Molten Chloride Research Articles

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.

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 hours. 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.

Electrowinning of Light Metals from Molten Salts Electrolytes

Light metals such as lithium, sodium, potassium, magnesium and aluminium may be produced by molten salts electrolysis. Aluminium is produced by the Hall-Heroult process by electrolysis in molten salts. It is by far the largest in terms of production volumes and for metals second only to iron and steel, but it carries a very high carbon footprint mainly due to the wide-spead use of electricity based on based on coal. Magnesium is currently produced by silicothermic reduction of MgO according to the Pidgeon process. However, electrolysis in molten chloride electrolytes using MgCl2 as feedstock may return as it is more sustainable. The current Kroll process for producing titanium is energy intensive and requires lots of work as well as having very low productivity. Sodium is produced by electrolysis in a chloride electrolyte. This process may be improved by using sodium oxide as raw material and inert oxygen evolving anode.

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

Efficient separation of Chromium(III) from Lanthanide(III) via reductive boronization in molten chloride

It is an easy way to recover metals in molten salt by the precipitation-based method, but the selectivity yet remains challenging. Herein, a new separation method employing reductive boronization is proposed, and it is validated by the separation of trivalent chromium and lanthanide ions in molten LiCl-KCl. Although no Cr(III) and La(III) separation was achieved by using NaBH4, Cr(III) was successfully removed via the formation of insoluble CrB2 when B powder was used as the boronization reagent with La(III) remaining entirely in the melt. More than 97% Cr(III) can be removed from the CrCl3-LaCl3-LiCl-KCl melt after a reaction time of 300 min, and the removal rate was retained when the La/Cr ratio varied from 0.1:1 to 10:1. Such reductive boronization method is also valid in other Ln(III)/Cr(III) systems. The boronization process involves the reduction of Cr(III) and the reaction between zero valent metal and boron rather than the simple combination of anions and cations to give precipitates as evidenced by the XPS analyses. The difference in reactivity between Cr(III) and La(III) toward B can be rationalized in terms of their significant difference in the ΔG values of the boronization reactions that mainly correlates with the much lower reduction potential of La(III)/La(0), which makes B an ideal boronization reagent for the separation of trivalent chromium and lanthanide ions in molten chloride.

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.

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

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.

Corrosion of Two Iron-Based Aluminaforming Alloys in NaCl-MgCl2 Molten Salts at 600 °C.

Molten salts have been used as heat transfer fluids since the middle of the 20th century. More recently, molten chloride salts have been studied for use in concentrated solar power plants or molten salt reactors. However, none of the materials studied to date has been able to withstand this highly corrosive environment without controlling the salt's redox potential. The alumina-forming alloy was a promising option, as it has not yet been widely studied. To investigate this possibility, two iron-based alumina-forming alloys were corroded in NaCl-MgCl2 eutectic at 600 °C for 500 h after being pre-oxidised to grow a protective layer of α-alumina on each alloy. A salt purification protocol based on salt electrolysis was implemented to ensure comparable and reproducible results. During immersion, alumina was transformed into MgAl2O4, as shown by FIB-SEM observation. Inter and intragranular corrosion were observed, with the formation of MgAl2O4 in the corroded zones. The nature of the oxides was explained by the predominance diagram. Intragranular corrosion was 2 µm deep, and intergranular corrosion 10 µm deep. Alumina formed at the bottom of the intergranular corrosion zones. The depth of intergranular corrosion is consistent with O diffusion control at the grain boundary.

Flow and Heat Transfer Experimental Study for 3D-Printed Solar Receiving Tubes With Helical Fins at Internal Surface

Abstract 3D-printing technology was applied to fabricate novel solar thermal collection tubes that have internal heat transfer enhancement fins and external surfaces with high solar absorptivity and low emissivity due to the ability to use different materials in one tube. Helical fins were selected to introduce circumferential flow and thus minimize the circumferential temperature difference of the tube that receives sunlight on one side. The structures of the helical fins were previously optimized from computational fluid dynamics (CFD) analysis with the objective of low entropy production rate by looking for high heat transfer coefficient and relatively lower pressure loss. High-temperature alloy, Inconel-718, was used to 3D print the tubes, which can resist corrosion for the potential application of molten chloride salts as heat transfer fluid. Experimental tests were carried out using water as the heat transfer fluid with the high heat flux provided by a tubular furnace heater. The tested Reynolds number ranges from 3.9 × 103 to 6.1 × 104. Heat transfer coefficients of up to 2.8 times that of the smooth tube could be obtained with the expense of increased pressure loss compared to that of the smooth tube. The total system entropy generation can be significantly reduced due to the benefit of heat transfer enhancement that is greater than the expenses of the increased pressure loss. The experimental results of the 3D-printed heat transfer tubes confirmed the CFD-based results of fin optimization. The novel heat transfer tube is recommended for application in concentrating solar power systems.

Assessing boronized and aluminized thermal diffusion coatings in molten chloride salt and molten sodium environments

The service of structural materials, such as steels and alloys, in molten salt and sodium-cooled reactors in nuclear power generation is associated with high temperature corrosion of these materials, which leads to degradation and failures of structural components. To prevent these issues and to extend the reactor components' service life, protective coatings on steels need to be employed. In the present work, thermal diffusion protective coatings based on borides and aluminides have been tested, for the first time, in molten Na-K-Mg-chlorides of eutectic composition and in molten sodium at 500 °C in high-purity Ar glovebox where the test conditions simulated molten materials environments in nuclear reactors. The structural and compositional examination of the proposed protective coatings revealed only their minimal changing and deterioration in the corrosive environments. Thus, no or minimal formation of the corrosive scale on the top of the coatings was observed with neither structure transformation, nor reduction of the protective layer after 30-days testing in the molten materials' environments. Furthermore, no penetration of the corrosive media to the steel substrates occurred. Due to promising test results, the selected protective coatings may be highly recommended for further long-term and higher temperature testing in modeling and actual molten salts and molten sodium conditions associated with the service in nuclear reactors.

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.

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.

Electrochemical measurement of the corrosion rates of structural materials in molten NaCl–MgCl2 at 580℃

Molten chlorides find varied applications in engineering disciplines, notably in the nuclear energy sector. However, the utilization of molten chlorides in molten salt reactors (MSRs) requires investigation of the corrosion behavior of the reactor's structural materials in such environments. In this study, we proposed a facile method for determining the corrosion rates of structural materials immersed in a high temperature NaCl–MgCl2 molten salt, based on electrochemical measurements by cyclic voltammetry. The method involves measuring the electrochemical signals of the soluble corrosion products as a function of time. EuCl3-induced corrosion of Inconel 625 and stainless steel 316 (SS 316) in molten NaCl–MgCl2 was monitored by electrochemical detection of Ni2+ and Fe2+. The corrosion rates were linearly dependent on the EuCl3 concentration, and first-order rate constants of 1.03 × 10-4 and 3.20 × 10-5 s−1 at 580 ℃ were obtained for Inconel 625 and for SS 316, respectively. Arrhenius plot analysis of the temperature dependence of the rate constant provided an activation energy of 43.3 kJ mol−1 for Inconel 625 corrosion in molten NaCl–MgCl2–EuCl3. The rate constant became temperature-invariant above 620 ℃ indicating the mass-transfer limitation of corrosion at higher melt temperatures. The proposed approach enables accurate and real-time evaluation of the corrosion rate in the molten salt, and accordingly, offers a straightforward solution for in-situ corrosion monitoring in MSR systems.