Chloride Molten Salt Research Articles

Impact of Rhenium Ion Stability on Coating Quality in Chloride Molten Salts

Impact of Rhenium Ion Stability on Coating Quality in Chloride Molten Salts

The Form of Electrodeposited Iridium Ions in a Molten Chloride Salt and the Effects of Different Iridium Concentrations

A molten salt system was prepared using an optimized method. We studied the complex structure of Ir3+ ions in the molten salt system and the influence their concentration had on the quality of the coatings prepared via electrodeposition. Using TG-DSC and in situ XRD experiments, we studied the high-temperature characteristics and properties of IrCl3 alongside its thermal stability. Using in situ XRD and Raman spectroscopy, we analyzed the Ir ions’ complex structure and the variation in the molten salt system at high temperatures. Finally, the changes in the Ir ion concentration in the molten salt system and the influence of the microstructure of the coatings’ surfaces were investigated under different anode conditions. IrCl3 easily decomposes above 400 °C, and temperature increases accelerate the rate of this decomposition. When the NaCl-KCl-CsCl system is in a high-temperature, molten state, IrCl3 forms stable complex structures (IrCl6)3− and (IrCl6)2−, and the valence state of Ir will be transformed with the increase in temperature. Generating these complex structures is conducive to improving the Ir coating quality. During the electrodeposition process, too few Ir ions in the molten salt can lead to concentration polarization, affecting the quality of the coating. Application of the molten NaCl-KCl-CsCl system is conducive to the electrodeposition of Ir coatings in a suitable temperature range. At the same time, using Ir as the anode can enhance the quality of the coatings.

Investigation of the corrosion behavior of 29NC alloy in LiCl-KCl melt at 500 oC depending on the content of Li2O and LiOH from 0 to 2 mol. %

Molten chloride salt electrolytes are promising for use as a working medium for the implementation of high-temperature technologies. Alkali metal chlorides are an aggressive environment in relation to structural materials. One of the possible methods of reducing the corrosion damage of a structural material is the method of oxygen passivation of the surface of a metal or alloy by introducing a certain amount of oxygen-containing additives into the melt. The article considers the effect of oxygen-containing impurities (lithium oxide and lithium hydroxide) on the corrosive behavior of a metal material — an alloy of the composition iron–cobalt–nickel. To assess the corrosion resistance of materials, gravimetric analysis, micro-X-ray spectral analysis (XRSA) of the surface and cross-section sections, and X-ray phase analysis (XRF) of the sample surface were used. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li2O and LiOH are presented. Based on the data set of gravimetric, MRSA and XRF data, it was found that 29NC alloy samples in the LiCl–KCl–nLi2O salt melt are not susceptible to corrosion, but in the LiCl–KCl–nLiOH melt, the speed of the 29NC alloy increases significantly due to the interaction of the LiOH additive with the most electronegative component of the alloy — iron.

Enhancing high-temperature chloride molten salts corrosion resistance of 310S steel with Si-added Ni–Al coating

Ni–Al based coating is one of the common high temperature and corrosion resistant protective coatings. In the present paper, the effect of Si content on the formation of Al2O3 layer and the corrosion resistance of Ni–Al coating to chloride molten salt was studied. The research results show that the Ni–Al coatings with varying Si content mainly form NiAl2O4 and Al2O3 oxide layers after pre-oxidation, and form a three-layer structure after molten salt corrosion. The appropriate amount of Si added to the Ni–Al coating can promote the formation of the α-Al2O3 layer during pre-oxidation and thermal corrosion, improving the corrosion resistance of the coating to high temperature chloride molten salt, and hindering the diffusion of Cr elements. Particularly, the Ni–Al coating with Si content of 1.2 at.% has the thickest and densest Al2O3 protective layer, showing the best corrosion resistance. The simulation results shows that compared with the Ni–Al coating without Si, the Cl atoms in the Si-added Ni–Al coating need to overcome greater energy barriers to diffusion during the molten salt corrosion process, which also indicates that the addition of Si can improve the corrosion resistance of the coating.

Interaction of the periodic layered structure formed on the hot-dip aluminized Fe-Cr-B cast steel with molten MoCl5 salt

The corrosion behaviors of MAX/MAB phases in molten chloride salt at high temperature were widely studied. However, the reaction between MAB phase and molten chloride salt at low temperature was still unknown. Here we investigated the interaction of the periodic layered structure, which consisted of alternatively arranged Cr-Al-B MAB phase and FeAl3, on the hot-dip aluminized Fe-Cr-B cast steel with molten MoCl5 salt at 250 ℃. The results revealed that the Al was selectively etched from the periodic layered structure coating, accompanied with the transformation into metastable Cr4AlB6 and pure Fe as intermediate corrosion products, ultimately led to the formation of Cr-B MBenes and holes in the corroded regions of Cr3AlB4 MAB phase and FeAl3, respectively, after immersed in molten MoCl5 salt.

Analysis of deliquescent chloride salt by laser-induced breakdown spectroscopy with controlled uniform precipitation

BackgroundThe electrolytic efficiency in magnesium (Mg) production by molten salt electrolysis is mainly affected by the chloride content, which is determined by the metal content in the cooled molten salt. Laser-induced breakdown spectroscopy (LIBS) is a potential element detection method in cooled molten salt element detection. However, the cooled molten chloride salt is easily deliquescent, which greatly affects the LIBS detection results. ResultsTo solve the problem, a liquid-phase precipitation method based on the addition of a dispersant named as controlled uniform precipitation (CUP) method was proposed to pretreat the samples. Compared with the conventional powder compacting method and precipitation method, the CUP obtained the highest spectral stability and detection accuracy. The average relative standard deviation (ARSD) of Mg, Ca, and Na decreased from the 51.80%, 77.04%, and 44.01% to 6.77%, 6.27%, and 29.97%, the determination coefficient of the calibration curve (R2) increased from 0.044, 0.084, and -0.109 to 0.974, 0.954, and 0.802, and the average relative error (ARE) decreased from 58.49%, 52.46%, and 99.78% to 8.58%, 11.28%, and 29.38%, respectively, compared with the powder compacting method. Further investigation showed that the CUP method suppressed the sample deliquescence and the coffee ring effect, leading to the performance improvement of LIBS quantitative detection. SignificanceThe CUP provided an effective method of detecting metal elements for deliquescent samples and showed applied potential for other precipitable elements detection.

Characterization of nickel cladding on type 316H stainless steel for enhanced corrosion resistance in molten chloride salts

This study explores the corrosion protection characteristics of nickel (Ni) cladding on Type 316H stainless steel in molten chloride salt environments, relevant to molten salt reactors (MSRs) which are gaining interest as advanced and sustainable nuclear energy sources. Type 316H stainless steel, while noted for its high-temperature strength and corrosion resistance, faces significant challenges when exposed to corrosive molten salts. To address this, Ni cladding was applied using gas tungsten arc (GTA) welding and directed energy deposition (DED) laser cladding. Characterization of the cladding layers was performed using field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS), identifying precipitates such as Ti(C,N) and Cr₂O₃. Mechanical integrity was verified through bending tests in accordance with ASTM E190-21, which revealed no cracks in the cladding layers. Corrosion resistance was assessed through immersion tests in a 57 mol% NaCl - 43 mol% MgCl₂ mixture at 650 °C for 502 h. Results demonstrated superior corrosion resistance of Ni-clad specimens compared to uncoated Type 316H stainless steel and Incoloy 800H, with the Ni cladding exhibiting a weight increase due to Fe deposition from the Fe-base alloys. The study concludes that Ni cladding, irrespective of the application method, enhances the durability and longevity of Type 316H stainless steel in the harsh environments typical of MSRs, suggesting a promising approach to improving the performance of structural materials in these systems.

Corrosion-mediated production of uranium(III) chloride from metallic uranium in molten LiCl–KCl salt contained within a stainless-steel crucible

Uranium (III) chloride (UCl3) is a crucial component of a potent nuclear recycling technology—pyroprocessing—and next-generation molten salt reactors. It is usually synthesized by reacting metallic uranium with chlorinating agents (e.g., CdCl2 and PbCl2) in molten chloride salts. In this study, we report the unexpected formation of UCl3 from metallic simulated fuel (simfuel) immersed in impure molten LiCl–KCl salt (in the presence of a small amount of residual H2O) in a stainless-steel (SS) crucible, without a chlorinating agent. We investigated various factors influencing UCl3 formation, including fuel type (metallic simfuel, pure U, oxide simfuel, or no fuel), crucible material (SS or alumina), salt composition (LiCl–KCl or LiCl), temperature (773 K or 923 K), and contact between fuel and SS crucible. UCl3 only formed when metallic fuels (simfuel or pure U) were immersed in molten salt in the SS crucible, with higher concentrations at elevated temperatures. Oxide fuels did not produce UCl3, nor did contact with the crucible affect formation. Our findings suggest that impurities, particularly moisture in the salt, corroded the SS crucible, releasing iron and chromium chlorides that reacted with metallic U to form UCl3. UCl3 formation was more pronounced in LiCl–KCl than in LiCl, and thermodynamic calculations helped establish the mechanism.

Comparative study of corrosion behaviors of SS310 stainless steel in NaCl-KCl-MgCl2 and NaCl-KCl-CaCl2 molten salts

Concentrated solar power plants using chloride molten salt as heat transfer fluids imply high corrosion risks. The corrosion behaviour of SS310 stainless steel in NaCl-KCl-MgCl2 and NaCl-KCl-CaCl2 molten salts was comparatively studied. MgCl2 exhibits remarkably higher hygroscopicity than CaCl2, inducing a significant difference in the concentration of HCl that directly results in the matrix corrosion. The hydrolysis of MgCl2 produces large amounts of MgO that destroys Cr2O3 film and produce less-protective MgCr2O4. Correspondingly, a heterogeneous oxide bilayer is formed in NaCl-KCl-MgCl2. In NaCl-KCl-CaCl2, a lamellar oxide layer composed of primarily Ni(Fe,Cr)2O4 can effectively block the corrosive species H2O and HCl.

Effect of Mg Addition on Molten Chloride Salt Corrosion Resistance of 310S Stainless Steel with Aluminum

Effect of Mg Addition on Molten Chloride Salt Corrosion Resistance of 310S Stainless Steel with Aluminum

Enhanced visible light photocatalytic activity of Pt cluster modified carbon nitrides for U(VI) removal

Photocatalysis has been regarded as one of the effective methods of removing U(VI) from radioactive wastewater in recent years, due to the benefits of being environmentally friendly and efficient. In this work, a Pt cluster modified carbon nitride photocatalyst (Pt-CN) was synthesized in zinc chloride molten salt. The photocatalysts were characterized by XRD, IR, XPS, UV–vis, PL and EIS. The result demonstrated that the doping of Pt makes Pt-CN show higher electronic conductivity and better charge separation and migration, leading to an enhanced photocatalytic activity and faster kinetics compared with bulk graphitic carbon nitride (g-C3N4). Moreover, for uranium contained solution photocatalytic treatment, Pt-CN reached 94.6 % removal efficiency after 300 min of illumination with Xe light, and the kinetics improved by a factor of three compared to that of g-C3N4. Furthermore, the good reusability and photostability of the Pt-CN was confirmed as the removal efficiency remained at 83.7 % even after being recycled and reused five times. The photocatalytic reaction product was characterized as (UO2)O2·2 H2O, and the mechanism was deduced as uranium species reacting with H2O2 generated by light illumination in an open system. This molten salt one-step synthesis and Pt doping method and the performance of U(VI) extraction in air showed a promising potential for the application of Pt-CN for U(VI) extraction and removal from water.

Microstructure and thermal properties of ternary chloride eutectic salts for high temperature thermal energy storage

Chloride molten salts are attractive candidate materials used for effective thermal energy transfer and storage in renewable energy system as concentrating solar power, but associated thermal properties at high temperature are still insufficient in research. In this work, eutectic point verification, microstructure and thermal properties of NaCl–KCl–ZnCl2 eutectic salts with four different contents are investigated by molecular dynamics simulation based on Born-Mayer-Huggins potential and experimental measurement. The density, specific heat capacity, viscosity, self-diffusion coefficient and thermal conductivity of four ternary eutectic salts are predicted with experimental validation and their correlations with different temperature and composition are proposed, and the mechanism of desirable thermal performance variation is revealed from the microscopic point of view. As ZnCl2 content increases, the thermal conductivity and specific heat capacity increase significantly, and eutectic salt #4 (18.6 %NaCl: 21.9 %KCl: 59.5 %ZnCl2) has maxima of density, specific heat capacity and thermal conductivity. The above phenomena can be attributed to the increase in potential energy and Columbic energy for more charge of Zn2+, which strengthens the interaction between atoms in the system and intensifies the collision. This work is expected to provide effective guidance on the design and application of molten salts for high temperature thermal energy storage.

Effects of Al Content on the Mechanical Properties and Corrosion Behavior of Ni-Al Coatings on 310S Heat-Resistant Steel for CSP Plants

Material selection is crucial in concentrated solar power technology due to the significant impact of high-temperature corrosion. The Ni-Al coating on the steel is an effective approach to corrosion resistance. The alterations in microstructure, hardness, toughness, adhesion, and corrosion inhibition properties of Ni-Al coatings were examined across varying Al concentrations after oxidation processes. Cracks appeared on the surface of coatings with preoxidation Al content ranging from 9.9 at% to 23.3 at%, and no continuous Al2O3 oxide film was formed. Notably, as the Al content rose to 41.0 at%, and a continuous α-Al2O3 oxide film was successfully formed. It showed the best toughness and elastic modulus (181.6 GPa), and relatively high coating hardness (8.0 GPa). The corrosion results show that when the Al content is 41.0 at%, the corrosion resistance of high-temperature molten chloride salt is the best. The oxidation layer formed by the preoxidation has a blocking effect on the migration of elements from the molten salt into the coatings. The results provide a route to fabricate Ni-Al coatings that possess integrated strength, toughness, and corrosion resistance suitable for protective coating applications in concentrated solar power systems.

Uncovering accurate values of the polarization resistance in molten fluorides using electrochemical impedance spectroscopy

High-temperature operation and the presence of oxidizing impurities in molten chloride and fluoride salts pose significant corrosion risks to structural alloys in molten salt nuclear reactors. Electrochemical impedance spectroscopy (EIS) offers an operando, real-time method to study the instantaneous corrosion behavior of these materials non-destructively. However, signal noises may originate from sources extraneous to the electrochemical systems that limit the ability to perform quality impedance measurements in low-frequency regime where polarization resistance (Rp) dominates. Using the exposure of pure Cr in molten LiF-NaF-KF containing 1 wt% EuF3 at 600 °C as a model corrosion system, the extent of Non-Uniform Current and Potential (NUCP) distribution during an AC perturbation associate with the disk electrodes is evaluated in terms of two dimensionless correction factors kfRct(=Rct/Reff) and kfCdl(=Cdl/Ceff). Results show that a small area Cr disk electrode increases the transition frequency that marks the dominance of Rp to a conventional terminational frequency above 10−2 Hz, enabling a reliable charge-transfer resistance (Rct) value to be determined via circle fitting of an electrical equivalent circuit model (ECM) model. Experimental results were supported by EIS simulations considering commonly used ECM models that factor in the NUCP effect of disk geometries.

Corrosion of 316 SS in chloride molten salt for thermal energy storage: Inhibitory effects of Al powder

Abstract MgCl2-KCl-NaCl is regarded as one of the most prospective high-temperature thermal energy storage mediums and heat transfer fluids (HTF) for 3rd generation concentrated solar power (CSP) systems. However, high corrosion to alloys limits its application. In this paper, corrosion tests were conducted on 316 SS, in MgCl2-KCl-NaCl at 800°C with different content (0 wt.%,1 wt.%, and 10 wt.%) of Al powder addition as a corrosion inhibitor. The impact of Al powder was assessed through electrochemical methods, specifically impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). Following corrosion tests, the morphologies and phase compositions of 316 SS were determined by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The addition of Al powder can significantly reduce the corrosion current density of 316 SS in MgCl2-KCl-NaCl at 800°C, which was 183.29 times higher than that with 10 wt.% without Al addition. Al and the degree increased with increasing content of Al. With the addition of 1 wt.% Al, the thickness of the diffusion layer is significantly reduced, which was 54.6 μm (100 h), 275.1 μm (200 h), 370.4 μm (300 h), and 500 μm (400 h), respectively. When the addition of Al reaches up to 10 wt.%, the inwards diffusion of Al caused the formation of Al enriched layer, which was identified as the FeAl phase, on the surface of 316 SS during the high-temperature corrosion processes. The thickness of the Al enriched layer was associated with the diffusion time of Al, and its depth was 40.4 μm (100 h), 45.3 μm (200 h), 103.5 μm (300 h), and 139.5 μm (400 h).

Assessment of dissimilar metal corrosion in molten chloride salt

Dissimilar metal corrosion between high nickel alloys and stainless steel was tested in purified molten chloride salt. Preliminary results have shown that corrosion rates are significantly affected for samples both when electrically connected and isolated. Suggesting that galvanic corrosion as well as non-galvanic dissimilar metal corrosion play a role in corrosion processes for multi component systems. These results suggest that even if efforts to prevent electrical contact are taken, close proximity of structural materials can still change corrosion rates in molten salt environments.

Modeling the Impact of Grain Size on Corrosion Behavior of Ni-Based Alloys in Molten Chloride Salt via Cellular Automata

Modeling the Impact of Grain Size on Corrosion Behavior of Ni-Based Alloys in Molten Chloride Salt via Cellular Automata

A Sustainable Process for Rare Earth Electrowinning in Chloride Based Molten Salts

Neodymium production has become increasingly important due to use of neodymium–iron–boron (NdFeB) magnets in green energy, consumer technology and defense applications. The state-of-the-art practice features a neodymium fluoride and lithium fluoride molten salt with a consumable carbon anode converting dissolved neodymium oxide and carbon to neodymium metal and CO2.1-2 The anode effect results in PFCs and CO emissions, which make the current neodymium electrowinning process unsustainable.3-4 Some attempts have been made to electrolyze the chloride salt of neodymium to produce chlorine at the anode. Chlorine is a value-added product which neither consumes the anode nor produces other deleterious GHGs. Additionally, the chloride of neodymium can be non-carbothermically produced through a spontaneous reaction with hydrochloric acid, producing only water as a side product. However, a common problem in chloride rare earth electrowinning is the low Coulombic efficiency (10-50%) obtained in lab-scale and pilot-scale efforts.5 The neodymium chloride electrolysis process notoriously encounters a two-step reduction producing an intermediate divalent (Nd2+) oxidation state which is stable in the chloride media.6-7 The intermediate neodymium species diffuses away from the cathode leading to Coulombic efficiency loss. Additionally, the kinetics of the chlorine evolution reaction (CER) in chloride molten salts are known to be relatively sluggish, causing significant anodic overpotentials which elevate the specific energy consumption during electrowinning. Furthermore, electrowinning from molten salts often produces spongy or dendritic deposits of metal requiring energy-intensive purification. In this talk, we will address all aforementioned technical hurdles and report on our efforts achieving high Coulombic efficiency (~85%), and compact deposits with superior as-electrowon neodymium metal purity (>99 wt.%). Furthermore, we will report the development of a new anode which catalyzes chlorine evolution, lowering the anodic overpotential considerably during electrolysis at high current densities of practical interest for electrowinning. Specific energy consumption is calculated for some example cases and found to be between 2.1 and 3.5 kWh/kg representing a significant improvement over the conventional oxyfluoride process. Figure 1

Does Sn2+ play well with Zn2+ as their molten chloride salts react with Cr–Al–B MAB phase?

Novel Zn-MAX phases like Ti2ZnC were synthesized in the literature by the reaction between Lewis acid salts like ZnCl2 and the MAX phase. The interactions between the Cr–Al–B MAB phase contained in the PLS coating formed during HDA of Fe–Cr–B cast steel and five types of molten chloride salts (ZnCl2 and SnCl2) were investigated. The results showed that the Cr-(Al, Zn)–B and Cr-(Al, Sn)–B MAB phase solid solution were formed as the PLS coating reacted with the molten ZnCl2 and SnCl2, respectively, through topochemical reaction. Zn and Sn could not coexist in the A-site Cr–Al–B MAB phase solid solution, and the replacement ability of Sn2+ contained in molten salt was much stronger than that of Zn2+. Zn and Sn whiskers could spontaneously grow on the Cr-(Al, Zn)–B and Cr-(Al, Sn)–B MAB phase solid solution respectively.

Corrosion behavior of alloys 600, 617, and hastelloy N in molten KCl salt

Molten chloride salts are promising fluids for use in molten salt reactors (MSRs) and concentrated solar power (CSP) systems. Hence, understanding the corrosion phenomenon caused by individual chloride salts is essential to determine their effect on the corrosion of structural alloys. This study aimed to explore the corrosion behavior of nickel-based alloys, namely alloys 600, 617, and Hastelloy N, in individual KCl salt. The findings demonstrated that the corrosion of all alloys in molten KCl salt primarily results from the preferential dissolution of the Cr element along grain boundaries, leading to the formation of pits. Furthermore, the corrosion rate was estimated to be 53.2 μm/year for alloy 600, 104.5 μm/year for alloy 617 with the fastest, and 34.8 μm/year for Hastelloy N with the slowest. These results indicate that Hastelloy N with the lowest pristine Cr content exhibits excellent corrosion resistance compared to alloys 600 and 617 in molten KCl salt.