Pierre Taxil

Preparation of oxygen evolving electrodes with long service life under extreme conditions

Among the numerous base metals tested for DSA® type electrodes (e.g., titanium and its alloys, zirconium, niobium etc.), tantalum is a potentially excellent substrate owing to its good electrical conductivity and corrosion resistance, and the favourable dielectric properties of its oxide. Nevertheless, a DSA® type electrode fabricated on a tantalum substrate would be very expensive due to the high cost of the metal. To prepare an anode combining the excellent properties of tantalum at reasonable price, a new material has been developed in our laboratory. This consists of a common base metal (e.g., Cu) covered with a thin tantalum coating. This tantalum layer was obtained by molten salt electroplating in a LiF–NaF–K2TaF7 melt at 800°C. Thus, an anode of the type Metal/Ta/Ta2O5–IrO2 with a surface load of 22gm-2 IrO2, submitted to the severe test conditions used in this work, exhibits a standardized lifetime tenfold greater than one made with ASTM grade 4 titanium base metal. Thus, this type of electrode might be advantageously employed as an oxygen evolution anode in acidic solutions.

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.

Formation of corrosion-resistant layers by electrodeposition of refractory metals or by alloy electrowinning in molten fluorides

The electrolytic treatment of less resistant metals such as iron, copper and nickel with tantalum or niobium has been carried out in K2TaF7-LiF-NaF or K2NbF7-LiF-NaF solutions in the 550 to 1050°C temperature range. Two kinds of experiments have been used.(i)At lower temperatures, electroplating with pure tantalum and niobium on inert cathodes was performed. The electrodeposition mechanism of each metal was studied and coherent electroplates were prepared which were tested in electrocatalytic applications.(ii)At higher temperatures (850–1050°C), using nickel cathodes, intermetallic compounds were obtained at more positive potentials than that for pure electrodeposition (Ta2Ni, TaNi, TaNi2, TaNi3, NbNi, NbNi3). The electrowinning of stable TaNi3 and NbNi3 layers was carried out by the metalliding process which makes these materials resistant to corrosion in various media. Further, a study of the kinetics of growth of the diffusion layer allowed a diffusion parameter to be determined which was in agreement with other results obtained by conventional methods.

Tantalum protective thin coating techniques for the Chemical Process Industry : Molten salts electrocoating as a new alternative

Abstract A comparison of corrosion resistance and basic properties of solid tantalum with other high-performance materials used in the Chemical Process Industry (CPI) is given. The corrosive chemicals taken into consideration are strong acidic media. Secondly, it is pointed out that tantalum, which exhibits excellent corrosion resistance, owing to a rapid build-up of passivating protective film in oxidizing conditions, also has good mechanical, thermal and electrical properties which suggest its use when little or no metallic corrosion is tolerated. Thirdly, tantalum thin-layer, coated onto a usual base metal, which offers the same protection as solid metal and avoids its expensive use, is treated. Fourthly, numerous tantalum-coating techniques for clad-vessel and CPI devices are reviewed and compared. Amongst these coating techniques, this paper focuses mainly on two techniques which give a very thin, protective coating against corrosion. Thus, Chemical Vapor Deposition (CVD) and Molten Salt Electro-deposition (MSE) are especially enhanced. Finally, MSE which is still not widely used for manufacturing clad-vessels is examined in greater detail.

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.

Electrochemical behaviour and codeposition of titanium and niobium in chloride–fluoride melts

Abstract Niobium–titanium alloys can be prepared by codeposition in NaCl–KCl–NaF melts. The electrochemical behaviour of niobium and titanium is investigated first separately, then together, using the following methods: linear sweep voltammetry, semi-integration and semi-differentiation of cyclic voltammograms, and square-wave voltammetry. The reduction process of niobium (V) and titanium (IV) are, respectively, Nb V →Nb IV →Nb and Ti IV →Ti III →Ti. Each of the first steps proceeds in the same potential range whereas niobium metal is obtained at more positive potential than titanium. In the case of mixtures of Ti and Nb ions in the melt, the electrochemical curves derive from the addition of each separated system and show three peaks (Nb V →Nb IV and Ti IV →Ti III ; Nb IV →Nb; Ti III →Ti) in the cathodic sense. Codeposition of Nb–Ti alloys was prepared by galvanostatic electrolysis and further analysed with a microprobe.

Electrolyte and water transfer through the porous electrodes of an immobilised-alkali hydrogen-oxygen fuel cell

This study concerns the performance of a hydrogen-oxygen fuel cell with an immobilized alkaline electrolyte, intended for use in space applications. After optimization of the hydrogen flow rate allowing the precise removal of the water produced by the overall reaction of the cell, the question of electrolyte transfer at the cathode is approached. During long-term operation of the cell, the reaction brings about the accumulation of OH− ions in the reaction zone and the expulsion of electrolyte into the oxygen compartment, known as the weeping effect. It is shown that this phenomenon can be avoided by dampening the oxygen at the cell inlet. The moisture level is a critical factor and optimization studies are also necessary if the performance of the cell is to be maintained.

The preparation of corrosion-resistant layers by the electrolytic deposition of tantalum on nickel and stainless steel

Abstract The deposition of tantalum by electrolysis in molten fluorides is applied to stainless steel and nickel. The deposit is made at 800°C and is controlled by measuring the cathode potential. Only with deposits on nickel is surface alloy formation observed. This effect is not observed in the case of deposits on stainless steel.

Preparation of uranium by electrolysis in chloride melt

The uranium deposits were prepared by electrolysis in a fused chloride melt. Whatever the electrolysis conditions, the deposit morphology was dendritic. The influence of the parameters current density, melt temperature and electrolyte concentration on the morphology was investigated using an optimal design. Our results show that increasing the temperature makes the coatings more compact. The dendritic morphology is attributed to transport phenomena (diffusion, convection, migration) promoted by the cation discharge at the substrate-coating interface.