Fluoride Salts Research Articles

Corrosion of Si, C, and SiC in molten salt

Corrosion of Si, C, and SiC in fluoride salt has been studied byab initio molecular dynamics. The standard dissolution potential for Si is found to be smaller (easier to corrode) than that of C. The dissolved Si attracts F−ions and forms SiF62-, whereas the dissolved C species forms neutral CF4molecules. A swapping mechanism is identified for the initial corrosion stage, where Si first comes to the surface and then is dissolved, leaving behind chain- and ring-like C structures. A strategy to suppress SiC corrosion is also discussed based on Be doping, including avoiding Be2C formation.

Development and verification of a model for generation of MSFR few-group homogenized cross-sections based on a Monte Carlo code OpenMC

A concept of molten salt fast reactor (MSFR) was proposed in EVOL to burn transuranium element discharged from pressurized water reactors. MSFR is featured by fast spectrum and using liquid fuel salt containing UF4 or ThF4. Some issues are presented, for instance, global material arrangement affects the local neutron spectrum due to long neutron free path, and fluoride salt (LiF-BeF2) has nonnegligible thermal neutron scattering effect. Thus, lattice code prepared for thermal-spectrum reactor is not suitable for MSFR calculation. In this study, “two-step” calculation scheme combining Monte Carlo method and deterministic method was prepared for MSFR calculation. A tool named TRANS was developed to transfer tally data from an open source Monte Carlo code OpenMC into few-group homogenized cross-sections, and one benchmark based on pressurized water reactor and two types of model based on MSFR were used for verification. Besides, the applicability of few-group parameters generated by different model to MSFR whole-core calculation was analyzed. Finally, MSFR neutronics characteristics at steady-state were calculated using MOREL. The results show that the few-group parameters generated by one-dimension (1D) and two-dimension (2D) model are correct, and it is feasible to use OpenMC to generate few-group parameters. In case of 1D homogenization model, few-group parameters by 1D model (b) can give more accurate results both for eigenvalue and flux distribution. In MSFR whole-core calculation, using few-group cross-sections generated by 2D model has better accuracy in flux distribution, however, using few-group cross-sections generated by 1D model has better accuracy in keff calculation. Moreover, the neutronics parameters of MSFR calculated by MOREL code agree well with that by other institutes.

Oxygen reduction reaction with efficient, metal-free nitrogen, fluoride-codoped carbon electrocatalysts derived from melamine hydrogen fluoride salt

In this study, we successfully demonstrate an efficient, metal-free nitrogen, fluoride-codoped carbon (NFC) oxygen reduction reaction (ORR) electrocatalyst, which is produced by directly pyrolyzing melamine hydrogen fluoride salt (using as a single N and F precursor for the first time) mixed with carbon black BP2000 in a N2 atmosphere. The ORR electrocatalytic performances are evaluated by rotating ring disk electrode experiments in 0.1 M KOH. The NFC electrocatalyst prepared at the optimized temperature of 1000 °C (NFC1000) demonstrates a high ORR electrocatalytic activity with a peak potential of 0.82 V (vs. RHE), half-wave potential of 0.82 V (vs. RHE), predominant direct 4-electron reaction pathway, and good durability and methanol tolerance. Transmission electron microscopy equipped with mapping, X-ray diffraction and X-ray photoelectron spectroscopy results indicate that NFC1000 possesses an amorphous carbon structure with a homogenous codoped distribution of N and F at 2.25 at% and 1.52 at%, respectively. N2 adsorption-desorption analysis reveals that the as-prepared NFC1000 has a high surface area of 1169 m2 g−1. This study provides a feasible approach to synthesize low-cost and highly efficient metal-free heteroatom-doped carbon-based electrocatalysts.

Chromium carbide coatings produced on ductile cast iron QT600-3 by thermal reactive diffusion in fluoride salt bath: Growth behavior, microstructure evolution and kinetics

A series of chromium carbide coatings were formed on ductile cast iron QT600-3 by thermal reactive diffusion (TRD) processes in fluoride salt bath at 850–980 °C for 10 min ~ 40 h under an argon atmosphere. Coatings were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). The results demonstrate that the microstructure of chromium carbide coatings formed on ductile cast iron QT600-3 at 850 °C is related to the treatment time. At first 10 min, there is only a thin layer of Cr7C3 in the coating. At 1 h, the outermost layer of the coating is a thin Cr23C6 layer, and there is a thin M3C layer beneath the M7C3 layer. With increasing treatment time, the thickness of all layers in the coatings increases. For the sample treated for longer than 1 h, the microstructure of coating is Cr23C6, M7C3 and M3C from the outermost layer to the substrate. The microstructure of chromium carbide coatings formed on high C content steel substrates via TRD processes is independent with the composition of the base salt. The growth kinetics of chromium carbide coating follows a parabolic law with an activation energy of 129.906 kJ/mol. The evolution of hardness of coatings is consistent with the microstructure evolution of coatings.

Theoretical study of fluorine-induced surface segregation of Cr in non-passivated Ni-based alloys

The effects of fluorine adsorption on the surface segregation behaviors of chromium in nickel-based alloys are systematically investigated by selecting three typical low-index Cr-doped Ni (111), (100), and (110) surfaces based on the first-principles calculations. The Cr doped in the nickel substrate is identified to possess a local trapping effect for fluorine in the full space and contributes to the more exothermic adsorption of fluorine. In turn, the chemisorption of fluorine on the Cr-doped Ni surfaces can dramatically incur the preferential top-surface segregation for Cr from nickel bulk. The increased adsorption energy of fluorine on Cr-doped Ni surfaces compensates for the larger surface energy of Cr relative to Ni. The strong F-Cr bonding from the deeper hybrid state of F 2p-Cr 3d helps to release strain energy caused by atomic size mismatch. At higher fluorine coverage, the driving force for Cr to segregate at nickel surfaces is significantly increased, as the fluorine starts to agglomerate in the vicinity of Cr. Such chemisorption-induced surface abnormal segregation of Cr to the top-most surface plays a critical role in the preferential dealloying of Cr at the early-stage corrosion of non-passivated nickel-based alloys in molten fluoride salts.

Corrosion behaviour of Ni-based superalloys in molten FLiNaK salts

ABSTRACTHigh-temperature corrosion tests of alloys, Nimonic 80A, Inconel 718 and Inconel C-276, were investigated at 680°C in molten alkali fluoride salt (LiF–NaF–KF: 46.5–11.5–42%) environment. In this work, techniques included were weight loss measurements and potentiodynamic polarisation curves measurements. Inconel C-276 and Inconel 718 showed better corrosion resistance, while Nimonic 80A exhibited comparatively lower corrosion resistance. The high-temperature corrosion behaviour was observed using measurements of the oxide morphology and thickness. The corrosion rates were determined by recording the weight changes of the sample alloys at different time intervals. Microstructural examination showed the depletion of Cr near the surface of the alloys and hence the significant weight loss in the early stages of corrosion tests. The corrosion mechanism of the alloys is discussed in detail.

Experimental study on heat transfer performance between fluoride salt and heat pipes in the new conceptual passive residual heat removal system of molten salt reactor

Abstract In order to improve the passive safety performance of molten salt reactor, a new conceptual design of passive residual heat removal system (PRHRS) has been proposed using heat pipe technology. In this paper, an experimental system has been constructed to verify the PRHRS design of molten salt reactors. An eutectic salt mixture of 46.5 mol%LiF-11.5 mol%NaF-42 mol% KF compositions called FLiNaK is selected as working fluid. Tests on the natural convection between the fluoride salt and a vertical array of heat pipes in the drain tank have been conducted. The temperature field of fluoride salt in the tank with different heat pipe spacing is analyzed. Comparisons have been made on the heat transfer characteristics between the natural convection with a single heat pipe and the vertical array of heat pipes. In addition, the empirical correlation for the Nusselt number is obtained within the range of Rayleigh numbers from 5.7 × 10 6 to 1.31 × 10 7 . A good agreement between the proposed correlation and the experimental data is obtained with the deviation of −2.9%–1.7%. This work could provide valuable experimental data and suggestions for the design and analysis of PRHRS of MSR.

Microstructure and properties of fine-grained isotropic graphite based on mixed fillers for application in molten salt breeder reactor

Fine-grained isotropic graphite (FG) derived from mixed fillers with different weight ratio of natural graphite flake (NG) and calcined coke was prepared by cold isostatic pressing method. The average diameters of NG and coke were both 10 μm. The structure and properties of FG were investigated and compared with IG-110 graphite from Toyo Tanso Co., Ltd. The structure and properties of FG changed regularly when the weight percentage of NG in filler varied from 0 to 100%. The average pore diameter and porosity respectively minimized from 1.05 to 0.183 μm and 17.1 ± 0.1% to 11.8 ± 0.1%. Nevertheless, thermal conductivity at 298 K and density respectively increased from 93 ± 2 to 130 ± 2 W/m·K and 1.74 ± 0.02 to 1.85 ± 0.02 g/cm3. Result of molten salt infiltration experiment indicated that FG exhibited much better molten salt barrier property than IG-110 once the NG in filler excess 50 wt%. Especially, only 0.085 wt% fluoride salt could be detected for the graphite derived from 100% NG even up to 10 atm, which implied it could nearly totally inhibit the molten fluoride salt permeation and NG might be used as filler to fabricate moderator or reflector graphite materials for molten salt breeder reactor.

Improvement of Corrosion Resistance of Hastelloy-N Alloy in LiF-NaF-KF Molten Salt by Laser Cladding Pure Metallic Coatings

The corrosion protection of Hastelloy-N alloy in LiF-NaF-KF (commonly referred to as FLiNaK) molten salt has been developed by pure Ni and Co coatings using the laser cladding technique. An immersion experiment with samples was performed in molten FLiNaK salt at 900 °C for 100 h. It was found that the corrosion rates of the pure Ni-coated specimen and the pure Co-coated specimen are 39.9% and 35.7% of that of Hastelloy-N alloy, respectively. A careful microstructural characterization indicates that a selective dissolution of the elemental Cr occurred in the surface of bare Hastelloy-N alloy, showing a severe intergranular corrosion. For pure metal-coated specimens, in contrast, only metal oxide formed during the laser cladding process dissolved into the molten fluoride salt. The dense pure metal (Ni or Co) coatings exhibit a slightly general corrosion and protect the Hastelloy-N substrate effectively. The possible corrosion mechanism for both coated and uncoated Hastelloy-N under the current experimental condition are discussed in this work.

Effect of Surface Decarburization on Corrosion Behavior of GH3535 Alloy in Molten Fluoride Salts

Corrosion test of a Ni–16Mo–7Cr alloy with a decarburized layer was conducted in FLiNaK salt at 700 °C. A detailed microstructure study was performed to investigate the corrosion behavior and mechanisms. The results show that the Fe-rich layers were formed on the corroded alloys with and without decarburization. The surface decarburization had little influence on the corrosion resistance of the alloy, whereas it caused more M2C carbide formation beneath the corrosion layer. That is attributed to the higher concentration of C gradient near the alloy surface, which was resulted from the increase in C content liberated from graphite crucible wall during the corrosion process.

Effect of SO42− on the corrosion of 316L stainless steel in molten FLiNaK salt

The effect of SO42− on the corrosion behavior of 316 L stainless steel (SS) in molten FLiNaK (LiF-NaF-KF: 46.5–11.5–42 mol.%) salt was studied. Results reveal that the SO42− in the salt remarkably accelerates the corrosion of 316 L SS by promoting the dissolution of Cr, leading to increasing intergranular corrosion of the alloy. On the other hand, the results of structural characterization indicate that the SO42− can react with Mn to form MnS at the grain boundaries of the alloy. The galvanic couple between MnS and steel matrix could further accelerate the intergranular corrosion of 316 L SS in molten fluoride salt environments.

Investigation on the phase stability of yttria-stabilized zirconia electrolytes for high-temperature electrochemical application

For the Fundamental studies on the phase stability of YSZ, we tried to approach the detailed influences of practical conditions through various hypotheses. The phase stability of cubic and tetragonal yttria-stabilized zirconia (YSZ) was studied in the specific electrochemical conditions of current loading and molten salt flux. The degradation of electrochemical performance in the cubic phase occurred more easily than in the tetragonal phase because of yttrium elution in the lattice. The fully stabilized cubic phase showed high electrical performance as an oxide ion-conducting electrolyte, but phase decomposition occurred more easily under applied currents or chemical loading conditions than for the partially stabilized tetragonal phase. When YSZ reacted with a molten salt fluoride flux in solid oxide membrane (SOM) processing, which is used for direct metal reduction, and high-temperature electrolysis in the molten salt, the electrical conductivity of cubic YSZ was decreased by ~37%, while that of tetragonal YSZ showed no change. Our findings have important implications for the use of YSZ as an oxygen conductor in various electrochemical devices.

Efficient Conversion of Carbon Dioxide with Si-Based Reducing Agents Catalyzed by Metal Complexes and Salts.

Homogeneous metal complex and salt catalysts were developed for the reductive transformation of CO2 with Si-based reducing agents. Cu-bisphosphine complexes were found to be excellent catalysts for the hydrosilylation of CO2 with polymethylhydrosiloxane (PMHS). The Cu complexes also showed high catalytic activity and a wide substrate scope for formamide synthesis from amines, CO2 , and PMHS. Simple fluoride salts such as tetrabutylammonium fluoride acted as good catalysts for the reductive conversion of CO2 to formic acid in the presence of hydrosilane, disilane, and metallic Si. Based on the kinetics, isotopic experiments, and in-situ NMR measurements, the reaction mechanism for both catalyst systems, the Cu complex and the fluoride salt, have been proposed.

Preparation of Neodymium Metal by Pyro-Metallurgical Process Using Fluoride Salt

The neodymium fluoride was reduced to metal by thermal reduction or pyro-metallurgical process using calcium as reducing agent. Thermal reduction was carried out in a sealed reactions chamber in inert atmosphere with a nickel crucible having dry calcium fluoride lining. The stoichiometric mixture of neodymium fluoride and calcium metal chips were used in the form of pallets for pyro-metallurgical reactions. Small iodine was also added to increase heat available during ignition and to yield a lower melting and more fluid slag. The contents were heated electrically at a predetermined rate until ignition took place. The reaction was self-supporting and neodymium metal was obtained with fairly good efficiency.

Dimensions for reducing the consumption of fluoride salts in the production of aluminum

This article is devoted to the dimensions for reducing the consumption of fluoride salts in the production of aluminum. The main dimensions are: production and use of alumina with a lower sodium content; processing of a part of the excess electrolyte using a new technology to produce aluminum fluoride; processing of the heat insulating part of the spent lining with the production of regenerative cryolite, which is processed into a low-modulus cryolite with the help of a new technology. With the use of new technologies, secondary wastes are used in the production of cement, and aluminum smelters can become completely non-waste ones.

In situ anticaries efficacy of dentifrices with different formulations – A pooled analysis of results from three randomized clinical trials

ObjectivesData generated from three similar in situ caries crossover studies presented the opportunity to conduct a pooled analysis to investigate how dentifrice formulations with different fluoride salts and combinations at concentrations of 1400–1450 ppm F, different abrasive systems and in some cases, carbomer (Carb), affect enamel caries lesion remineralization and fluoridation. MethodsSubjects continuously wore modified partial dentures holding two gauze-covered partially-demineralized human enamel specimens for 14 days and brushed 2×/day with their assigned dentifrice: Study 1: sodium fluoride (NaF)/Carb/silica, NaF/silica, NaF + monofluorophosphate (MFP)/chalk; Study 2: NaF/Carb/silica, NaF + MFP/dical, amine fluoride (AmF)/silica; Study 3: NaF/Carb/silica, NaF + stannous fluoride (SnF2)/silica/hexametaphosphate (HMP). All studies included Placebo (0 ppm F) and/or dose-response controls (675 ppm F as NaF [675F-NaF]) ±Carb. Specimens were evaluated for percentage surface microhardness recovery (SMHR) and enamel fluoride uptake (EFU). ResultsAll 1400–1450 ppm F dentifrices except NaF + SnF2/silica/HMP provided significantly greater lesion remineralization than Placebo (p < 0.0001): differences in SMHR ranged from 17.46% (NaF + MFP/dical) to 26.66% (AmF/silica). For EFU (back-transformed log EFU), all 1400–1450 ppm F dentifrices gave significant fluoride uptake compared to Placebo (p < 0.0001): increases in EFU ranged from 4.95 μg F/cm2 (NaF + SnF2/silica/HMP) to 16.32 μg F/cm2 (NaF/carb/silica). Dentifrices containing NaF or AmF as sole fluoride source provided the greatest remineralization and fluoridation; Carb addition did not alter fluoride efficacy; some excipients appeared to interfere with the cariostatic action of fluoride. Treatments were generally well-tolerated with ≤4 treatment-related adverse events per study. ConclusionCommercially available fluoride dentifrices varied greatly in their ability to remineralize and fluoridate early caries lesions. Clinical significanceFluoride dentifrices are the most impactful anticaries modality worldwide. While clinical caries trials have not consistently shown the superiority of one formulation over another, these findings using a sensitive in situ caries model indicated that dentifrices containing NaF or AmF as the sole fluoride source provided the greatest remineralization and fluoridation benefits.

Effect of concentration of Cr3+ in LiF-NaF-KF salt on the corrosion of SiC

Effect of the concentration of Cr3+ in molten LiF-NaF-KF salt on the corrosion of SiC was investigated. Results reveal that increasing initial concentrations of Cr3+ in salt can enhance both the loss of Si and the deposition of Cr on SiC, which results in the weight change of SiC from loss to gain. The corrosion of SiC in molten LiF-NaF-KF salt with Cr3+ should be attributed to the oxides in SiC, the oxygen-containing impurities and Cr3+ in salt. The oxygen-containing impurities in salt react with SiC to form oxides that can be corroded by fluoride salt. The Cr3+ in salt can react with SiC and the oxides (SiOx).

Novel acridine-based thiosemicarbazones as 'turn-on' chemosensors for selective recognition of fluoride anion: a spectroscopic and theoretical study.

New thiosemicarbazide-linked acridines 3a–c were prepared and investigated as chemosensors for the detection of biologically and environmentally important anions. The compounds 3a–c were found selective for fluoride (F−) with no affinity for other anions, i.e. −OAc, Br−, I−, HSO4−, SO42−, PO43−, ClO3−, ClO4−, CN− and SCN−. Further, upon the gradual addition of a fluoride anion (F−) source (tetrabutylammonium fluoride), a well-defined change in colour of the solution of probes 3a–c was observed. The anion-sensing process was studied in detail via UV–visible absorption, fluorescence and 1H-NMR experiments. Moreover, during the synthesis of acridine probes 3a–c nickel fluoride (NiF2), a rarely explored transition metal fluoride salt, was used as the catalyst. Theoretical studies via density functional theory were also carried out to further investigate the sensing and anion (F−) selectivity pattern of these probes.

Corrosion of Structural Alloys in High-Temperature Molten Fluoride Salts for Applications in Molten Salt Reactors

Hastelloy N®, a nickel-based alloy, and 316 stainless steel are among the candidate structural materials being considered for the construction of the molten salt reactor (MSR). Most of the proposed MSR concepts use molten fluoride salts as coolant which can be quite corrosive to structural alloys. The results of studies on the corrosion behavior of the two alloys in molten Li2BeF4 (FLiBe) salt at 700°C are discussed. This salt is being considered as the primary coolant for MSR designs featuring solid fuel particles, but the reported results also provide insights into the corrosion in MSR designs where the uranium fuel is dissolved in the molten fluoride salt. Corrosion was observed to occur predominantly by de-alloying of Cr from the alloy surface and into the molten salt, with more pronounced attack occurring along the grain boundaries than in the bulk grains. The magnitude and the mechanisms of corrosion were different for corrosion tests performed in graphite and metallic capsules, a result warranting recognition given the coexistence of structural alloys and graphite in the molten salt medium in the MSR.

Cold/Hot Deformation Induced Recrystallization of Nickel-Based Superalloys for Molten Salt Reactors

A new special structural nickel-based alloy for use in molten fluoride salt environments in molten salt reactors (MSR) began to be developed by the COMTES FHT Company in the Czech Republic as early as 2001. The outcome of this development was the MoNiCr alloy, an alternative to Hastelloy-N. The present study was carried out on two experimentally-manufactured nickel alloys: MoNiCr and HN80MTY. Its purpose was to activate recrystallization processes in the as-cast microstructure of these alloys. In addition, experiments were performed to find a temperature which produces complete recrystallization. Static and dynamic recrystallization was studied in both alloys as well. An important aspect was to determine the lowest amount of deformation which still ensures complete recrystallization and provides as uniform recrystallized grains as possible. Such microstructure is well-suited for subsequent forming operations. Specimen microstructures were characterized using light microscopy (LM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD). Furthermore, corrosion tests of the MoNiCr alloy were conducted.