204 publications from this institution
Corrosion resistance of VDM Alloy С-4, Hastelloy G-35 and ХН62М (KhN62M) nickel based alloys, and AISI316L type steel was studied in alkali chloride melts containing uranium (III) chloride and metallic zinc. These alloys are considered as prospective construction materials in pyrochemical reprocessing of spent nitride nuclear fuels including decladding fuel rods by liquid zinc dissolution process. Rates of corrosion of these materials were determined at 550 °C.
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Indirect methods for determining K : Al mole ratio in molten chloroaluminates were proposed to obtain correct data and perform possible required adjustment of the acid-base properties of KCl-AlCl 3 electrolytes. Potentiometric calibration curves for different compositions of chloroaluminate melts were obtained at temperatures ranging from 350 to 450 o C. It is shown that cyclic voltammograms measured in melts with different K : Al mole ratios also can be used for estimating acidity/basicity of KCl-AlCl 3 melts. Full profile Rietveld analysis of X-ray diffraction patterns was suggested for characterization of quenched chloroaluminate electrolytes.
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-AlCl 3 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 AlCl 3 content in the melt leads to increase of the red-ox potential due to formation of Al 2 Cl 7 - 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 AlCl 3 hydrolysis and result in decreasing «actual» aluminum chloride concentration in the melt. At increasing K : Al mole ratio (for example, due to evaporation of AlCl 3 ) 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-AlCl 3 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-AlCl 3 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 o C. They strongly depend on acid-base properties of KCl-AlCl 3 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-AlCl 3 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 Al 2 Cl 7 - species leads to the appearance of additional maxima on the voltammograms. The influence of AlCl 3 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 AlCl 3 phases (present in small amounts) with the lines of major KAlCl 4 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.
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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.
THE DENSITY AND ELECTRICAL CONDUCTIVITY OF MOLTEN SALT MIXTURES OF BERYLLIUM FLUORIDE WITH ALKALINE METALS CHLORIDE
The results of spectroscopic studies of Nb anodic dissolution, dissolution of NbCl5 and reaction of various niobium oxides with HCl in different chloride melts at 450-7500C are presented. Nb(III) species, NbCl63-, is the major product of niobium anodic dissolution at current densities up to 40 mA/cm2. Addition of NbCl5 to alkali chloride melts predominantly leads to the formation of Nb(V) species, NbCl6-. Upon increasing Nb(V) concentration NbCl6- tend to decompose to Nb(IV) and Cl2. Reaction of NbO and NbO2 with HCl proceeds via an intermediate oxygen-containing Nb(IV) complexes. Oxygen-free Nb(IV) ions, NbCl62-, were obtained by exchange reactions between metallic niobium and melts containing Bi(III), Ag(I) or Ni(II) chloro-ions. Electroreduction of Nb(III) on a tungsten working electrode below -1.45 V vs. Cl2/Cl- led to the formation of metallic niobium, no evidence of intermediate reduction products was obtained.
A novel method of forming niobium-based coatings by currentless deposition in molten salts was developed. The protective coatings were produced on nickel and various nickel-based alloy substrates. The corrosion resistance of the obtained coatings was tested and characterized in fused niobium- and uranium-containing chloride electrolytes at different temperatures. The method of protecting nickel-based alloys by currentless niobium deposition can be used in industrial technologies of rare earth and refractory metal electrowinning from molten salt baths.
No abstract is provided for this article.
Electrode processes in 3LiCl–2KCl–UCl3 melts were studied at different temperatures and uranium concentrations using a variety of electrochemical techniques. It was found that the anodic process leads to the formation of U(IV) species while uranium metal is the only product of uranium(III) ions electroreduction. Characteristic potentials and limiting current densities were determined both for potentiostatic and galvanostatic regimes of uranium electrodeposition. Varying uranium concentration and temperature did not influence the mechanism of the cathodic reaction.
