission electron microscopy analysis and the corrosion mode is homogeneous corrosion with Cr and Fe diffusion, followed by Mo and Ni elution.

Electrochemical behavior and electromotive force measurements of U3+/U0 in molten MgCl2-NaCl

The electrochemical behavior of UCl3 was investigated in molten MgCl2NaCl at 550 °C. Using electromotive force measurements, we have measured the formal reduction potential (Eo’) at 550 °C of U3+/U0 as +0.276 V vs. Mg/MgCl2 (-1.400 V vs. Ag/AgCl 5 mol%) on the molal scale. We also present the design and construction of a sealed Mg/MgCl2 reference electrode and have determined this system demonstrates ideal nonpolarizable electrode behavior, confirming it as an appropriate choice of reference electrode in molten chloride salts.

Effect of Al content on in-situ formation of Al2O3 protective layer on Ni-Al coatings in high-temperature chloride molten salt

The microstructure, strength, toughness, and resistance to chloride molten salt corrosion of Ni-Al coatings with varying Al content were investigated. Increasing the Al content from 9.9 to 61.1 at.% resulted in a decrease in adhesion strength and toughness, while an increase in corrosion rate. Contrary to previous reports, it is noteworthy that Ni-Al coatings with the lowest Al content exhibited superior resistance corrosion due to the formation of an oxidation layer which effectively blocked corrosion. This protective layer was primarily in-situ formed through the reaction between crystalline water present in the molten salt and an appropriate amount of Al. The balance between Al and O content is precisely regulated, resulting in the in-situ formation of Al2O3 protective.

Investigation of the Calciothermic Reduction of TiO<sub>2</sub> for the Green Production of Ti and TiH<sub>2</sub> Powders

To develop an eco-friendly titanium (Ti) production process, the calciothermic reduction of titanium dioxide (TiO<sub>2</sub>) was investigated. The mechanism involved in the reduction of TiO<sub>2</sub> using calcium (Ca) was examined. The influence of a molten salt, calcium chloride (CaCl<sub>2</sub>), on the calciothermic reduction was investigated by conducting experiments at 1123 – 1223 K in an argon (Ar) atmosphere for the duration of 0.5 – 9 h. When CaCl<sub>2</sub> was used as a molten salt, the oxygen (O) concentration in Ti decreased to 0.162 mass% owing to the removal of calcium oxide (CaO) from the surface of the Ti particles. In addition, the use of a CaCl<sub>2</sub> – lithium chloride (LiCl) molten salt decreased the reaction temperature to 1073 K and the O concentration in the obtained Ti reached 0.333 mass%. Furthermore, after the calciothermic reduction of TiO<sub>2</sub> at 1173 K using a perforated crucible, the residual Ca-containing salt was sufficiently separated in-situ from the crucible, and a mixture of Ti and TiH<sub>1.924</sub> was produced via hydrogenation at 1023 K by changing the atmosphere gas from Ar to hydrogen gas (H<sub>2</sub>). The results of this study demonstrate the feasibility of the calciothermic reduction of TiO<sub>2</sub> for the eco-friendly production of Ti and TiH<sub>2</sub> powders.

Electrolysis in Chloride Molten Salts for Sustainable Critical Metals Production and Recovery

Critical materials, such as rare-earth metals, are essential to numerous applications, including clean energy; however, the present industrial practices for producing rare-earth metals involve environmentally damaging and thus unsustainable chemical and electrochemical processes. An alternative moderate-temperature chloride-based molten salt electrolysis process can address these issues, providing energy efficient and sustainable metal production. While it is being developed presently for rare-earth electrowinning, one can easily envision its broader application to rare-earth electrorefining and the electrolytic production of high-volume metals like Fe and Al. Presently, these high-volume metals industries account for nearly 10% of global greenhouse gas emissions. Thus, the chloride MSE process presents a huge opportunity for truly achieving sustainability if it is developed further for producing Fe, Al, Ti, Mg, and other commodity metals.

Hot corrosion behavior of Co-W coated ferritic stainless steel in molten chloride salt

The hot corrosion behavior of CoW multilayered oxide-coated FSS430 in molten NaCl-KCl (44.53–55.47 wt%) at 700 °C in an air atmosphere was investigated. The effects of CoW multi-layer oxide coating on the oxidation resistance and hot corrosion resistance of the substrate were studied by analyzing the mass change, corrosion products, surface morphology, and cross-sectional structure. The results showed that the CoW multi-layer oxide coating suppressed the interdiffusion of elements, and thus, effectively reduced the corrosion rate under the experimental conditions from 4.98 × 10−8 to 2.42 × 10−9 (g2·cm−4·s−1). Also, the coating was proved to inhibit the outward diffusion of Cr in the substrate and reduced the hot corrosion caused by the loss of Cr into the molten chloride salts.

High-temperature corrosion of a Si3N4/W composite exposed to molten MgCl2-NaCl-KCl salts

The high-temperature corrosion behavior of a Si3N4/W composite under the molten MgCl2-NaCl-KCl salts has been investigated. It is suggested that the molten salts diffuse along the physical surface defects and enter the interior of adjacent Si3N4/W grains, leading to the 9.1% bending strength loss after 400 hours for corrosion. In air atmosphere, oxidation products WO2, WO3, and corrosion product MgWO4 caused a 34.3% bending strength decrease after 400 hours for corrosion. This study elucidates the high-temperature molten chloride salt corrosion mechanisms of the Si3N4/W composite material under two different atmospheres. The composite demonstrates relatively excellent corrosion resistance in service environments.

Chloride molten salt derived attapulgite with ground-breaking electrochemical performance

The existence of adsorbed water and structural water in the crystal structure of attapulgite (ATP) endows it with poor capability to store lithium ions. Herein, the chloride molten salt method was developed to function ATP materials based on theoretical calculations, which exhibit ground-breaking electrochemical performance. After the modification process, the metal ions in chloride molten salt occupy the vertices of the Mg-O octahedral structure from the liberation of structural water and hydroxyl groups in ATP, forming MaMgbAlcSixOy (M = Li, Na, or K). Using LiCl molten salt-modified ATP (Li-ATP) as a proof-of-concept, the detailed phase transition, physicochemical properties, and lithium storage capacity were investigated. Compared to the original ATP, Li-ATP achieves a nearly 7-fold increase in lithium storage capacity (498 mAh/g), featuring a promising low-cost polyanionic type anode material.

Design of a Continuous Electrochemical Purification Reactor for Corrosion Mitigation in Molten Chloride Salt Systems

The utility of molten chloride salts in concentrating solar power (CSP) is dependent on the ability to maintain low corrosive impurities. Without proper purification, the corrosion rates of containment alloys exceed the industry standard of corrosion—under 20 μm/year. An extensive body of literature focuses on the initial purification stage of molten chloride salts. However, occurrences such as maintenance, leaks, and so forth expose the salt to the atmosphere and reintroduce impurities. Therefore, it is critical to develop a purification strategy that can control impurity levels during CSP plant operation. Here we present the design and fabrication of a purification vessel to continuously remove impurities out of flowing molten salt.

Spectroscopic and theoretical analyses of the reaction of SrO in molten chloride and fluoride salts

In this paper, the reaction of SrO in molten chloride and fluoride systems is investigated using Raman spectroscopy, combined with density functional theory calculations. The formation of Sr4OCl6 was observed in various salt compositions, including LiCl, LiCl-KCl, and LiCl-LiF, at temperatures ranging from 298 K to 923 K. Thermodynamic calculations estimated that Sr4OCl6 is more stable than SrO and SrCl2, while no stable strontium oxyfluoride compound was found. The results were also supported by X-ray analyses: Extended X-ray absorption fine structure analysis indicated that the local structure of Sr in LiCl-KCl and LiCl-LiF consists of one oxygen and six chlorides, corresponding to a complex with a similar local structure of Sr4OCl6. Additionally, Sr4OCl6 was found in LiCl-LiF, and no strontium compound was observed in LiF-KF using X-ray diffraction. These results provide a comprehensive understanding of the dissolution structures of strontium oxyhalide in molten chloride and fluoride systems.