Electrodeposition of titanium from chloride melts

The kinetics of the electrodeposition and electrocrystallisation of titanium were studied in alkali chloride melts. Voltammograms showed two distinct anodic peaks for the oxidation steps Ti/Ti(II) and Ti(II)/Ti(III) at 70 mV and 300 mV, respectively, referred to titanium. The cathodic reduction Ti(III)/Ti(II) was very irreversible, showing an extended cathodic wave and a shoulder on the Ti(II)/Ti reduction peak. The Ti(II)/Ti reduction was found to be quasi reversible. The rate constant at 450'C was 5 × 10^–3 cm s^–1 . The diffusion coefficient in KCl-LiCl eutectic was 1 × 10^–5 cm²s^–1 at 450° C rising to 3.5 × 10^–5 cm²s^–1 at 650° C. Potential step measurements gave curves indicating slow instantaneous nucleation and growth (I against t ^1/2) followed by slow diffusion control (I against t ^–1/2). During constant current steady state deposition the nature of the deposit depended on the composition of the melt, the temperature and the cd. In KCl-LiCl melts coherent deposits were obtained for cds of 60–120 mA cm² while higher current densities gave dendritic and spongy deposits.     

Study of Ti(III) solutions in various molten alkali chlorides. I. Chemical and electrochemical investigation

The effect of alkali metal cations in molten chloride baths on the behaviour of Ti(III) in these melts has been studied. With caesium chloride the percentage of total dissolved titanium present as Ti(II) after 72 hours at 700° C is 0.1%, whereas in lithium chloride it is 2%. In the presence of excess titanium metal the proportion of Ti(II) is always much higher. In LiCl it depends strongly on the area of titanium exposed and may reach 85%. In CsCl, on the other hand, this factor has no effect since a fine dispersion of powdery titanium appears at the very beginning of the reaction. The amount of Ti(II) remains close to 50%. In all the baths studied, the electrochemical reduction of Ti(III) at 700° C occurs in two steps: Ti(III) + e Ti(II)E _1/2 = – 1.6V versus Cl^–/Cl₂ Ti(II) + 2e Ti(O)E _1/2=–2.1V versus Cl^–/Cl₂. These results are different from those previously obtained in CsCl-LiCl (40–60 mol%) at 400° C, where reduction is a one-step process.     

Role of indium in promoting anodic dissolution of Al-In alloys in non-aqueous electrolyte

Exploratory work on the anodic dissolution behaviour of aluminium and aluminium binary alloys in electrolytes in non-aqueous organic solvents is reported. Commonly used electrolytes for non-aqueous battery systems were selected on the basis of their conductivities and activities for anodic dissolution of Al and Al-alloy anodes. It is found that Al–In alloy electrodes exhibit an exceptionally active anodic dissolution behaviour in a 1 M solution of AlCl₃ in anhydrous acetonitrile. The steady-state Tafel polarization plots for dissolution of pure Al, Al–Sn, Al–Ga and Al–In alloy anodes are compared, and a.c. impedance spectra for an Al–In alloy anode in 1 M solution of AlCl₃ in CH₃CN are evaluated and discussed. The In component, like Ga or Hg, interferes with passivation of Al during its anodic dissolution and thus promotes an active condition on the metal surface leading to relatively high anodic dissolution current-densities at substantially negative electrode potentials.     

High-temperature chlorine corrosion of technical carbons Part II. Anodic corrosion in chloride melt

Medium (200 to 400°C) to high (600 to 800°C) temperature corrosion of technical carbons (Acheson graphites) have been investigated in alkali chloride melts at chlorine evolving anodes. At low temperature in chloride melts containing free Lewis acid (AlCl₃) no chlorine is evolved — even at high current densities — because chlorine, together with aluminium chloride, instantaneously form intercalation compounds with graphite and, as a consequence, the carbon desintegrates very rapidly. At 200°C carbon is consumed anodically in a C/Cl of molar ratio 70/1. With increasing temperature Scheson graphites become more stable so that at 700°C short term destruction cannot be observed in melts which contain free Lewis acid. Chlorine corrosion of carbon electrodes in purified basic alkali chloride melts, which are free of oxygen carriers and, in particular, free of water at temperatures between 600 and 800°C in basic chloride melts, is an electrochemical reaction proceeding at low current densities slower than anticipated from thermodynamic data for carbon chlorination equilibria. The anodic carbon corrosion reaction has an activation energy of only 50 kJ mol^–1 and its rate increases with increasing anode potential, or anodic current densities (rate: exp (i)). At a technical current density of 0.4 A cm^–2 at 700°C the corrosion rate is estimated to be of the order of centimeters per year, rendering carbon anodes dimensionally unstable. Most important is to note that apart from CCl₄, chlorinated carbon compounds (olefins and arenes) are generated as side-products which are noxious and ecologically dangerous and must not be released from processes which use carbon anodes for chlorine evolution from salt melts.This paper is dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.     

Electrochemical studies of the reaction between titanium metal and titanium ions in the KCl-NaCl molten salt system at 973 K

The reaction between titanium metal and titanium ions in the KCl?NaCl molten salt system was investigated by means of electrochemical and physical methods at 973 K in an inert gas atmosphere (argon). It was found that the reaction between titanium metal and Ti³⁺ in the molten salts with TiCl₃ followed a simple reaction (I). However, in the case of the concentration of K₂ TiF₆ being higher than 2.7 mol % the reaction was (II); in the case of it being less than 0.8 mol %, reactions (III) and (III′) were followed. $$\begin{gathered} 2Ti^{3 + } + Ti = 3Ti^{2 + } (I) \hfill \\ 3Ti^{4 + } + Ti = 4Ti^{3 + } (II) \hfill \\ Ti^{4 + } + Ti = 2Ti^{2 + } (III) \hfill \\ 3Ti^{2 + } = Ti + 2Ti^{3 + } (III')(on cooling) \hfill \\ \end{gathered} $$ It was also found that these reactions were controlled by charge transfer and diffusion simultaneously.     

Dissolution of BaZrO3 refractory in titanium melt

Although numerous investigations have studied BaZrO₃ as a crucible refractory for melting titanium alloys, the interaction mechanism between them has not been clarified. In this study, a set of three designed alloys with different Ti composition (TiNi, Ti_1.5Ni, and Ti₂Ni) were melted in the BaZrO₃ crucibles. By using the X‐ray diffraction, optical, and scanning electron microscopy analysis, the interactions between the BaZrO₃ crucibles and the titanium melts were investigated. It was found that dissolution of the BaZrO₃ refractory into the titanium melts resulted in the crucible erosion and the melt contamination, the degree of which were both increased with the increasing of Ti content in the melts. The dissolution reaction could be determined as follows: . The oxygen content dissolved in TiNi, Ti_1.5Ni, and Ti₂Ni melts was thermodynamically calculated as 0.0055, 0.1922, and 0.2263 wt%, respectively, which were in agreement with the experimental results.     

Electrochemical study of titanium in chloride-fluoride melts

The study of electrochemical reduction of titanium in molten NaClKClK₂TiF₆ by means of linear sweep voltammetry and the quasi-steady-state method has revealed that the reduction of Ti(IV) to Ti proceeds reversibly in two consequent steps

The temperature dependence of the equilibrium constant for the metal—salt interaction is established. The effect of the fluorine ion concentration on the interaction mechanism of Ti metal with chloride—fluoride melts containing tetravalent ions of titanium is considered.     

Corrosion protection of steel in molten Li2CO3–K2CO3 and Na2CO3–K2CO3 mixtures in a hydrogen-containing atmosphere

The electrochemical behaviour of TiN-, TiN–AlN-, Cr- and CrN-coated 316L stainless steel in molten Li₂CO₃–K₂CO₃ and Na₂CO₃–K₂CO₃ melts in a reducing gaseous atmosphere (10% H₂–90% N₂) was studied using voltammetry and scanning electron microscopy combined with energy-dispersed X-ray analysis in the temperature range of 600–730 C. To facilitate the identification of the electrochemical reactions the voltammetric behaviour of stainless steel, titanium, nickel and gold was also investigated. Voltammetric characteristics obtained at AlN–TiN coated electrodes showed no anodic reactions at potentials more negative than that of CO^2– ₃ oxidation. Cr- and CrN-coated electrodes demonstrated a suppressed anodic dissolution after the first steady state voltammetric cycle. The voltammograms obtained for the other electrodes studied displayed the corresponding anodic metal-dissolution waves. TiN, AlN, Cr and CrN coatings seem to be the most promising as corrosion-resistant materials for the anodic compartments of molten carbonate fuel cells.     

Anodic dissolution of titanium in NaCl-containing ethylene glycol

Anodic dissolution behavior of titanium in NaCl-containing ethylene glycol has been examined to obtain electropolished titanium surface. During anodic polarization in 1 mol dm^?3 NaCl ethylene glycol solution at 293 K, the titanium electrode covered with oxide dissolves with gas evolution at potentials higher than 10 V (Ag/AgCl) while it is in passive state at potentials lower than 5 V. However, after removal of the oxide layer by pre-polarization at gas-evolving potentials, no gas evolution is observed, and the titanium electrode shows a limiting dissolution current as tetravalent species at potentials higher than 5 V, producing a smooth surface. The polarization of the rotating disk titanium electrode reveals that the kinetics of the mass transfer reaction for electropolishing of titanium is controlled by titanium species dissolved into the solution, not by chloride ions or water containing in the solution. Repetition of dynamic polarization gives a well-electropolished surface.     

Electrochemical treatment of effluents: A preliminary study of anodic oxidation of simple sugars using lead dioxide-coated titanium anodes

The electrolysis of simple sugar solutions, containing 0.5% NaCl or Na₂SO₄, using lead dioxide-coated titanium mesh anodes in a simple electroflotation cell indicated that electrode corrosion and dissolution of polluting Pb²⁺ ions from the electrode material were negligible even after prolonged use; the sugars were concomitantly electrooxidised at such anodes. The rate of electrooxidation followed linear removal kinetics and the relative ease of electrochemical destruction was in the order sucrose>maltose～glucose. Such oxidation processes represent added advantages in the use of these electrodes in electrochemical treatment of effluent containing the sugars. Cyclic voltammetric analysis of the glucose solution indicated that oxygen evolution on the lead dioxide-titanium anode appeared to be promoted by the presence of the sugar. A much more detailed study is required to give further insight into the mechanism of the anodic oxidation of these compounds.     

Solvent extraction separation of titanium(IV), vanadium(V) and iron(III) from simulated waste chloride liquors of titanium minerals processing industry by the trialkylphosphine oxide Cyanex 923

The solvent extraction of magnesium(II), aluminium(III), titanium(IV), vanadium(V), chromium(III), manganese(II) and iron(III) from hydrochloric acid solutions has been investigated using the trialkylphosphine oxide Cyanex 923 (TRPO) in kerosene as extractant. The results demonstrate that titanium(IV), vanadium(V) and iron(III) are extracted into kerosene as TiOCl₂·2TRPO, VO₂Cl·TRPO and HFeCl₄·2TRPO, respectively. On the other hand magnesium(II), aluminium(III), chromium(III) and manganese(II) are not extracted with TRPO from hydrochloric acid solutions (1.0–4.0 mol dm^?3) under the experimental conditions. IR spectral studies of the extracted complexes were further used to clarify the nature of the extracted complexes. The effect of the diluent on the extraction of titanium(IV), vanadium(V) and iron(III) has been studied and correlated with the dielectric constant. The loading capacity of the TRPO system has been evaluated and the potential for the separation and recovery of titanium(IV), vanadium(V) and iron(III) from simulated waste chloride liquors of the titanium minerals processing industry has been assessed. Copyright © 2004 Society of Chemical Industry     

Comparison of the behaviour of glassy carbon and some metals for use as nonconsumable anodes in alumina-cryolite melts

Current interest exists in development of nonconsumable anodes for the Hall-Heroult process of aluminium production and also in situ analytical probes for determination of Al₂O₃ content in the cryolite melts used in this process. A comparison of the behaviour of glassy carbon and metals such as tungsten, tungsten carbide, nickel and stainless steel (SS-316) used as anodes in alumina-cryolite melts is investigated by means of electrochemical transient techniques (cyclic voltammetry and chronoamperometry) and Tafel anodic polarization experiments. The results show that only glassy carbon could be used as a successful sensor electrode for an in situ determination of Al₂0₃ in alumina-cryolite melts and that the metals investigated are unresistant to anodic attack in such melts. Consequently, the metals investigated cannot be used as sensor electrodes for in situ electro-analytical determination of alumina in alumina-cryolite melts, nor as anodes in the production of aluminium by the Hall-Heroult process.     

Electrochemical intercalation of aluminium chloride in graphite in the molten sodium chloroaluminate medium

Aluminium chloride intercalation in graphite was studied by anodic oxidation of compacted graphite (rod) and graphite powder electrodes in sodium chloroaluminate melt saturated with sodium chloride at 175 °C. The studies carried out by employing both galvanostatic and cyclic voltammetric techniques had shown that the intercalation reactions take place only beyond the chlorine evolution potential of +2.2 V vs. Al on both the electrodes. The extent of intercalation reaction was directly related to the anodic potential and probably to the amount of chlorine available on the graphite anodes. In the case of graphite powder electrode, a distinctly different redox process was observed at sub-chlorine evolution potentials and this was attributed to the adsorption of chlorine on its high surface area. This finding contradicts a report in the literature that the intercalation reactions occur at potentials below chlorine evolution in the chloroaluminate melt.     

Reactivite electrochimique de quelques oxydes de vanadium en milieu HCl 1 M

The voltammograms recorded for the oxides V₂O₅, NaVO₃, VOSO₄, NaVO₂ and V₂O₃, when incorporated in carbon paste electrodes with conducting binder, show that the electronic transfer occurs between the electrode and the soluble species. The V(V)/V(IV) and V(III)/V(II) systems are reversible and the electrochemical reactions give anodic and cathodic peaks with quite identical potentials, close to that of the standard potential of these systems. The V(IV)/V(III) couple is irreversible and the reduction, shifted towards negative potentials, coalesces with the V(III) to V(II) reduction process. For the oxides containing V(V), the reduction to V(IV) leads to two peaks: the first one is reversible and due to the reduction of the species coming from the instantaneous dissolution of the finest particles, the second one is irreversible and due to the progressive dissolution of the largest particles occuring during the cathodic sweep.     

Fabrication of core-shell structured MWCNT-Ti(TiC) using a one-pot reaction from a mixture of TiCl3, TiH2, and MWCNTs

Multi-walled carbon nanotubes (MWCNTs) coated with metal titanium or carbide layers were prepared by heating a mixture of TiCl₃, TiH₂, and MWCNTs under vacuum. The resulting MWCNT-Ti(TiC) materials had a uniform and conformal core-shell structure. The thickness of the Ti(TiC) layer was determined by the deposition temperature and time. The amount of TiC in the shell depended primarily on the deposition temperature. Whether a metal or carbide coating was obtained was determined by controlling the deposition temperature. Ti(TiC)-coated MWCNTs showed better field emission properties than did pristine MWCNTs. This deposition method should prove to be a versatile route for fabricating other one-dimensional core-shell materials.     

Cr(III) oxidation with lead dioxide-based anodes

A procedure for preparing PbO₂-based electrodes with a titanium substrate is proposed. A platinum underlayer is first deposited on Ti by metal organic chemical vapor deposition (MOCVD), followed by the electrodeposition of the PbO₂ layer. The prepared Ti/Pt/PbO₂ anodes were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) before being used for oxidation of Cr(III) in sulphuric acid. The current efficiency was determined for that electrodes and the results were compared with those obtained with Pb/PbO₂ and Ebonex^®/PbO₂ electrodes with different pH conditions. The Ti/Pt/PbO₂ were found to have a very good electrochemical behaviour (current efficiency: φ=0.93 for pH 2), and may be used as dimensionally stable anodes for the oxidation of Cr(III).     

Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in 1-butyl-3-methylimidazolium chloride ionic liquid

Electrochemical behavior of ruthenium (III), rhodium (III) and palladium (II) in 1-butyl-3-methylimidazolium chloride (bmimCl) and their ternary and binary solutions in bmimCl was studied at various working electrodes at 373 K by cyclic voltammetry and chronoamperometry. Ruthenium (III) chloride forms a stable solution with bmimCl and the cyclic voltammogram of ruthenium (III) in bmimCl recorded at glassy carbon electrode consisted of several redox waves due to the complex nature of ruthenium to exist in several oxidation states. Electrolysis of ruthenium (III) chloride in bmimCl at the cathodic limit of bmimCl (−1.8 V (vs. Pd)) did not result in ruthenium metal deposition. However, it was deposited from bmimPF₆ and bmimNTf₂ room temperature ionic liquids at −0.8 V (vs. Pd). The electrochemical behavior of ruthenium (III) in bmimCl in the presence of palladium (II) and rhodium (III) was studied by cyclic voltammetry. The presence of palladium (II) in bmimCl favors underpotential deposition of ruthenium metal. The nuclear loop at −0.5 V (vs. Pd) was observed in all solutions when palladium (II) co-existed with other two metal ions. Nucleation and growth of the metal on glassy carbon working electrode was investigated by chronoamperometry. The growth and decay of chronocurrents has been found to follow the instantaneous nucleation model with three-dimensional growth of nuclei.     

Corrosion of oxide-coated titanium (DSA) anodes in organic solvents (especially methanol)

In certain organic solvents, notably alcohols (and in particular methanol), DSA-type anodes deteriorate rapidly under anodic conditions. The active RuO₂-based layer is itself quite resistant to corrosion; however, rapid dissolution of the titanium support through pores and cracks in the active oxide coating, eventually results in detachment of the surface layer. A survey of the titanium dissolution reaction in methanol, and (to a lesser extent) other common solvents, is reported first. Reaction was quite rapid in methanolic solutions at low pH, almost negligible in solutions of high pH, and inhibited in the former case on addition of increasing quantities of water or sulphuric acid. In similar experiments with oxide-coated titanium, the rate of metal dissolution in acidified methanol was reduced by addition to TiO₂ to the RuO₂ layer, increasing the active oxide loading, increasing the water or acid content of the solvent, use of low oxide annealing temperature, and operating at low current density. If this type of anode is operated in anhydrous alcohol systems, the use of a noble metal, or noble metal clad titanium, support would be worth considering. Of the other non-alcoholic solvents investigated, acetonitrile and propylene carbonate appeared to be more resistant to oxidation than dimethyl formamide or dimethyl sulphoxide — no trace of titanium dissolution was observed with any of these solvents.     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.