Molten Salts

The electrochemical properties of solution of molybdenum ions in molten alkali halide electrolytes were studied at 1023-1123 K. The chemical and electrochemical stability of solutions is an important parameter, which must be controlled to ensure reproducible electrochemical measurements. Polarization and voltammetric curves at tungsten electrode revealed the existence of a reversible electrochemical process between the species Mo(III) and Mo(0) with a standard potential equal to 812 mV vs. Ag/AgCl reference electrode. The reduction process Mo(III) to Mo(0) appears as a single reversible three-electron exchange couple with significant crystallization problems.

In the present paper we studied the conditions of obtaining sintered ceramic anodes and we tested their behaviour in a laboratory aluminium electrolysis cell. The material chosen for this study consisted of SnO 2 doped of the optimum composition from the viewpoint of the densification and electrical conductivities was 96 wt% SnO 2 + 2 wt% Sb 2 O 3 + 2 wt% CuO. Starting from the studies of granulometry and the pressability curves, cylindrical electrodes were obtained by isostatic pressing. In order to establish in which case the sintering process was complete, a structural analysis of two samples (S 1 and S 2 ) was made by SEM. The fine granulation of the powder for S 2 generates a higher grade of compactness by pressing and so a better qualitative synterizing is achieved. In order to observe their behaviour the ceramic materials (S 1 , S 2 ) were tested in aluminium electrolysis cell. Good results for current efficiency, corrosion, anodic polarization and overvoltage were obtained. Long term aluminium electrolysis has shown that the current and voltage varied in very close limits, which means that those anodes give good results from the point of view of the stability of electrolysis parameter.

In this paper are presented studies on SnO 2 -based ceramics a new possible material for inert anodes. Results on structure and behavior of this material are discussed. By constructing the x-y-z diagram of ternary SnO 2 -Sb 2 O 3 -Cu 2 O -O system, we could separate 5 monovariant equilibriums with SnO 2 -solid solution and vapour. From the compositions range of the ternary SnO 2 -Sb2O3-CuO system we choose only 2 for the electrochemical studies. The measurements of decomposition potential (E dec ), current efficiency, cyclic voltammetry (CV) and corrosion were used to investigate these materials as inert anodes in a cryolite-alumina melt used for aluminum Hall-Heroult process at 970-1000°C. From the obtained data we conclude that the anodic process on these anodes takes place by a very simple process with no secondary reactions. Low alumina concentrations led to catastrophic corrosion of these anodes and the corrosion rate was found to increases with increasing current density. The corrosion rate of SnO 2 -based anodes showed only a slight increase (of 0.0004 gcm 2 h ) when the anode-cathode distance was decreased from 5 to 2 cm. The mechanism of corrosion was elaborated by correlating the corrosion data with solubility data of SnO 2 -anodes in the Hall-Heroult bath. * Corresponding author: popescuamj@yahoo.com using an non-consumable anode the emission of the green gases CO 2 , CO, CF 4 , C 2 F 8 and most of the sulfurous gases (SO 2 ,COS, CS 2 , H 2 S) will be also eliminated and the process of producing aluminum will be an ecological one.

The errors in viscosity measurements by the oscillating cup method were calculated as a function of the limit of accuracy imposed by the uncertainty in determining the constants of the oscillating system R, I, T 0 , δ 0 and the limit of precision resulting from errors in determining the experimental parameters δ , T , h, ρ.

Some types of anodes that could replace the usual carbon anodes in aluminium production by the Hall-Heroult process are based on SnO 2 . The present investigation deals with SnO 2 -Sb 2 O 3 -CuO ceramics having an SnO 2 content of 96%), Sb 2 O 3 and CuO being dopants. The ceramic pellets, thermally treated at 1400 C for 4 hours, were analysed by X-ray diffraction and IR spectroscopy. The structural analysis of the samples evidenced an SnO 2 (ss) type solid solution. All samples were electronically conductive (small negative values of the Seebeck coefficients), having an activation energy ranging within 0.02 -0.3 eV.

The electrical conductivity, density, and viscosity of the molten MgCl 2 --CaCl 2 -NaCl-KCl quaternary system have been investigated over a temperature range of 100 К in the composition range of 6-18 mass % MgCl 2 . Very low temperature coefficients and smooth isotherms were ob tained over the whole temperature and respectively composition range investigated. Frenkel equation was also verified, which proves that the acting forces upon the moving ions are mutually compensated in the mix tures. An optimum composition of the electrolyte for magnesium electrowinning was recommended.

The current state of research on the titanium and boron electrochemistry as well as high-temperature electrochemical synthesis of titanium diboride and titanium intermetallic compounds with the iron triad metals is analyzed. The prospects for the development of these studies are discussed. Primary attention is paid to the electrochemical properties and stability of halide systems containing titanium in various oxidation states together with boron species.

This work continued our general research program on obtaining metallic cerium by electrodeposition from NaCeF 4 dissolved in different molten fluorides. The structure of NaCeF 4 (cubic or hexagonal depending on the way of preparation) was established by DTA analysis, IR spectra and X-ray diffraction. The heat capacity (C p ) of NaCeF 4 was measured by differential scanning calorimetry in the temperature range of 300-1093 K using the "step-method". The C p was fitted by an equation with a satisfactory result. Heat capacity was compared with that calculated from the Neumann-Kopp rule (NKR) and the deviations observed were consistent with the stability of the NaCeF 4 compound.

Electrolytic deposition of molybdenum and tungsten boride particles from ionic melts is studied. Conditions are found for preparing different boride phases. If the anode material is graphite and the voltage in the bath does not exceed 2.5 V the cathode deposit consists mainly of molybdenum and tungsten metals. A mixture of phases (M, M 2 B, MB, MB 2 , M 2 B 5 ), is produced on the cathode with U = 2.5 ÷ 3.5 V while with U = 3.5 ÷ 4.5 V the deposit consists of the B higher content boride MB 4 . On the whole, the process of electrochemical synthesis for molybdenum and tungsten borides is governed by the following interconnected parameters: electrolytic bath composition, voltage applied to the bath, temperature, and duration of electrolysis.

This study aims to determine the principal electrochemical characteristics of the electrodeposition of cerium metal from molten fluoride systems. The cathodic process of Ce 3+ ions in LiF-NaF and LiF-NaF-CaF 2 molten salts was studied using electrochemical techniques as steady state and cyclic voltammetry methods. The decomposition potential (E d ) and the overvoltage( ) were determined for NaCeF 4 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-CaF 2 . It was also found that the ohmic drop potential (E ) was not dependent on NaCeF 4 concentration and it rose as the current intensity increased. The overvoltage ( ) 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 Ce 3+ , 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 Ce 3+ ion was actually a reversible process on the molybdenum electrode and cathodic reduction of Ce 3+ took place in one single step involving three electron exchanges.

This study deals with the obtaining of corrosion protection Zn-Ni-P films deposited on steel products. The protective thin layers were deposited by both electrochemical and electroless processes in aqueous solutions. The development of such deposition techniques of films with high mechanical and chemical resistance will contribute to the gradual replacement of the conventional corrosion protection method through thermal zinc plating. The obtained Zn-Ni-P films have been characterized by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) in an attempt to understand the chemistry and structure of the films before and after the corrosion process. The corrosion behaviour in 3.5 wt % NaCl solution of different samples of thin Zn-Ni-P films deposited on steel (which differ by the way of preparation) was determined by open circuit potential vs. time curves, linear polarization and Tafel plots and compared with the behaviour of steel support. The corrosion parameters were calculated and the obtained results were correlated with the micrographic images of the studied samples. The influence of Zn/Ni ratio and P content of the alloy on the corrosion protection properties was evaluated.

The present paper is a preliminary study on preparation and properties of some choline chloride based ionic liquids. Three mixtures (1:2 molar ratio) of choline chloride (2-hydroxy-ethyl-trimethyl-amonium chloride) with urea, malonic acid and citric acid were prepared and characterized. The obtained ionic liquids were clear and colorless at room temperature. Some physical properties, namely, density, electrical conductivity and viscosity of these mixtures were investigated as a function of temperature within a of 25-80 o C. The Arrhenius theory and the Frenkel equation were used for understanding the transport phenomena in these ionic liquids.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were used for studying both cathodic and anodic processes on Pt electrode in a choline chloride containing ionic liquid with copper (II) ions. The investigations were conducted at 343K and 353K temperatures in choline chlorideurea (1:2 molar ratio) deep eutectic solvent in which CuCl 2 (anhidrous) salt was dissolved. The electrodeposition processes of copper ions at different concentrations (0.05-0.38M) and scan rates ) were evidenced by CV technique. Supplementary measurements were performed for determining the electrochemical window for background electrolyte. Experiments with either CuCl salt or CuCl 2 salt introduced in the same choline chloride-urea deep eutectic solvent have shown identical shapes of voltammograms, proving a disproportionation process between

Tin dioxide based ceramics, for their excellent corrosion resistance and heat resistant properties, are considered to be promising good materials for manufacturing non-consumable electrodes, in order to replace the carbon anodes in industrial aluminum electrolysis. Our studies in this latter field were focused on the SnO2-based ceramics doped with Sb2O3 and CuO. Different samples of this material were obtained by ceramic method as cylindrical pellets. By X-ray diffraction, and Scanning Electron Microscopy the phase composition and microstructure of the studied materials were investigated. Electrical conductivity was performed in the 293-1273K temperature range into the especial laboratory cell. The electrochemical behavior of tin dioxide -based ceramic as inert anodes was examined in a laboratory scale aluminum electrolysis cell by anodic polarization and preliminary cyclic-voltammetry measurements. The obtained results allow to select the ceramic material to be used as inert electrode in aluminium electrolysis.

Electroless Zn-Ni-P thin film alloys deposits on steel substrate from chloride bath (ZnCl 2 , NiCl 2 ) containing ammonium chloride as a buffer agent and sodium citrate as a complexing agent were investigated. The effects of deposition conditions (temperature, bath composition, time) on the final deposit composition were studied. It was found that the percentages of zinc and phosphorus in the obtained Zn-Ni-P alloys never reach high values. Some physical characteristics such as morphology and corrosion properties of Zn-Ni-P coatings were assessed. Corrosion tests were done in 3.5 wt.% sodium chloride aerated solution. Corrosion rate values were determined to be 0.1283-0.0297 mm·year -1 comparative to 0.2104 mm·year -1 for the uncoated steel substrate.The surface analysis of the thin film samples before / after corrosion was performed by X-ray photoelectron spectroscopy (XPS) in an attempt to understand the chemistry of the obtained thin films.

In order to make reasonable selection of systems for tungsten-molybdenum alloy electrodeposition, effects of molybdenum (VI) oxide concentration and cationic composition of melt on cathode product nature and characteristics were studied. Experiments were carried out in Me 2 WO 4 -Na 2 MoO 4 melts (Me= Li,Na,K and Rb) at 1173 K with cathode current density equal to 5 A/dm 2 . It was found that, for high MoO 3 concentration in melt, the cathodic deposit was molybdenum (VI) oxide instead of metallic molybdenum. The lowest concentration of MoO 3 deposition changes from 2.0 to 8.5 mol% as alkaline metal cation radius increases in the following sequence: Li 2 WO 4 < Na 2 WO 4 <K 2 WO 4 <Rb 2 WO 4 . Irrespective of the cationic composition, with MoO 3 concentration lower than 1.0 mol. %, Mo-W alloys (with 1-5 at. % W content) were deposited at the cathode. At higher MoO 3 concentrations (but still lower than the lowest concentration of MoO 3 deposition), metallic molybdenum deposit occurs. Thus, MoO 3 concentrations ranges for pure molybdenum obtained are 1-2, 1-4.5, 1-7, and 1-8.5 mol. % for melts based on Li 2 WO 4 , Na 2 WO 4 , K 2 WO 4 , and Rb 2 WO 4 , respectively. Effects of MoO 3 concentration, temperature, and cathodic current density on structure and composition of deposited molybdenum-tungsten alloys were also investigated. It was found that Mo content in alloy increases with MoO 3 concentration and with decrease of both WO 3 concentration and cathodic current density. Finally, some SEM determinations were done on the the obtained deposits of W-Mo alloy.