Pierre Chamelot

Using square-wave voltammetry to monitor molten alkaline fluoride baths for electrodeposition of niobium

This study implemented an electrochemical analysis method, square-wave voltammetry, to monitor the molten fluoride (LiFNaF) baths which are used for electrolytic deposition of niobium. This method makes it possible to perform qualitative and quantitative analyses of the niobium species in situ at different degrees of oxidation and to determine the content of oxygenated niobium compounds. Square-wave voltammetry was first compared with the cyclic voltammetric technique and then its applicability area was defined in terms of frequency range. Using the standard addition method, the calibration straight lines were constructed giving the species reduction peak differential current densities leading to the metallic deposition of niobium and of the niobiums oxyfluorinated compound according to their respective concentrations and the disproportionation constant for NbIV into NbV and Nb was estimated for a temperature of 780°C. Finally, an overall mass balance was performed on the niobium to verify the coherence of the results obtained in this study on the system in equilibrium.

Voltammetric studies of tantalum electrodeposition baths

Abstract Deposits of tantalum were prepared at 780°C by reduction of K 2 TaF 7 , the solvent being a eutectic LiF-NaF mixture. Cyclic voltammetry and square-wave voltammetry were used to demonstrate the competition exerted on the reduction of the TaF 2− 7 ions leading to deposition of tantalum oxide by reduction of the tantalum oxyfluoride ions [TaOF ( n−3 ) n , probably with n = 5]. The use of square-wave voltammetry enabled the attribution of a peak to each of the reactions and, from the peak current produced by TaF 2− 7 its calibration curve to be established. This curve was used in monitoring the level of TaF 2− 7 during the deposition process.

Studies of Niobium Electrocrystallization Phenomena in Molten Fluorides

The electrodeposition of niobium from NbCl{sub 5} or K{sub 2}NbF{sub 7}, dissolved in molten alkaline fluorides (LiF/NaF), in the 700--800 C temperature range has been studied by using cyclic voltammetry, chronopotentiometry, and chronoamperometry. A linear relationship between the Nb{sup IV} diffusion coefficient and the temperature was found in the cyclic voltammograms and chronopotentiograms. Scanning electron micrography shows that the shape of the nuclei is hemispherical. Chronoamperometric results show that the deposition process involves instantaneous nucleation with diffusion-controlled growth of the nuclei. The influence of temperature, overpotential, and substrate (copper, stainless steel, vitreous carbon) on the nuclear site densities is also considered.

Oxidation of Stoichiometric Nickel Ferrites Used as Inert Anodes for Aluminium Electrolysis in Molten Cryolite Mixtures

Thanks to their good electronic conductivity and their low solubility in cryolite melts, nickel ferrites are considered to be among the most suitable ceramic materials that could be used as inert anodes in the electrowinning of aluminium. In this work, electrodes composed of single phased and stoichiometric nickel ferrite NixFe3-xO4 (0≤x<1) have been studied by electrochemical techniques (linear voltammetry and potentiostatic electrolysis) in a molten cryolite mixture at 960°C. The aim was to understand the oxidation reactions susceptible to take place inside the material under anodic polarization and oxygen evolution. Ex situ characterization of the electrodes by SEM-EDX and microprobe analysis allows proposing a degradation mechanism of the pure nickel ferrites: the formation of haematite Fe2O3 at the grain boundaries was evidenced and it resulted in a slow degradation. The influence of the Ni content in the ceramic phase was investigated, and it was shown that rich-Ni compositions exhibit a better resistance to corrosion.

Electrochemical study of the corrosion of metals in molten fluorides

Resistance to corrosion of the structural materials is a key factor for nuclear applications that use molten fluorides. Low chromium, nickel-base alloys are regarded as the most suitable metallic materials. In a first approach, corrosion of some pure metallic constituents Ni, Mo, W and Fe, was studied by electrochemical techniques. Linear voltammetry was applied in LiF-NaF and LiF-AlF3, in the temperature range 900-1100°C. The relative stability of the metals in LiF-NaF is established. To determine the corrosion current density, three methods are presented, two based on the Tafel extrapolation method and the third one being the polarization resistance method. Results regarding corrosion rates are compared. Two corrosion behaviors are observed: on the one side, Ni, Mo and W and on the other side Fe. The difference might come either from different corrosion mechanisms or from a different number of exchanged electrons. The corrosion rate increases with temperature following the Arrhenius law. However, further experiments are needed in order to identify the key parameters that influence the corrosion in the different melts.

Niobium Electrodeposition in Molten Fluorides Using Pulsed Electrolysis

that the nucleation pro-cess of niobium in molten LiF-NaF is instantaneous, three-dimen-sional, and closely controlled by linear diffusion. Under these con-ditions a columnar structure of coarse grains is expected. This situa-tion prevents obtaining a coating with a compact structure withoutdendrites if too high a current density is used. Consequently, indus-trial applications of the process would be hindered by the requiredslow deposition rates.However, pulsed electrolysis may be explored to improve theprocess, since pulsed electrodeposition of various metals in aqueousmedia have given good results in terms of high electrodepositionrates and smoothness of the coatings.

Lanthanides Extraction Processes in Molten Fluoride Media

Abstract This chapter presents a scope of different pyrochemical processes (precipitation, electrolysis on inert electrode, liquid or solid reactive electrodes, or codeposition) in molten fluorides in view of lanthanide extraction. The lanthanide extraction has two major interests, which are primary preparation of lanthanide metals or alloys and nuclear spent fuel reprocessing. The different studies summarize the extraction feasibility and provide extraction efficiency evaluation. Precipitation with oxide ions, codeposition with Al(III), and reduction on copper reactive cathode appear to be the most promising processes.

Silver as anode in cryolite—alumina-based melts

The anodic behaviour of silver was investigated in cryolite—alumina-based melt. Silver has a lower melting point (ca. 960°C) than the other metals considered as possible inert materials for aluminium electrolysis. The working temperature used in aluminium industry is approximately 960°C, depending on the melt composition. Therefore, the stability of silver during the anodic process was tested at 870°C in an acidic electrolyte consisting of 65.5 mass % Na3AlF6 + 22.9 mass % AlF3 + 5.7 mass % CaF2 + 3.9 mass % LiF + 2 mass % Al2O3 with the melting point ca. 850°C. The electrolyte without alumina was prepared as well, with the melting point ca. 860°C. The resulting cryolite ratio (CR = n(NaF)/n(AlF3)) for both electrolytes was equal to 1.6. The behaviour of the silver anode was studied by voltammetry measurements. The electrochemical study showed that an oxidation reaction occurred at a potential below the oxygen evolution potential. Silver was not found to be stable under oxygen evolution. The degradation of the silver anode was apparent after electrolysis.

Technology of Electrodeposition from Molten Salts for the Preparation of Electrode Materials Based on Refractory Metals

This paper reviews all the characteristics of surface coating by refractory metals or alloys including Molten Salts Electrodeposition: Selection of the fluorides as electrolyte; experimental conditions of obtaining massive coatings or alloys; applications of the coatings, focused on the preparation of electrode materials. According to the results in the literature, it seems that the process in some cases can be competitive with other techniques.