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The crystal structures of three polymorphs of Rb3ScF6 have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; α) and medium-temperature (β) structures are tetragonal, with space groups I41/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) A, c = 36.5160(0) A and a = 14.4093(2) A, c = 9.2015(1) A at RT and 187 °C, respectively. The high-temperature (γ) structure is cubic space group Fm3m (Z = 4) with a = 9.1944(1) A at 250 °C. The temperatures of the phase transitions were measured at 141 and 201 °C. The three α, β, and γ Rb3ScF6 phases are isostructural with the α, β, and δ forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the β polymorph, the dynamic rotations of the ScF6 octahedra of both Sc crystallographic sites have been detailed.

A family of three- and four-coordinated silver(I) complexes of formulas [Ag(PPh

Molten halides can be effectively employed for electrorefining of refractory and radioactive metals with relatively negative reduction potentials and high melting temperatures. In the present study electrorefining of uranium was studied for developing pyrochemical reprocessing technology of metallized spent nuclear fuels. 3LiCl-2KCl molten salt was chosen as the working electrolyte. Uranium containing electrolytes were prepared by dissolving UCl3 in the solvent salt melt in an argon filled glove box to obtain various desired concentrations of uranium in the electrolyte. The working temperature in the refining experiments varied between 450 and 750 °C. The experiments on uranium electrorefining were carried out in a stainless steel semi-industrial water-cooled electrolyser. Crude uranium metal was loaded in a specially designed molybdenum anodic basket. Glassy-carbon crucible was used to hold the melt. Molybdenum rod was employed as a cathode. The main goal of the electrolytic refining process is obtaining high-purity uranium metal. Specific energy consumption is also very important. The results of the electrolytic refining depend on various parameters. In the present study current density, temperature, uranium concentration in the electrolyte, and specific quantity of electricity passed through the cell were selected as variables, and the current efficiently was chosen as a response factor. Dense and compact uranium deposits were obtained under low current densities, while application of relatively high current densities led to the formation of uranium dendrites. Intermediate values of current density allowed producing more coherent deposits that could be easily scraped off the cathode. Increasing temperature led to a sharp change of the cathodic deposits morphology. The reason for this is the formation of tetragonal beta-uranium instead of the low-temperature orthorhombic alpha modification. The values of cathodic current efficiency in the most of experiments confirmed the three-electron scheme of uranium reduction. Cathodic current density under certain conditions exceeded 95 %. The amount of salt retained in the cathodic deposit varied from 7 to 25 %, values typical for dendrite electrolytic metals.

Molten fluoride salts can be used as the fuel and coolant for molten salt reactors (MSR) and electrolytes for spent nuclear fuel (SNF) reprocessing. 46.5%LiF-11.5%NaF-42%KF (FLiNaK) melt is the prospective media for these purposes due to their desirable thermophysical and nuclear properties. Finding construction materials with sufficient corrosion and mechanical resistance is the most challenging task for practical implementation of MSR concept. In the present study the corrosion and mechanical properties of different types of construction materials were investigated. The materials included various low carbon Ni-Cr-Fe-Mo, Ni-Cr-Mo, and Ni-Mo alloys, and metals with relatively positive electrode potentials. The corrosion experiments were performed in FLiNaK melt at different temperatures (from 550 to 750 °C) in the specially designed stainless steel cells under high-purity argon atmosphere. Corrosion properties of studied materials were investigated under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. In a special series of experiments fluorides of typical fissile nuclides and fission products were added to the salt electrolyte to estimate the influence of the red-ox potential on the corrosion resistance of the materials. Corrosion rates were determined from the weight loss measurements and chemical analysis of quenched melts. Surface and microstructure of corroded samples was examined by various microscopic techniques. Mechanical properties of investigated materials were also studied at the ambient temperature and at 600 °C. Advantages and limitations of different types of construction materials were evaluated on the basis of data obtained. The effect of temperature on corrosion and mechanical properties of the studied materials were determined. Possible mechanisms of corrosion of various materials in fluoride melts were proposed. The alloys based on Ni-Cr-Mo and Ni-Mo systems, and molybdenum and its alloys were selected for further long-time tests under dynamic conditions to determine the resource of materials in contact with molten fluorides for MSR and SNF recycling technologies.

Solubility of rare earth oxides in mixtures of alkali or alkaline earth halides was determined by the method of isothermal saturation at the temperatures up to 1400oC. The melts studied included CaCl2–CaF2 (20 or 75 mol. % CaF2), BaCl2–BaF2 (15 or 73 mol. % BaF2), equimolar CaF2–BaF2 and eutectic NaCl–NaF mixtures. Melting points of the salt mixtures were determined by differential thermal analysis and effect of added rare earth oxide on the melting point was considered. Solubility of yttrium, lanthanum, cerium, praseodymium, neodymium, and samarium oxides was determined. The effect of temperature, melt composition, and presence of a mixture of rare earth oxides on solubility was assessed.

The corrosion of various metallic materials was investigated in 3LiCl–2KCl and 3LiCl–2KCl–UCl3 (5 wt. % U) melts under inert atmosphere. Metallic tantalum and molybdenum, nickel-chromium-molybdenum alloys (VDM® Alloy C-4, Hastelloy® G-35®, and specially designed KhN62M alloy) were investigated. The corrosion tests run for 100 h, and the temperature was varied from 450 to 750°C. The corrosion rates were determined by gravimetric technique and chemical analysis of quenched melts.

Modern aerospace engineering relies on Al-based alloys with enhanced high-temperature strength properties and oxidation resistance. These requirements can be fulfilled by modifying alloys’ structure employing rare earths metal doping. Direct addition of metallic rare earths to aluminium is complicated due to high chemical activity of molten rare-earth metals and significant vapor pressure of aluminium at elevated temperatures. To overcome these drawbacks, lanthanide-containing master alloys with relatively low REE content can be employed instead of pure scandium. Currently there are three methods for preparing aluminium-based alloys containing rare earths, i.e. direct fusing, molten salt electrolysis and exchange or metallothermic reduction reactions. In the last case, Al–Gd and Al–Sc master alloy can be prepared by aluminothermic reduction of GdF3 or ScF3 in a molten binary NaF–KCl salt mixture at 800–1050 °С. In the present work we suggest the modification of the above mentioned method for aluminium–cerium and aluminium–neodymium master alloy production with a lanthanide concentration of 1 to 7 wt. %. Based on thermodynamic calculations we proposed that the high-temperature exchange reaction between metallic aluminum and corresponding fluoride containing salt electrolyte should be carried out at temperatures from 740 to 780 ° C to provide the higher recovery of REE. The influence of the time of the exchange reaction and «aluminium–salt» ratio on the process to obtain ligatures of various compositions is analyzed. The yield higher than 85 % is achieved. The microstructure of the alloys represents a metal matrix of aluminum in which intermetallic compounds of Ln3Al11 compositions are distributed In the case of an exchange reaction between aluminum and cerium fluoride, the formation of a new phase, presumably related to the Al4Ce intermetallic compound, is found.

Molten salt nuclear fast nuclear reactor (MSNFR) is one of the prospective design in frame of Generation IV concept. This technology is also required for reprocessing of spent nuclear fuel (SNF) in molten salts to extract valuable components. Recycling SNF consists of extraction uranium and plutonium with the disposal of minor actinides and fission products. These stages need the materials that retain their corrosion and mechanical properties for long time under the influence of high temperatures, radiation fields and contact with molten salts. Various steels and alloys were widely studied to assess their use under such extreme conditions. Alternative materials include ceramics, composite and carbon materials, which have high corrosion resistance in various media and can be used up to 1000 °C. In the present work the corrosion and mechanical properties of nitride ceramics (Si3N4 and BN) and carbon-based materials (carbon-carbon composite material (C/C) and high-density carbon) were studied in a molten mixture of lithium, sodium, and potassium fluorides (FLiNaK) in the temperature range of 550–750 °C under inert atmosphere. Corrosion tests were performed under static conditions, and the duration of each test was 100 h to enable the comparison of the experimental data. It was found that carbon-containing materials (C/C and high-density carbon) showed high corrosion resistance in the melt. C/C samples had lower corrosion rates (less than 0,1 mm/year in the entire temperature range), while the high-density carbon showed better mechanical properties. The main disadvantage of these materials is their impregnation by molten salt due to relative high porosity. However, C/C composite and high-density carbon are promising structural materials for salt media based on FLiNaK in the selected temperature range. Tested nitride ceramics (BN, Si4N3) demonstrated relatively low corrosion resistance in fluoride systems compare to carbon-containing and metallic materials. The corrosion rates of these materials in FLiNaK exceeded the value of 1 mm/year. The plastic properties of nitride ceramics were also poor. Possible application of such materials for MSNFR and SNF reprocessing is under further investigation.

The relation between the microstructure and the corrosion behavior of Hastelloy® G-35® in a KCl – AlCl3 salt solution after different deformation and heat treatments (DHT) is studied. It is shown that DHT may be used for creating a structure providing a corrosion rate lower than that of the traditional single-phase structure formed in the alloy by quenching.

A family of three- and four-coordinated silver(I) complexes of formulas [Ag(PPh3)2L], [Ag(PPh3)L], and [AgL]n with N-thiophosphorylated thiourea and thioamide ligands of general formula RC(S)NHP(S)(OPri)2 [R = Ph, PhNH, iPrNH, tBuNH, NH2] have been studied by solid-state 109Ag and 31P CPMAS NMR spectroscopy. 109Ag NMR spectra have provided valuable structural information about Ag coordination, which is in good accordance with the available crystal structure data. The data presented in this work represent a significant addition to the available 109Ag chemical shifts and chemical shifts anisotropies. The silver chemical shift ranges for different P,S-environments and coordination state were discussed in detail. The 1J(31P–107/109Ag) and 2J(31P–31P) values were determined and analyzed. © 2022 John Wiley & Sons Ltd. Kazan Federal University: Priority-2030 Financial support from the IR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. This paper has been supported by the Kazan Federal University Strategic Academic Leadership Program (Priority-2030). Financial support from the IR-RMN-THC Fr3050 CNRS for conducting the research is gratefully acknowledged. This paper has been supported by the Kazan Federal University Strategic Academic Leadership Program (Priority-2030).

International audience; A combination of high field solid-state MAS NMR spectroscopy, X-ray diffraction, and firstprinciples calculations is used to elucidate the crystalline structure of CsSc3F10. At room temperature, this phase was found to crystallize in the Pmma (n°51) space group with a = 8.0837(1) Å, b = 7.5764(1) Å, and c = 6.8127(1) Å. The remarkable feature of CsSc3F10 is an unusual high cesium coordination number of 18. 45 Sc-19 F D-HMQC NMR method has been employed to investigate the connectivity of scandium with fluorine atoms. NMR parameters were determined using first principle DFT calculations and compared with experimentally obtained data.

International audience; In situ high temperature NMR spectroscopy was used to characterize the NaF-ScF3 melt over a wide range of compositions. 19 F, 23 Na, and 45 Sc NMR spectra were acquired in NaF-ScF3 melts of up to 70 mol% of ScF3. The interpretation of all experimental results obtained in situ in the melt is significantly enhanced by the contribution of Molecular Dynamics (MD) calculations. A new interatomic potential for the molten NaF-ScF3 system was developed by using a Polarizable Ion Model (PIM). The potential parameters were obtained by force-fitting to density functional theory (DFT) reference data. MD simulations were combined with further DFT calculations to determine NMR chemical shifts for 19 F, 23 Na, and 45 Sc. The agreement between the experimental NMR data and the corresponding calculated data from our applied computational protocol indicated the polymerization and network formation in the melt. Additionally, the density and the electrical conductivity in the molten state were calculated from the statistical analysis of ionic trajectories obtained through MD simulations.

The discovery of new magnetic materials is a big challenge in the field of modern materials science. We report the development of a new extension of the evolutionary algorithm USPEX, enabling the search for half-metals (materials that are metallic only in one spin channel) and hard magnetic materials. First, we enabled the simultaneous optimization of stoichiometries, crystal structures, and magnetic structures of stable phases. Second, we developed a new fitness function for half-metallic materials that can be used for predicting half-metals through an evolutionary algorithm. We used this extended technique to predict new, potentially hard magnets and rediscover known half-metals. In total, we report five promising hard magnets with high energy product (| BH|MAX), anisotropy field ( H a), and magnetic hardness ( κ) and a few half-metal phases in the Cr–O system. A comparison of our predictions with experimental results, including the synthesis of a newly predicted antiferromagnetic material (WMnB2), shows the robustness of our technique.

The crystal structures of three polymorphs of Rb

Bismuth–lithium alloys are considered as primary candidates for the reductive extraction step of on-line reprocessing of a molten salt reactor fuel. The corrosion behavior of pure metals and nickel-based alloys was studied in a liquid Bi–Li (5 mol.%) alloy at 650 °C. The tantalum, molybdenum, and corrosion-resistant alloys VDM® Alloy C-4, Hastelloy® G-35®, KhN62M, VDM® Alloy 59 were studied as prospective materials for this liquid metal media. The corrosion rates were determined by gravimetric method as well as chemical analysis of corrosion products in Bi–Li alloy. Microstructure and chemical composition of samples of the materials and Bi–Li alloys containing the corrosion products after the tests were evaluated using inductively coupled plasma–atomic emission spectroscopy, X-ray fluorescence analysis, scanning electron microscopy, and energy dispersive spectroscopy. Metallic tantalum and molybdenum do not chemically interact with liquid Bi–Li alloy, the corrosion rate of these metals is determined only by the solubility in this medium. The corrosion rates of Ta and Mo at 650 °C were 0.09 and 0.07 mm/year, respectively. Nickel alloys are subjected to severe corrosion in liquid Bi–Li alloys due to dissolution of nickel in liquid bismuth. Alloys of this type cannot be used in such an environment.

The corrosion behavior of austenitic steels AISI 316L (analog of 03Kh16N15M3), 12Kh18N10T (analog of AISI 321) and 06KhN28MDT and ferritic and ferritic–martensitic steels 12Kh13 (analog of AISI 410), 08Kh17T (analog of AISI 439), and 16Kh12MVSFBR is studied in chloroaluminate melts at a temperature of 550°C. The corrosion rates of these steels in a KCl–AlCl3 electrolyte are determined. The corrosion damage of the stainless steel surfaces is shown to have a continuous inhomogeneous character and to be characterized by etching off the most electronegative elements, namely, chromium, iron, and manganese.

Alkali chloride based melts can be employed as working media in spent nuclear fuels pyrochemical reprocessing. Oxygen is on of common impurities in technology and oxygen species can seriously influence speciation of metals in molten electrolytes. From the other hand, one of the possible ways of separating uranium from rare earth fission products by selective precipitation of uranium oxide. The present work was aimed at studying reactions taking place in chloride melts containing neodymium (III) and uranium (III and IV) ions upon addition of oxygen species. In particular, reactions with oxygen and oxide ions (introduced in form of lithium oxide) were considered. Thermodynamic modeling of possible reactions was first performed to estimate the possibility of separating uranium from rare earths. The experiments were then conducted in the melts based on LiCl–KCl eutectic, NaCl–KCl equimolar mixture and individual LiCl at 550 oC (for LiCl–KCl) and 750 oC for all the melts). Reaction of the melts containing NdCl3, UCl3 or UCl4 with Li2O (at various O2– to Nd3+ (U3+, U4+) mole ratios) and with O2 was investigated. High temperature electronic absorption spectroscopy was employed to determine the kinetics of the reactions taking place. Phase composition of the solid phases was characterized by X-ray powder diffraction analysis. Size of the particles in the precipitates formed at various conditions was also determined to assess the feasibility of subsequent separation of the solids from the technological melts.

The corrosion of austenitic 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 steels and their components (Cr, Fe, Ni, Mo) in NaCl-KCl-VCl2 melts with 5 wt % V at 750°C is studied. The rates and mechanisms of corrosion of the materials under these conditions are determined. The processes that occur during contact of the metals and steels with vanadium-containing chloride electrolytes are investigated.

Corrosion resistance of such ceramic materials as BN, Si3N4, YSZ (yttria stabilized zirconia) was studied in fused KCl–LiCl mixtures containing 8 wt. % UCl3. The rate and the mechanism of the corrosion of the materials studied were determined. The processes taking place during the interaction between perspective materials and the melt were investigated.

A mixed nitride U-Pu SNF pyrochemical reprocessing technology was suggested. It includes the following basic operations: dissolution of key components using the CdCl2 or PbCl2 oxidizer in chloride melt and their subsequent deposition as oxides. The obtained product, which is a mixture of actinides and rare-earth metals, may be additionally purified using hydrometallurgical processes (combined technology) or using pyrochemical methods. The mixture of actinides oxides is reduced to metal (“metallization”), additionally purified from fission products during electrorefining and then Am and Cm are separated using the potentiostatic electrolysis in order to obtain products that may be used for pure fuel fabrication.

Corrosion properties of type 625 alloys (Inconel 625 and Nicrofer 6020) in chloroaluminate melts were investigated in a wide range of temperatures and various exposure time intervals using gravimetric tests and energy- dispersive X-ray spectroscopy of corroded samples surface. It was shown that corrosion processes have electrochemical nature and the increase of temperature up to 650 °C leads to change of corrosion mechanism from gradual etching towards intergranular corrosion. Intergranular destruction is associated with the formation of secondary phases at the grain boundaries of alloys. “Time-temperature-precipitation” diagram was constructed to predict the possible conditions of type 625 alloys application. It was found that Inconel 625 and Nicrofer 6020 alloys can be used in contact with chloaluminate melts at temperatures less than 600 °C.

A comprehensive study of processes taking place during synthesis and subsequent thermal treatment of hafnium oxide was performed. Synthesis of HfO2 involves following stages: stepwise dehydration of hydrated hafnium hydroxide, crystallization of metastable tetragonal phase of HfO2 and subsequent irreversible transition to the stable monoclinic modification. Preliminary heat treatment lowered temperature of crystallization of HfO2 and resulted in changing process mechanism.

Melts based on mixtures of alkali and/or alkaline earth halides are considered as prospective media for electrowinning rare earth metals as well as for pyrochemical reprocessing spent nuclear fuels. Fluoride or mixed fluoride-chloride baths can be operated at high temperatures yielding molten rare earth metals (REMs) thus simplifying separation of the metal and salt. One of the problems in using fluoride melts is possible formation of fluorine at the anode. This can be avoided by adding a rare earth oxide to the melt both as the source of REM and oxide ions. The latter will be oxidized to oxygen producing carbon mono- or dioxide at the anode. The limiting factor for feeding the electrolysis bath with REM oxides is their solubility in the halide melt. The aim of the present study was determining the effect of temperature and melt composition on solubility of REM oxides in fused halides. The experiments were performed in CaCl2–CaF2 mixtures containing 20 or 75 mol. % CaF2; BaCl2–BaF2 mixtures containing 15 or 73 mol. % BaF2; equimolar CaF2–BaF2 mixture and NaCl–NaF eutectic mixture (34 mol. % NaF). Solubility of REM oxides was determined by the method of isothermal saturation. Time required for reaching the equilibrium between solid REM oxides and fused salts was determined in a preliminary set of experiments. To compare the behavior of 4f- and 5f-elements, solubility of uranium dioxide was also measured. The measurements were performed at the temperatures up to 1400 oC under argon atmosphere. The lower limit of the temperature range varied from 700 to 1100 oC depending on the melting temperatures of the salt mixtures used. Oxides of yttrium, lanthanum, cerium, praseodymium, neodymium and samarium were selected for the study. To assess a possible mutual influence of rare earth elements on solubility of their oxides in fused salts the solubility of a mixture of REM oxides was determined in a separate series of experiments and concentrations of individual REMs in the melt was determined. Solubility of REM oxides increased with increasing temperature and an example of effect of temperature on solubility of neodymium oxide in various melts is shown in Fig. Fig. Concentration of neodymium in alkali and alkaline halide based melts saturated with Nd2O3. Melt: CaCl2–CaF2 20 mol. % (1); CaCl2–CaF2 75 mol. % (2); BaCl2–BaF2 73 mol. % (3); BaCl2–BaF2 15 mol. % (4); CaF2–BaF2 50 mol. % (5; and NaCl–NaF 34 mol. % (6). Figure 1

The temperature dependence of the elastic modulus of nickel-chromium-molybdenum alloy C4 was investigated by the method of dynamic mechanical analysis. At all heating rates, an abnormal increase in the elastic modulus is observed upon heating in the temperature range of 200-300 °C. It is shown that an increase in the heating rate shifts the beginning of the observed effect to the region of higher temperatures. The temperature anomaly in the elastic modulus of the C4 alloy is explained from the standpoint of the formation of a short-range order in a solid solution. The thermodynamically equilibrium temperature of the “disorder – short-range order” transition in C4 alloy is determined to be 205 °C.

Interaction between metallic uranium and rare earth oxides, i.e. neodymium oxide, were investigated using thermodynamic modeling and experimental exposures at 1300–1500 °C. It was shown that metallic uranium partly reduced neodymium oxide resulting in formation of uranium dioxide film that prevented a uniform ingot formation. Increasing temperature led to higher degree of reduction and further complicated the rare earth oxide separation from uranium metal. Direct smelting cannot be used for fissile materials extraction from a mixture of metallic and oxide components obtained during nuclear fuel reprocessing.

Extended information is provided about 19F MAS NMR and XPS spectroscopic data on 11 ternary fluorido-zirconates (Li2ZrF6, Rb2ZrF6, Cs2ZrF6, Na3ZrF7, (NH4)3ZrF7, K3ZrF7, Li4ZrF8, K2ZrF6, Na5Zr2F13, Na7Zr6F31, and (NH4)2ZrF6), which display different coordination numbers of the zirconium atom as well as different modes of fluorine atom bonding. XPS data show that binding energies on fluorine 1s electrons are sensitive to its coordination properties (ionic, terminal or bridging coordination mode) while the binding energies of zirconium orbitals are not sensitive to its coordination properties (six-, seven- or eightfold coordination). 19F chemical shifts correlate with the ionic radii of counter cations along the series Li2ZrF6, Rb2ZrF6, Cs2ZrF6 and K3ZrF7, (NH4)3ZrF7 and Na3ZrF7. This reflects the polarizability properties and shielding potential of the cations surrounding the fluorine atoms. Additionally, calculations of NMR parameters for the compounds were also carried out using the CASTEP DFT code. The correlation between the experimental isotropic chemical shifts and calculated isotropic chemical shieldings established for the studied compounds allows us to predict the 19F NMR spectrum of crystalline compounds with good accuracy. A unique technique of low temperature 19F MAS NMR experiments combined with CASTEP calculation on (NH4)2ZrF6 enabled the assignment of all 12 crystallographically non-equivalent fluorine atoms to NMR signals.

The applicability of the active criteria for estimating the metastability of austenite for predicting the operating capacity of high-alloy corrosion-resistant alloys based on iron and nickel in a single-phase preliminarily quenched condition in a temperature range of up to 500 – 650°C is analyzed. The temperature and time ranges of formation of excess phases are determined and fragments of C-curves are plotted. The effect of preliminary cold plastic deformation on the kinetics of aging and on the morphology of segregation of particles of second phases is studied.

The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solid-state 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assig...

Corrosion tresistance of EP-823 (16Kh12MVSFBR) alloy as a prospective construction material in reprocessing spent nitride nuclear fuels was considered. Corrosion rates of EP-823 steel in a number of fused salt systems at 500 and 650 °C were determined. Changes in the material’s structure due to hight temperature influence, and type of corrosion destruction were established. Composition of the corrosuion products in the molten salts was determined on the basis of chemical analysis and corrosion resistance of EP-823 steel in the melts with and without iranium ions prtesent was compared.

The thermodynamics of metallothermic reduction of rare earths was analyzed with different reductants and initial starting compounds. Alkali metals and calcium were found as the most suitable reagents for reducing rare earth fluorides. Thermodynamic calculations showed the advantages of lithium as the reducing agent. Processes of metallothermic reduction of neodymium, lanthanum, and cerium fluorides by lithium metal were studied on a semi-industrial scale.

Reaction of solutions of uranium tri- and tetrachloride with lithium oxide was studied in the melts based on the eutectic mixture of lithium and potassium chlorides. The experiments were conducted at 550 and 750 °C, and the atomic ratio of oxygen-to-uranium in the melt varied from 0.5 to 2.0. Degree of uranium precipitation from the melt and the phase composition of the solids formed were determined.

The phase stability of nickel-based solid solution VDM® Alloy C-4 was studied, and time-temperature-precipitation diagram was built. The processes of heat treatment of sheets and tubes made of VDM® Alloy C-4 were investigated. The following regime of heat treatment was chosen – annealing at 1100 °C for 10 min/mm thickness with further rapid cooling in air. It was also shown that the cooling rate should be higher than 0.1 °С/s to prevent sigma-phase formation along the grain boundaries.

Some of the reports presented at the Sixteenth International Conference on High Temperature Materials Chemistry (Yekaterinburg, July 2–6, 2018) and related to the topics of journal Rasplavy are published in this special issue.

Chloroalumninate melts are prospective media for the second loop of molten salt nuclear reactors and low-temperature electrowinning and electrorefining processes for metals that can not be obtained from aqueous solutions. Application of chloroaluminates in such technologies is limited by the problem of corrosion resistance of construction materials. The alloys and steels corrosion resistance strongly depends on KCl-AlCl3 melt composition (K : Al mole ratio). When the mole ratio of potassium-to-aluminum is high (basic melts) precipitation of insoluble potassium chloride can cause the abrasion of the material. From the other side increasing AlCl3 content in the melt leads to increase of the red-ox potential due to formation of Al2Cl7 - species (acidic melts). Thus the importance of knowledge and ability to control K : Al mole ratios is undoubted for the industrial application of chloroaluminate melts. Chemical analysis based methods allow to determine the general content of potassium and aluminium in the melt. However chloroaluminate melts are very sensitive to moisture that can cause AlCl3 hydrolysis and result in decreasing «actual» aluminum chloride concentration in the melt. At increasing K : Al mole ratio (for example, due to evaporation of AlCl3) precipitation of solid potassium chloride can take place and this may lead to erroneous results of hemical analysis. Therefore indirect methods for determining K : Al mole ratio in molten chloroaluminates are needed to obtain correct data and perform possible required adjustment of the acid-base properties of KCl-AlCl3 electrolytes. In the present work potentiometry (with aluminum working electrode), cyclic voltammetry (with tungsten working electrode) and X-Ray diffraction analysis were suggested for indirect control of K : Al mole ratio. The electrochemical measurements were conducted using an Autolab PGSTAT 302N potentiostat/galvanostat. The electrode potentials were measured vs. aluminum reference electrode (ARE) consisting of aluminum wire lowered into KCl-AlCl3 melt containing solid KCl and separated from the working electrolyte by a diaphragm. X-ray diffraction patterns were recorded on an X’PERT PRO MPD X-Ray spectrometer with high speed 3D detector PIXCEL and an inert chamber Anton Paar HTK 1200N. Calibration curves of aluminum working electrode vs. ARE in studied chloroaluminates were obtained in temperature range from 300 to 500 oC. They strongly depend on acid-base properties of KCl-AlCl3 electrolytes and thus can be used for K/Al molar ratio determination. Reproducibility of the obtained values with temperature and salt composition was demonstrated. Cyclic voltammetry can be effectively used for determining acid-base properties of molten КСl-AlCl3 systems. In case of basic melts the cyclic voltamogramm consists of two edge peaks at 2.2-2.3 V and -0.3-0 V that correspond to dissolution of tungsten and deposition of aluminum. The presence of acidic Al2Cl7 - species leads to the appearance of additional maxima on the voltammograms. The influence of AlCl3 concentration on the position of these peaks and their intensity was investigated. The use of modern X-ray diffraction methods allows to perform high-precision qualitative and quantitative analysis. However the determination of low concentrations of «free» potassium and aluminum chlorides in quenched chloroaluminate electrolytes is complicated due to overlapping of weak lines of KCl and AlCl3 phases (present in small amounts) with the lines of major KAlCl4 monoclinic phase. High inclination of salt crystals to preferential orientation during sample preparation constitutes another difficulty for determining K : Al mole ratio by X-ray diffraction. In the present work we suggest using full profile Rietvelds analysis that allows to separate the contribution of diffraction patterns of different crystal phases even in the case of their overlapping. The results of X-ray diffraction determination of the composition of potassium chloroaluminate melts agree well with the electrochemical data.

Molten chlorides can be used for niobium electrowinning and electrorefining. Understanding niobium electrochemical properties and speciation in chloride melts is essential for designing or optimizing the industrial electrochemical process. Apart from the technological value, the information on the kinetics of the electrode reactions can also be used for verifying theoretical predictions concerning the nature of the heterophase electrochemical reactions. In the present study a variety of electrochemical techniques (polarization curves measurements (PCM), chronoamperometry (CA), chronopotentiometry (CP), linear (LVA), cyclic (CVA) and square-wave voltammetry (SWVA), impedance spectroscopy) were applied for studying electrode processes involving niobium species in NaCl-KCl-based melts. In addition the most valuable and reliable information about niobium speciation was obtained by combining electrochemical and spectroscopic (measuring electronic absorption spectra, EAS) methods. The results of spectroelectrochemical experiments showed that the major product of niobium anodic dissolution in chloride melts at current densities up to 40 mA/cm2 were niobium(III) species, NbCl6 3-. In the potentiostatic regime of anodic dissolution of niobium metal niobium(III) complex ions were formed at potentials below -1.15 V vs.chlorine reference electrode. The polarization curves characterizing the anodic dissolution of niobium consist of three or more waves. The first wave is associated with reaching limiting current density of niobium ionization according to the three-electron scheme. Further increase of applied current leads to the oxidation of Nb(III) to Nb(IV) ions. These conclusions were confirmed by determined values of potentials of niobium or indifferent electrodes in the equilibrium with the melts containing Nb(III) and Nb(IV) ions. When studying anodic dissolution of niobium we found that at high current densities there is sudden increase of the ohmic part of voltage drop sometime accompanied by increase of the anode potential. As a result stationary and non-stationary polarization dependencies exhibit unstable behavior. This is explained by the salt passivation, SP. The product of the SP of niobium is unstable NbCl4, which decomposes to NbCl3.13 and volatile NbCl5. The results of electrochemical experiments (PCM, CVA, LVA, CA, CP and SWVA) showed that soluble niobium(V) species were formed during Nb(IV) electrooxidation and represented the highest oxidation state of niobium in NaCl-KCl melts. We noticed that peak potential value (both cathodic and anodic) on CV changes with scan rate, whereas the ip/ν1/2 value remains constant. The formation of Nb(V) species was confirmed by the disappearance of absorption bands in the visible part of the spectrum due to Nb(V) species formation during elecrooxidation of niobium-containing melts at potentials up to -0.5 V. The quasireversibility of Nb4+↔Nb5+reaction was explained by the instability of niobium(V) species. Niobium(IV) ions were formed when potentiostatic electrolysis of NbCl3-NaCl-KCl melts (prepared by anodic dissolution of niobium metal) was performed at -(1.0÷0.8) V vs.chlorine reference electrode on a glassy-carbon anode, Fig. 1. The electroreduction of niobium on a tungsten electrode in NbCl3-NaCl-KCl melts at potentials below -1.45 V led to the formation of metallic niobium (Fig. 2), no evidence of any intermediate product formation was obtained. Linear voltammetry measurements showed that intermediate products can be formed during elecroreduction of NaCl-KCl-NbCln (n=3.8-4.1) melts. Most likely the electroreduction of Nb(IV) results in the formation of niobium(III) ions. The shape of cyclic voltammograms recorded in niobium containing melts depended on the scan rate. Similar sets of cyclic voltammograms were obtained at different temperatures and niobium concentrations. SWVA measurements performed on different electrodes also confirmed that there is more than one step of electroreduction. The obtained results were explained by overlapping of the following electrode reactions: Nb(IV) → Nb(III), Nb(IV) → Nb(0) and Nb(III) → Nb(0).

The corrosion resistance of Haynes 230, Hastelloy S and X high-temperature alloys and Hastelloy N, B-3, G-35, C-2000 corrosion-resistant alloys was investigated in a wide temperature range (450–650 °C) in fused KCl-AlCl3 mixtures. It was found that the mechanisms of corrosion of high-temperature alloys and corrosion-resistant alloys in KCl-AlCl3 based melts are different. At a critical temperature phase structure of the high-temperature alloys changes after contact with chloroaluminate melts resulting in formation of intermetallic or carbon-containing phases along the grain boundaries that result in increasing strength of the alloys but initiate intense intergranular corrosion. The structural changes in most of the corrosion-resistant alloys take place at higher temperatures but they also can cause structural changes in the materials. It was shown that both types of alloys undergo intergranular corrosion up to the critical conditions. Formation of different secondary phases was detected and their influence on the corrosion processes mechanism is analyzed.

The corrosion of austenitic 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 steels and their components (Cr, Fe, Ni, Mo) in NaCl-KCl-VCl2 melts with 5 wt % V at 750°C is studied. The rates and mechanisms of corrosion of the materials under these conditions are determined. The processes that occur during contact of the metals and steels with vanadium-containing chloride electrolytes are investigated.

High-temperature corrosion of VDM® Alloy C-4 was investigated in 3LiCl–2KCl and acidic KCl–AlCl3 melts in a wide temperature range (450–750 °C). The corrosion resistance of the alloy in molten salts depends on the oxidizing properties of the electrolyte and temperature. Holding VDM® Alloy C-4 above 650 °C may lead to the formation of secondary phases at the grain boundaries inducing undesirable intergranular corrosion. The secondary phase is represented by intermetallic sigma-phase; its chemical composition corresponds to the stoichiometric Ni(Mo,Cr)2. Time-temperature-precipitation and time-temperature-sensitization diagrams describing formation of sigma-phase in VDM ® Alloy C-4 were built to predict maximal working temperature and time of the material's application. The phenomenon of simultaneous decreasing ductility and increasing strength of the alloy caused by the long range order transformation was noticed after prolonged exposure of the material in the range from 550 to 650 °C. It was shown that unlike precipitation at the grain boundaries, formation of the long range ordered Ni2(Cr,Mo) phase did not increase the corrosion rate of VDM® Alloy C-4 in halide melts due to absence of micro galvanic pairs and growing of interatomic interaction force.

The mechanism of corrosion of austenitic steels 12Kh18N10T, 10Kh17N13M2T, and 03Kh17N14M3 and metals Cr, Fe, Ni, and Mo in a NaCl-KCl-NbCln (n = 3.5, Nb content is 5 ± 0.1 wt %) melt at 750°C is studied. The metal and steel corrosion rates under these conditions are determined. The character of material fracture and the mechanisms of material corrosion are found.

The corrosion resistance of Hastelloy G-35 and VDM® Alloy 625 nickel alloys was studied in a wide temperature range (750–1100 °C) in fused LiCl, CaCl2, NaCl–KCl, LiF mixtures containing REE. The rates and the mechanisms of the corrosion of the materials studied were determined. The processes taking place during the interaction between alloys and melts were investigated.