Chloride-fluoride Melts Research Articles

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

Abstract The paper presents the results of neodymium electrochemical behavior in chloride and chloride-fluoride melts. It was shown that the process of neodymium electroreduction in the NaCl-KCl-NdCl3 melt proceeds in two stages. By diagnostic criteria of voltammetry it was established that the discharge process of Nd(III) to Nd(II) at a sweep rate in the range of 0.6 ≤  ≤ 1.0 V s-1 is not complicated by disproportionation reaction. In this study diffusion coefficients, activation energy of diffusion for Nd(III) chloride complexes and standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple in the NaCl-KCl melt were determined. The nature of the working electrode on the standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple has been studied by cyclic voltammetry and electrochemical impedance spectroscopy. The formal redox potentials E*Nd(III)/Nd(II) in the NaCl-KCl melt were obtained from the cyclic voltammetry data. It was shown that the addition of fluorine anions into the NaCl-KCl-NdCl3 melt leads to stabilization of the higher oxidation state of neodymium in chloride-fluoride melts and the intermediate oxidation state Nd(II) does not exist in these melts.

ЭЛЕКТРОХИМИЧЕСКОЕ ПОВЕДЕНИЕ ТИТАНА В ХЛОРИДНО-ФТОРИДНЫХ РАСПЛАВАХ РАЗЛИЧНОГО СОСТАВА В ПРИСУТСТВИИ КАТИОНОВ ЩЕЛОЧНОЗЕМЕЛЬНЫХ МЕТАЛЛОВ

The electrochemical behavior of titanium and the influence of alkali earth metal cations (Mg2+, Ca2+, Sr2+ and Ва2+) on the charge transfer kinetics of the Ti(IV)/Ti(III) redox couple were studied by cyclic voltammetry method in chloride-fluoride melts of different composition. Standard rate constants of charge transfer and the activation energies of the charge transfer process were calculated.

On the Ratio of Cathodic and Anodic Processes during Electroreduction-Electrooxidation of Hafnium in Molten Salts

Investigation of hafnium cathodic and anodic processes in chloride melts containing HfCl4 revealed that two-hafnium electroreduction processes are corresponded two electrooxidation processes. The only difference between these processes was the presence of salt passivation under certain dissolution conditions. In chloride-fluoride melts containing K2HfF6 the number of cathodic and anodic processes can be different due to the amount of hafnium ions passing into the melt during anodic dissolution exceeding the amount of fluorine anions necessary to bind them into fluoride complexes. Based on the experimental data it was found that the conclusion about the mirror reflection of cathodic and anodic processes is invalid in the case of a heteroligand composition of melts. It was determined that the dissolution in the NaCl-KCl-K2HfF6 (10 wt%) melt at the anodic current density of >0.02 А∙cm−2 was accompanied by the formation of Hf(IV) fluoride complexes and Hf(II) chloride complexes. It was shown that the appearance of hafnium complexes with different valence during anodic process in the NaCl-KCl-K2HfF6 (10 wt%) melt is a highly undesirable phenomenon, which leads to the formation of metallic film on the surface of the melt decreasing the current efficiency for coating deposition.

Electrodeposited from Ion-Organic Melt Ti-B Protective Coatings on Al-Foil

The great demand for advanced batteries in applications such as mobile devices, the transportation sector, and stationary storage has spurred intensive research and development on rechargeable lithium batteries. The corrosion behavior of Al-foil, as material for theirs current collectors, has long been a concern due to uniform or localized pitting corrosion occurs in contact with an organic electrolyte. Pitting corrosion is attributed to inhomogeneous current flow through the aluminum / electrode interface and results in local breakdown of the passive film on Al-foil. Particular attention among different protective coatings of Al-foil excites Ti-B coatings as conductive and electrochemically inactive protective layers.The main objectives of the present work are investigation of regimes for electrodeposition of Ti-B protective coatings onto Al-foil and examination of the corrosion behavior of Al-foil with such electrodeposited Ti-B protective coatings in contact with an organic electrolyte under various electrodes charging conditions.Low melting point of aluminium (tm = 6600C) made impossible electrodeposition of Ti-B coatings from chloride-fluoride melts (tel = 7000C). But it is possible from ion-organic melts at 70−2000C. In this work was investigated electrodeposition of Ti-B coatings from carbamide melt contains titanium and boron oxides. Cyclic voltammetry has shown the limit current density and potential of electrochemical synthesis. Ultra fine Ti-B coatings were electrodeposited onto Al-foil at 1300C. Figure shows the surface morphologies of an obtained Ti-B coating. Such cost-effective method of obtaining Ti-B protective coatings on Al-foils gives excellent results under theirs using as current collectors in rechargeable lithium batteries. Figure 1

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

An investigation of compounds of those few elements in the oxidation state (II), by opposition to the general trivalent lanthanides is of special interest. The present work follows our previous investigations on samarium, europium ytterbium chloride systems and, in particular, the electrochemical behavior of the Sm(III)/Sm(II), Eu(III)/Eu(II) Yb(III)/Yb(II) redox couples. It deals with neodimium compounds.The electrochemical behavior of NdCl3 in molten salts was studied in few works, but only melts with a low melting point LiCl-KCl and LiCl-KCl-CsCl were investigated.The electroreduction of NdCl3 in an equimolar NaCl-KCl mixture was studied in the temperature range 973-1123 K by different electrochemical methods. It was shown that the discharge of Nd(III) to Nd(II) is complicated by the disproportionation reaction (DPP):3Nd(II) ↔ 2Nd(III) + Nd (1)Ignoring of reaction DPP can lead to incorrect determination of values diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple. The relatively narrow interval of polarization rate was found when the recharge process was reversible and not accompanied by the DPP reaction. At these scan rates the diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple were determined by cyclic voltammetry.The standard rate constants for the redox reaction Nd(III) + e- ↔ Nd(II) were calculated on the basis of cyclic voltammetry at sweep rates corresponding to the quasy-reversible process. The nature of working electrode on the rate of charge transfer for the Nd(III)/Nd(II) redox couple was studied. It was found that the values of k s determined at a molybdenum electrode were higher than those at a glassy carbon electrode.The introduction of fluorine anions into a chloride melt at a molar ratio of F-/Nd(III) = 5 leads to the disappearance of the recharge wave and only an ascending section associated with joint discharge of neodymium complexes and sodium and potassium cations was observed. Thus, the introduction of fluorine anions leads to stabilization of the highest oxidation state of neodymium in chloride-fluoride melt and neodymium in the oxidation state +2 does not exist in the melt.Introducing of fluoride ions to the melt NaCl-KCl containing Nd(II) accompanied by reaction of disproportionation:3NdCl4 2- + 12F- ↔ 2NdF6 3- + Nd + 12Cl- (2)The standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple were compared with the values obtained for other redox couples of lanthanides and these results were discussed in connection with the strength and stability of complexes.

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

The paper presents results of neodymium electrochemical behavior in chloride and chloride-fluoride melts. It was shown that the process of neodymium electroreduction in the NaCl-KCl-NdCl3 melt proceeds in two stages. It was established by diagnostic criteria of voltammetry that the discharge process of Nd(III) to Nd(II) at a sweep rate in the range of 0.6 ≤ n ≤ 1.0 V s-1 is not complicated by reaction disproportionation. In this study diffusion coefficients, activation energy of diffusion for Nd(III) complexes and standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple in the NaCl-KCl melt were determined. It was shown that the addition of fluorine anions into the NaCl-KCl-NdCl3 melt leads to stabilization of the higher oxidation state of neodymium in chloride-fluoride melts and neodymium in the intermediate oxidation state +2 in these melts does not exist.

Anodic Processes during the Electrowinning of Niobium from Chloride-Fluoride Melts and the Influence of Oxide Ions on Them

Anodic processes were studied in chloride-fluoride, fluoride melts by voltammetry and gas chromatography mass spectrometry method. It was found that the reason for the occurrence of the anodic effect during the electrolysis of the KCl-KF-K2NbF7 melt is the formation of the compound CxFy on the electrode surface, which has electrical insulating properties. Influence of oxide ions on the anodic processes in the KCl-KF-K2NbF7 melt was investigated. It was determined that oxide ions and oxofluoride complexes of niobium oxidize at a glassy carbon electrode at more negative potentials than fluoride ions. The anodic effect in the chloride-fluoride melt containing oxide ions and oxofluoride complexes of niobium can be reached if the anodic current density will be higher than the limiting current density of electrooxidation of oxide ions and niobium oxofluoride complexes.

Charge Transfer Kinetics of the Ti(IV)/Ti(III) Redox Couple in Chloride–Fluoride Melts

Charge Transfer Kinetics of the Ti(IV)/Ti(III) Redox Couple in Chloride–Fluoride Melts

Электрохимический синтез интерметаллических соединений кобальта с неодимом в хлоридных расплавах.

The electrodeposition of neodymium on a cobalt substrate in a chloride-fluoride melt NaCl-KCl — 10 wt. % NaF — 5 wt. % NdF3was studied. It is shown that during electrolysis, a coating with a crystal size of 2–5 microns is formed. Microanalysis of the coating determined the electrocrystallization of the intermetallic compounds Co17Nd2 and Co5Nd on a cobaltsubstrate.

Microstructural Master Alloys Features of Aluminum–Erbium System

Aluminum master alloys with rare earth metals are widely studied by many scientists around the world, but research on the production of Al-Er master alloys is still limited. The purpose of this work is to study the microstructure parameters of aluminum-erbium master alloys obtained by metallothermic reduction of salt mixtures containing erbium oxide or fluoride. The structural features were investigated by optical and scanning electron microscopy, and the dependence of the microhardness of the eutectic and solid solution fields of obtained master alloys on the content of erbium in the master alloy was determined. Studies have shown that master alloys obtained by metallothermic reduction of erbium compounds from chloride–fluoride melts are characterized by a uniform distribution of Al3Er intermetallic compounds in the volume of double eutectic [(Al) + Al3Er] and have a strong grain refinement effect. The analysis of the microstructure showed that the structure of the master alloys varies depending on the content of erbium. When the content of erbium in the master alloy is up to 6 wt.%, the eutectic structure is preserved. When the content of erbium in the master alloy is 8 wt.% or more, the structure becomes a solid solution with individual inclusions of various shapes and intermetallic compounds.

Obtaining of Nd-Co films by electrolysis in a chloride-fluoride melt

The electrodeposition of neodymium on a cobalt substrate in a chloride-fluoride melt NaCl-KCl-10 wt.% NaF-5 wt.% NdF3 was studied. It is shown that during electrolysis, a coating with a crystal size of 2-5 microns is formed. Microanalysis of the coating determined the electrocrystallization of intermetallic compounds Co17Nd2 and Co5Nd on a cobalt substrate.

Electrodeposition of Tantalum Coatings on Nitinol Stent and Composition of Intermetallic Compounds Forming During Electrolysis

The electroreduction of tantalum complexes in a chloride-fluoride melt was studied on tungsten, and nitinol electrodes. It was found that on a nitinol electrode in addition to the peak associated with the discharge of fluoride tantalum complexes, several peaks on the voltammogram related to the formation of intermetallic compounds of nickel and tantalum were observed. The electrodeposition of tantalum coatings from the NaCl-KCl-NaF(10 wt.%)-K2TaF7(10 wt.%) melt onto nitinol stent was investigated. The formation during electrolysis of several intermediate layers between the deposited layer and substrate was found and their microstructure and chemical composition were determined. It was determined that the surface layers of the nitinol are depleted in titanium due to corrosion in the melt. During electrodeposition a barrier intermetallic layer TiNi2.4Ta0.6 is formed, which prevents the penetration of tantalum into the substrate, but does not prevent the diffusion of nickel and titanium from the substrate into the deposited layer of tantalum.

Secondary Reduction of Refractory Metal near Smooth Cathode during the Salt Melt Electrolysis. 3. Numerical Reproducing of Natural Experiment with the Nb Cathodic Reduction in KNaCl2–NaF–K3NbF7 Melt

In terms of the model of steady-state process of a refractory-metal ion secondary reduction with an alkali (or alkali-earth) metal formed on a smooth cathode during salt melt electrolysis, numerical reproducing of a natural experiment is carried out. The model calculations are based on the polarization measured during the cathodic reduction of Nb in chloride―fluoride melt: [Nb] = 2.4 mol %, 800°C, current density i up to 5.4 A/cm2. On a smooth cathode in the overvoltage range from 0.21 to 0.41 V (i = 0.75―1.17 A/cm2), the reduction mechanism is shown to completely change from primary to the secondary reduction. The secondary reduction zone is located inside the diffusion layer and moves away from the cathode with increasing i. Even at maximal i, this zone remains inside δ (δ is the thickness of the diffusion layer): with i = 0.96 A/cm2, the coordinate of the maximum of the secondary reduction rate xm = 0; with i = 5.4 A/cm2, xm = 0.93δ (in the experiment, the value of i at which xm ≥ δ was not achieved). At i = 5.4 A/cm2, the fraction of secondary reduction in the bulk is insignificant, ≈10−5 A/cm2. The δ vs. E curve is calculated: when E is shifted from the equilibrium value toward the negative values, |i| increased from zero and higher, whereas δ first decreased by an order of magnitude and then increased. Correspondingly, the limiting diffusion current is not constant at a constant Nb concentration in the bulk. This is the reason of uncertain limiting current on steady-state cathodic polarization curves during the electrolysis of metals in melts. A hypothesis is proposed on the dependence of the Nernst diffusion layer thickness near the cathode on the current density exceeding the limiting diffusion current density under the conditions of secondary reduction.

Electroreduction of Hafnium and Electrodeposition of its Protective Coatings on a Molybdenum Substrate in Chloride-Fluoride Melt

The electroreduction of HfF62− complexes to Hf on a molybdenum electrode in the NaCl-KCl-K2HfF6-NaF (5 wt%) melt in the temperature range 973–1123 K was studied by cyclic voltammetry and impedance spectroscopy. It was found that discharge of Hf(IV) complexes to Hf occurs in one step and is irreversible. The diffusion coefficients of Hf(IV) fluoride complexes were determined by cyclic voltammetry. The impedance spectroscopy was used for the determination of the standard rate constants of charge transfer. Electrodeposition of hafnium coatings on a molybdenum substrate was performed in the NaCl-KCl-K2HfF6 (10 wt%)-NaF (5 wt%) melt at the cathodic current density 5–100 mA cm−2 and temperatures 973, 1023 and 1073 K. The influence of the current density and temperature on the roughness of hafnium coatings was studied. It was determined that hafnium coatings with significantly higher purity than that of the hafnium anode were obtained confirming that metal impurities were removed during electrodeposition. The oxidation kinetics data were obtained on a sample with hafnium and a sample of pure molybdenum, which were tested in a water/oxygen mixture at 773 K. The hafnium coating improves the corrosion resistance of coated plates in comparison with that of pure molybdenum.

Electrodeposition of Tantalum Coatings on Nitinol Stents

The Ni-Ti shape memory alloys are currently a topic of notable interest in medicine. They provide a unique opportunity to make novel surgical implants and instruments for vascular and orthopedic surgery. However, a high nickel content in these alloys could cause the problem of biocompatibility because of nickel toxic effect. In turn, a tantalum is successfully used in medicine as a wire or sheet. It does not irritate the living tissue and does not harm the functioning of the organism, but it has a high specific weight. Therefore, using of tantalum coatings on various materials is more appropriate than utilization of a bulk metal. Thus, it is necessary to improve a corrosion resistance of nickel-titanium alloys (nitinol) that can be achieved with applying of protective porousless tantalum coating.For obtaining of tantalum coatings the NaCl-KCl-NaF(10 wt%)-K2TaF7(10 wt%) melt was used at a temperature 1023 K. Direct (DC), unsteady state (USC) and pulsed current (PC) were supplied for electrodeposition. A cathodic current density was changed from 5 to 200 mA/cm2 at the galvanostatic mode. The same cathodic current densities and the time of electrolysis (tel = 0.5-5.0 s) with pauses (tp = 1-5 s) were used at unsteady state and pulsed electrolysis. It was found that a pulsed electrolysis allow to produce coatings with lower roughness than galvanostatic electrolysis.Electroreduction of tantalum complexes in chloride-fluoride melt at a nitinol electrode was investigated. A voltammetric curve registered at a nitinol electrode except of the electroreduction peak of tantalum fluoride complexes to tantalum has several peaks corresponding to the formation of intermetallic compounds of nickel and tantalum.In the case of formation intermetallic compounds between a substrate and coating is an important aspect of the coating adhesion to substrate. The adhesion was measured by cross-sections method using a tester of adhesion Elcometer 107. The measurements showed that coatings obtained by DC and USC could be classified as adhesion to international standards ISO (1) and ASTM (4B). At the same time, coatings deposited on nitinol by PC have a maximum class of adhesion ISO (0) and ASTM (5B). These results were obtained for intermetallic compounds with a thickness around 1-2 μm.Additional experiments were carried out for obtaining of thick layers of intermetallics and study their composition by SEM and EDX.The porosity of tantalum coatings was determined by Erhard’s technique based on measuring a current dissolution at a certain potential, in which dissolves a substrate material and coatings remain passive.Corrosion resistance of the composition tantalum coating-nitinol was studied in different corrosion media.

Synthesis of Chromium Silicides in Ionic Melts

We study the currentless diffusion saturation, metal-thermal reduction, and the electrochemical behaviors of chromium- and silicon-containing melts and establish the conditions of synthesis of chromium silicides in the form of superfine powders and coatings. It is shown that the role of limiting stage of the process of diffusion saturation of chromium, molybdenum, and tungsten with silicon in chloride-fluoride melts is played by the diffusion of silicon atoms in the solid phase. In the course of currentless transfer of silicon to metals of group VIB, silicide coatings are formed on their surfaces. The combined reduction of chromium (III) chloride and sodium fluorine-silicate by metallic sodium or magnesium makes it possible to obtain silicide powders. We formulate the conditions required for getting powders of chromium silicides by electrolysis of chloride-fluoride melts containing fluorine-silicate and potassium chromate. The oxidation resistance of the obtained silicide powders is explained by the formation of silicon-oxide layers on their surfaces.

Influence of the Second Coordination Sphere on the Roughness of Niobium and Tantalum Coatings Obtained in Chloride-Fluoride Melts

The influence of the second coordination sphere complexes on the roughness of niobium and tantalum coatings was studied. It was found that the discharge potential of niobium oxofluoride complexes has a more positive potential than fluoride ones in the NaCl-KCl-NaF (10 wt%)-K2NbF7 melt. For the СsCl-CsF (10 wt%)-CsNbF6 molten system the inversion of potentials corresponding to discharge of oxofluoride and fluoride complexes was observed. Due to micropassivation by oxygen impurities as niobium suboxides and oxygen-niobium solid solution the coating roughness in the NaCl-KCl-NaF (10 wt%)-K2NbF7 (8 wt%) melt diminishes in comparison with СsCl-CsF (10 wt%)-CsNbF6 (8 wt%) melt. In molten salts NaCl-KCl-NaF (10 wt%)-K2TaF7 and СsCl-CsF (10 wt%)-CsTaF6 tantalum oxofluoride complexes are discharged at more negative potentials than fluoride ones and changes in the composition of the second coordination sphere from sodium to cesium led to a lower level of roughness. For melts containing in the second coordination sphere cesium cations, the roughness of the tantalum coatings was less than that of niobium due to the greater strength of the fluoride complexes of tantalum. It was found that pulsed current electrolysis for deposition of niobium and tantalum coatings has significant advantages in comparison with direct current electrolysis.

Electrochemical Production of Tungsten Powders from Tungsten Hardmetal Waste

Hardmetals intended to perform in harsh operating conditions are developed with the use of metallic powders, in particular, tungsten powders. The growing consumption of tungsten in industry necessitates the improvement of techniques employed to produce it from industrial waste. The objective of this research effort was to reduce metallic tungsten (as a powder) from chloride and chloride–fluoride melts and determine the factors influencing its particle size. The starting material was tungsten oxide (WO3) produced electrochemically from processed raw material (VK-6 tungsten carbide pseudoalloy). The melts were KCl–NaCl–CsBr and NaCl–KCl–CsBr–Na3WO3F3 eutectic mixtures. When the melt with a high WO2F42− content was subjected to electrolysis, tungsten precipitated as a superfine powder. The precipitate morphology showed that the tungsten particle size was dependent on electrolysis conditions. A relatively inexpensive and effective method of producing tungsten powders was developed upon the research.

Effect of the Technological Parameters of the Aluminothermic Reduction of Erbium Oxide in Chloride–Fluoride Melts on the Transition of Erbium to a Master Alloy

The methods used for the preparation of an Al–Er master alloy as a promising addition to aluminum alloys for enhancing their physical and mechanical characteristics are analytically reviewed. Thermodynamic analysis of the processes that occur during master alloy fabrication in chloride–fluoride melts containing erbium oxide is carried out. Since the data on erbium complexes are lacking or restricted, the thermodynamic values are determined by indirect methods. The study of the phase composition of the chloride–fluoride melts shows that the melting of a salt mixture results in the fluorination of erbium oxide and the formation of the KEr3F10 complex, which is a precursor for erbium reduction and intermetallic compound Al3Er formation. The effect of the flux composition and the technological parameters of aluminothermic reduction on the transition of erbium to the master alloy is studied.

Features of Phase Formation During Aluminothermal Preparation of Aluminum-Erbium Master Alloy

A lack of domestic technology for preparing aluminum-erbium master alloy intended for improving the physicomechanical properties of aluminum alloys gives rise to the importance of creating technology for aluminothermal reduction of erbium compounds from chloride-fluoride melts. Thermodynamic analysis of aluminothermal reduction of erbium fluoride and oxide taking account of intermetallic compound Al3Er formation makes it possible to determine the thermodynamic possibility of the process. The phase composition of fluxes used for preparing master alloy of the Al–Er system from erbium fluoride and oxide is studied. Experimental data are provided for aluminothermal preparation of Al–Er master alloy from erbium fluoride and oxide. The effect of mixing melt and preliminary remelting a mixture of salts and zernolite, and the structure and phase composition of the master alloy specimens obtained are studied.