Corrosion-electrochemical behavior of zirconium in molten alkali metal carbonates

The corrosion and electrochemical characteristics of zirconium during its interaction with molten lithium, sodium, and potassium carbonates containing from 1 to 5 wt % additives to the salt phase are studied in a temperature range of 500–800°C using gravimetry, corrosion potential measurement, and anodic polarization. The substances decreasing the corrosion losses due to the strengthening and thickening of an oxide film (lithium, sodium, potassium hydroxides) are used as passivators. Sodium chloride, fluoride, and sulfate serve as corrosion stimulators (activators).     

Occlusion and ion exchange in the molten (lithium chloride-potassium chloride-alkali metal chloride) salt + zeolite 4A system with alkali metal chlorides of sodium, rubidium, and cesium

Interaction between molten salts of the type LiCl-KCl-MeCl (Me = Na, Rb, Cs, x _MeCl = 0 to 0.5, x _KCl/x _LiCl = 0.69) and zeolite 4A have been studied at 823 K. The main interactions between these salts and zeolite are molten salt occlusion to form salt-loaded zeolite and ion exchange between the molten salt and salt-loaded zeolite. No chemical reaction has been observed. The extent of occlusion is a function of the concentration of MeCl in the zeolite and is equal to 11±1 Cl⁻ per zeolite unit cell, (AlSiO₄)₁₂, at infinite MeCl dilution. The ion-exchange mole fraction equilibrium constants (separation factors) with respect to Li are decreasing functions of concentration of MeCl in the zeolite. At infinite MeCl dilution, they are equal to 0.84, 0.87, and 2.31 for NaCl, RbCl, and CsCl, respectively, and increase in the order Na<Rb<Cs at identical MeCl concentrations. The standard ion-exchange chemical potentials are equal to −(0.0±0.5) kJ·mol⁻¹, −(0.4±0.3) kJ·mol⁻¹, and −(6.5±0.5) kJ·mol⁻¹ for Na⁻, Rb⁻¹, and Cs⁺, respectively.     

Electrode process of La(III) in molten LiCl-KCl

The electrode process of La(III) at Mo electrode in the molten LiCl-KCl for temperatures ranging from 683 K to 773 K was studied by cyclic voltammetry and chronopotentiometry, respectively. The results showed that in the molten LiCl-KCl, reduction of La(III) occurred in a step with a global exchange of three electrons. Cyclic voltammetry studies indicated that at a sweep rate lower than 0.2 V/s, the electroreduction of La(III) to lanthanum metal was reversible and controlled by diffusion of La(III). However, the process became under a mixed control of both diffusion and electron transfer when sweep rate exceeded 0.2 V/s. And an empirical temperature dependence of the diffusion coefficient of La(III) was proposed: lnD_La(III)=7.742–1.441×10⁴/T. And the relation between the formal potential of La(III)/La versus Ag/AgCl reference electrode and temperature was described in the following equation: E^θ'=–1.402–4.689×10²/T.     

Electrochemical formation of Mg-Li-Y alloys by co-deposition of magnesium, lithium and yttrium ions in molten chlorides

An electrochemical approach for the preparation of Mg-Li-Y alloys via co-reduction of Mg, Li, and Y on a molybdenum electrode in LiCl-KCl-MgCl2-YCl3 melts at 943 K was investigated. Cyclic voltammograms (CVs) illuminated that the underpotential deposition (UPD) of yttrium on pre-deposited magnesium led to the formation of a liquid Mg-Y alloy, and the succeeding underpotential deposition of lithium on pre-deposited Mg-Y led to the formation of a liquid Mg-Li-Y alloy. Chronopotentiometry measurements indicated that the order of electrode reactions was as follows: discharge of Mg(II) to Mg-metal, electroreduction of Y on the surface of Mg with formation of ε-Mg24+xY5 and after that the discharge of Li+ with the deposition of Mg-Li-Y alloys. X-ray diffraction (XRD) indicated that Mg-Li-Y alloys with different phases were formed via galvanostatic electrolysis. The microstructure of different phases of Mg-Li-Y alloys was characterized by optical microscope (OM) and scanning electron microscopy (SEM). The analysis results of inductively coupled plasma atomic emission spectrometer (ICP-AES) showed that the chemical compositions of Mg-Li-Y alloys corresponded with the phase structures of the XRD patterns, and the lithium and yttrium contents of Mg-Li-Y alloys depended on the concentrations of MgCl2 and YCl3 .     

Capacitance of the double electrical layer on the copper-group metals in molten alkali metal halides

The electrochemical impedance is measured to study the capacitance of the double electrical layer of metallic Au, Ag, and Cu as a function of potential and temperature in nine molten salts, namely, the chlorides, bromides, and iodides of sodium, potassium, and cesium. The C–E curve of a gold electrode has an additional minimum in the anodic branch. This minimum for silver is less pronounced and is only observed at low ac signal frequencies in cesium halides. The additional minimum is not detected for copper in any salt under study. This phenomenon is explained on the assumption that the adsorption of halide anions on a positively charged electrode surface has a predominantly chemical rather than an electrostatic character. The specific adsorption in this case is accompanied by charge transfer through the interface and the formation of an adsorbent–adsorbate covalent bond.     

Electrochemical studies on cerium(III) in molten fluoride mixtures

This study aims to determine the principal electrochemical characteristics of the electrodeposition of cerium metal from molten fluoride systems. The cathodic process of Ce³⁺ ions in LiF-NaF and LiF-NaF-CaF₂ molten salts was studied using electrochemical techniques as steady state and cyclic voltammetry methods. The decomposition potential (E_d) and the overvoltage(η) were determined for NaCeF₄ using current-potential curves under galvanostatic conditions. The E_d was found to be 2.025 V in LiF-NaF and 2.045 V in LiF-NaF-CaF₂. It was also found that the ohmic drop potential (EΩ) was not dependent on NaCeF₄ concentration and it rose as the current intensity increased. The overvoltage (η) was determined from the polarization curves and the Tafel coefficients and kinetic parameters were calculated on the assumption that the process constitutes of direct discharge of Ce³⁺, with no solvent-solute interaction. In order to elucidate the cathodic process the investigation by cyclic voltammetry technique was finally used. From the evolution of the voltammograms we concluded that the electrochemical reduction of Ce³⁺ ion was actually a reversible process on the molybdenum electrode and cathodic reduction of Ce³⁺ took place in one single step involving three electron exchanges.     

Formation of oxide layers on aluminum, niobium, and tantalum in molten alkali metal carbonates

The electrochemical synthesis of niobium, tantalum, and aluminum oxide nanolayers is studied in the melt of lithium, sodium, and potassium carbonates with various additives to a salt phase in an oxidizing atmosphere at a temperature of 773 and 873 K. A scheme is proposed for high-temperature anion local activation of the process.     

Wetting of molybdenum with molten Cu-O alloys

Translated from Poroshkovaya Metallurgiya, No. 10 (346), pp. 72–78, October, 1991.     

Metallization of diamonds in a gaseous atmosphere of molybdenum chlorides

Conclusions Thermodynamic calculations have shown that coatings can be deposited on diamonds in a gaseous phase of molybdenum chlorides at temperatures below 1273°K under conditions preventing graphitization of diamonds. In an experimental study it was found that deposition for O.25–2.0 h in the temperature range 973–1273°K resulted in the formation of 0.03-to 1.15-m-thick coatings on diamonds. No graphite was detected.Translated from Poroshkovaya Metallurgiya, No. 4(256), pp. 40–44, April, 1984.     

Effect of the electrolyte composition on the corrosion of 12Kh17 steel in molten alkali metal carbonates

The corrosion of 12Kh17 steel in the melts of alkali metal carbonates containing additions of various chemical types is studied using anodic potentiostatic polarization. The morphology of the 12Kh17 steel surface is analyzed by electron-probe microanalysis. The introduction of corrosion passivators and activators affects the mechanism of local corrosion damages. The corrosion of the steel proceeds according to an electrochemical mechanism. Carbonate anions serve as oxidizers of the steel along with oxygen dissolved in the melt. Treatment of a corrosive medium can be used to protect metallic constructions against corrosion in high-temperature salt electrolytes. Combined introduction of corrosion activators and passivators in a salt phase makes it possible to form protective layers with a high corrosion resistance.     

Equilibria between ferrous and ferric chlorides in molten chloride salts

Equilibria between ferrous and ferric chlorides in molten salts have been studied for improving magnesium electrolysis and molten salt chlorination. The apparent equilibrium constants,K, of reaction FeCl_2(melt)+0.5Cl_2(gas)=FeCl_3(melt) were obtained. Measured values ofK were in good agreement with computed ones from regression equations. The composition of the melts, the partial pressure of chlorine, and the temperature were found to have important effects onK, and the effect of dissolved iron was smaller than that of other factors. At identical other conditions, the largest values ofK were observed in system 3, which suggested that the current efficiency for electrolysis of MgCl₂ should be lower when carnallite was used as electrolyte and that catalysis of iron species in molten salt chlorination would be better when molten salt systems containing high potassium chloride were used.     

Luminescent properties of dysprosium(III) ions in LaAlO_3 nanocrystallites

The absorption and emission spectra as well as decay time profile of Dy3+ ions in LaAlO3 nanocrystals were analyzed.The crystal structure of LaAlO3 was confirmed from XRD measurement.The emission peaks from blue to red came from main emitting level of dyspro-sium 4F9/2 to the ground and other excited levels of Dy3+ ions.Cross relaxation process led to non-radiative quenching of luminescence,so that the lifetime of the 4F9/2 energy level ions decreased with increasing amount of doped Dy3+ ions.The cross rela...     

Electrical conductivity of individual molten rare-earth element chlorides: III. Structure of the melts and electricity transport mechanism

Reported data are analyzed, and the structure of individual molten rare earth metal chlorides is found to represent a loose ionic network with a local octahedral configuration of every cation. Electricity in such melts is carried by both Ln³⁺ cations and Cl_- anions. Electrotransport occurs via the following two main mechanisms: translational motion of individual ions and translational transfer in which several ions simultaneously hop into neighboring equilibrium positions.     

Electrochemical reduction of arsenic (III) with cadmium metal

The kinetics of aqueous arsenic (III) reduction to amorphous arsenic with cadmium metal are presented. Two reaction mechanisms are observed in this system. With arsenic concentrations greater than approximately 0.2 gm/1 in solutions containing one molar sulfuric acid, the reduction of arsenyl ion on the cadmium surface is rate controlling. When arsenic concentrations are less than this value, reaction rate is mass-transport controlled. The reduction of arsenic (III) with cadmium powder at pH 2 is also presented.