Anodic Dissolution of Metals in Oxide-Free Cryolite Melts

The anodic behavior of metals in molten cryolite-alumina melts has been investigated mostly for use as inert anodes for the Hall–Héroult process. In the present work, gold, platinum, palladium, copper, tungsten, nickel, cobalt and iron metal electrodes were anodically polarized in an oxide-free cryolite melt (11%wt. excess AlF₃ ; 5%wt. CaF₂) at 1273 K. The aim of the experiments was to characterize the oxidation reactions of the metals occurring without the effect of oxygen-containing dissolved species. The anodic dissolution of each metal was demonstrated, and electrochemical reactions were assigned using reversible potential calculation. The relative stability of metals as well as the possibility of generating pure fluorine is discussed.     

Specificity of anodic processes in cyclic voltammetry to the type of carbon used in electrolysis of cryolite-alumina melts

Anodic processes associated with oxidation of carbon anodes used in electrolysis of cryolite-alumina melts, simulating the Hall-Héroult process, were studied by means of cyclic voltammetry in a comparative way at four graphitic carbon materials and at glassy carbon. Conditions were sought that give a current response function characteristic of diffusion-controlled oxidation of the anode by O^2– or oxyfluoride complex anions. Only at glassy carbon anodes are such conditions realized with a linear relation between response current in cyclic voltammetry and Al₂O₃ content in the melt. At the graphitic materials, complex mixed activation and diffusion controlled processes arise that are also relatively irreproducible from one experiment to the next, probably due to irreversible changes of the graphite surfaces. The effects of aluminium metal dissolved in the melt, to simulate practical smelter cell conditions, were also evaluated.     

Aluminium electrodeposition from AlCl3-NaCl melts on glassy carbon,platinum and gold electrodes

The electrochemical deposition and dissolution of aluminium on glassy carbon, platinum and gold electrodes in chloraluminate melts have been investigated using linear sweep voltammetry and potentiostatic pulse techniques. It was shown that deposition of aluminium on the glassy carbon electrode at low overpotentials takes place by 3-D progressive nucleation and growth, with the incorporation of atoms in the crystal lattice as the rate-determining step. At overpotentials higher than –100 mV vs Al, in the melts containing more than 52 mol % of AlCl₃, diffusion of Al₂Cl ₇ ^– , takes over the control of deposition of aluminium. Alloying of platinum and gold electrodes with aluminium from the melt occurs in the underpotential region.     

Electrochemistry of metals and semiconductors in fluoride media

The electrochemical surface transformations and diverse applications of a variety of metals and semiconductors in a wide range of fluoride media such as aqueous, non-aqueous media, liquid HF media, room temperature fluoride melts and molten fluoride media with a melting range covering 50–1000°C are reviewed. Nickel shows excellent corrosion resistance in the absence of water. The anodic performance of this metal in electrochemical perfluorination and NF₃ production is discussed. Compact carbon materials serve as anodes in fluorine generators. In high temperature melts, they perform as consumable anodes. Graphitic carbon undergoes intercalation/de-intercalation process and related battery applications. Cu/CuF₂ couple is a good reference electrode. Pt and vitreous carbon materials are the inert electrodes of choice for electro analytical applications. Electrodeposition of Lithium as a non-dendritic uniform phase is important in Lithium metal based secondary batteries. High temperature fluoride melts are used in electro-deposition of valve metals such as Nb, Ta, and Ti. The stability and decomposition of fluoride complexes in these media are of interest.     

Electrochemical deposition of aluminium on tungsten in cryolite based melts

Aluminium deposition and redissolution on tungsten electrodes in cryolite melts of different NaF/AlF₃ ratios have been investigated by potential sweep and galvanostatic techniques. The amount of charge necessary for anodic reoxidation of deposited aluminium was determined as well as the rate of spontaneous corrosion. The qualitative results obtained can be interpreted by assuming a mass transport controlled loss reaction involving metal dissolution in the melt.     

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 behaviour of titanium(II) and titanium(III) compounds in molten lithium chloride/ potassium chloride eutectic melts

The nature of the open circuit potentials of Ti and TiC electrodes in titanium(II) and titanium(III) chloride solutions, the apparent valency of the titanium ions in the melt immediately after the anodic dissolution of TiC and Ti, as well as the anodic dissolution of Ti, have been investigated in order to explain the oxidation-reduction process of titanium in lithium chloride-potassium chloride eutectic melts. It is shown that the standard electrode potential for TiC/Ti(III) exceeds that for the oxidation of Ti(II) to Ti(III). The anodic dissolution of TiC anodes give stable Ti(III) species at the standard electrode potentials the contrary to the behaviour of Ti metal anodes where the stable species are Ti(II) ions. Titanium electrodes in TiCl₃ solutions of molten lithium chloride-potassium chloride melts behave according to the Ti(III)/Ti(II) redox electrode potential of the reaction: Ti+TiCl₃3TiCl₂. An anodic dissolution mechanism compatible with all the experimental facts is proposed.     

Anodic oxidation behaviour of Al–Ti alloys in acidic media

The anodic oxidation behaviour of Al–Ti alloys in several acidic solutions was investigated. The influence of conditions such as alloy composition, electrolytic solution, electrolyte temperature and formation current density on the formation rate of oxides on Al–Ti alloys and the dielectric properties of the anodic films were analysed. It was shown that the oxide formation rate for the Al–Ti alloy containing 54 at % aluminium was the highest and the dielectric property of its anodic oxide was also the best. In addition, by means of several surface analytical techniques, the chemical composition of the films were determined as (TiO₂) _n (Al₂O₃) _m . AES (Auger electron spectroscopy) profiling analysis data showed that Al–Ti alloys had preferential oxidation behaviour, that is, the aluminium was oxidized preferentially.     

The mechanism of electrodeposition of acrylic resin on aluminium

A mechanism for the electrodeposition of acrylic resin on aluminium is proposed, based on experimental studies of acid value, anodic gas evaluation and anodic film resistance. The mechanism can be expressed as Alf Al³⁺ + 3e 2Al³⁺ + 3H₂Of Al₂O₃ + 6H⁺ 2Al³⁺ + 6H₂Of 2Al(OH)₃ + 6H⁺ H⁺ + RCOC^– f RCOOH. This is different from the mechanism for zinc and steel, where it is metal ions from anodic dissolution which neutralize the macro-ions and cause a deposit on the anode surface.     

On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.     

Behaviour of tungsten carbide hydrogen-diffusion electrodes in chloride etching solutions

Studies were performed of tungsten carbide hydrogen-diffusion electrodes operating as anodes in electrolytic baths for regeneration of etching solutions of CuCl₂ and FeCl₃. Under conditions of electrolytic regeneration of copper chloride solutions (i = 40 mA cm^–2, 40° C) after 1500 h operation the electrode polarization increased by about 200 mV. Maximum current efficiency of 60–65% was obtained at I _k = 80 mA cm^–2. It is demonstrated that the replacement of the standard carbon anodes with tungsten carbide hydrogen-diffusion electrodes and the elimination of the ion exchange membrane separating the anodic from the cathodic space leads to a 2–4 V decrease of the electrolytic bath voltage. The regenerated solutions of CuCl₂ and FeCl₃ can be reused as etching agents after adding 7–10 ml 30% solution of hydrogen peroxide per litre.     

Aluminium deposition and dissolution in aluminium chloride—n-butylpyridinium chloride melts

Glassy carbon and tungsten electrodes were used to investigate the mechanism of aluminium deposition and dissolution reactions in the AlCl₃—n-butylpyridinium chloride melts at ambient temperature. Chronopotentiometric results indicate that the electrodeposition of aluminium from the melt is kinetically complicated. Current reversal chronopotentiometry showed that the corrosion rate of aluminium is linearly proportional to the acidity of the melt at a given temperature. Besides impurities, the major cause of the corrosion of the deposited aluminium over a wide composition range of the melt was also found to be due to the organic butylpyridinium cation (BuPy+) serving as an electron acceptor in the corrosion process when freed from ion-pair interaction with AlCl⁻₄.     

Electrolytic reduction of oxygen at solid electrodes in aprotic solvents-the superoxide ion

Cyclic voltammetric behavior of oxygen reduction to superoxide ion in DMF and CH₃ CN in presence of tetrabutyl ammonium hexafluorophosphate as supporting electrolyte at glassy carbon, rhodium, palladium, lead and tungsten electrodes was investigated. The first reduction step to superoxide ion was found to approach reversibility with an average rate constant of 2.2 × 10^?2 cms^?1 at the glassy carbon electrode whereas at rhodium and palladium, the behavior was irreversible exhibiting high cathodic currents on the anodic scans which presumably is caused due to surface disproportion of superoxide ion at these electrodes. At lead and tungsten electrodes one irreversible reduction step was observed in both aprotic solvents.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

Aluminium dissolution in NaF-AlF3-Al2O3 systems

The rate of dissolution of electrolytically deposited aluminium was determined by the method of current reversal chronopotentiometry at a tungsten electrode in NaF?AlF₃?Al₂O₃ melts of varying NaF/AlF₃ molar ratios or cryolite ratios (CR). The temperature was maintained at 1031±3°C and the alumina content at 4 wt%. More accurate data were obtained by introducing delay times of various lengths (at zero current) between the cathodic and anodic current pulses, compared to direct current reversal chronopotentiometry with varying forward (deposition) times. The rate of aluminium dissolution increased with increasing NaF/AlF₃ molar ratio, the curve showing an inflexion in the vicinity of CR=3. This inflexion indicates two dissolution mechanisms, one being predominant depending on the CR. The main reaction in acidic melts (CR<3) may be represented by $$2Al(l) + AlF_6^{3 - } \rightleftarrows 3Al(I)F_x^{1 - x} + (6 - 3x)F^ - $$ while in basic melts (CR>3) $$Al(l) + 3Na^ + \rightleftarrows 3Na(soln) + Al(III)$$ is the likely dominant mechanism. For 0.8?7 mol cm^?2s^?1.     

The electrochemical and electrocatalytic behaviour of glassy metals

The suitability of a selection of amorphous alloys as electrocatalysts or as inhibitors for hydrogen evolution (HE) was investigated in 1 m KOH at 25 °C. Mild basic conditions were chosen so as to make direct comparison with other data, where available. The alloys studied were the known glassy alloys Fe₆₇Co₁₈B₁₄Si₁, Co₆₆Fe₄Si₁₆B₁₂Mo₂, Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆ and an entirely new glassy alloy Zr_73.22Ti_19.71Cu_1.24Fe_5.83. The electrochemical techniques of slow sweep anodic and cathodic polarisation were used, in conjunction with the surface analysis techniques of scanning electron microscopy (SEM) and X-ray analysis, to characterise the alloys and new data has been obtained for all alloys. The glassy alloys were tested in their as-polished state, as well as after surface activation, by ex situ chemical (acid etching) and in situ electrochemical (anodic oxidation in base) pre-treatment. The least corrosion resistant composition, Fe₆₇Co₁₈B₁₄Si₁, displayed the highest activity for HE in the as-polished state and only a minor improvement resulted from surface pre-treatment. Corrosion resistance was partly characterised by the degree to which the passive region increased and the passive region current decreased as a function of pre-treatment. The most corrosion resistant alloy, Zr_73.22Ti_19.71Cu_1.24Fe_5.83, displayed the poorest activity for HE in the as-polished state, but a significant improvement resulted from surface activation by in situ anodic oxidation in basic media. Surface activation by acid pre-treatment reduced the corrosion resistance of the Zr_73.22Ti_19.71 Cu_1.24Fe_5.83 alloy and was, therefore, a non-viable and destructive procedure. However, acid pre-treatment was effective in substantially activating the glassy Co₆₆Fe₄Si₁₆B₁₂Mo₂ and Fe₄₀Ni₄₀P₁₄B₆ alloys towards HE and did not alter the corrosion properties of these compositions. A novel technique for mounting thin alloy specimens has been developed, using an insulating photo-resist coating, resulting in sharply defined electrode edges.     

Oxide anodes in electro-organic oxidation. Oxidation of maleic addon tungsten oxide anodes

The electrochemical oxidation of maleic acid on tungsten anodes has been investigated. Glyoxal and carbon dioxide were the main products together with tartaric acid and acetaldehyde. Glyoxal is also obtained as the main product from the oxidation ofd-tartaric acid. Under the same conditions succinic acid is completely oxidized to carbon dioxide and water. The anodic dissolution of tungsten and the oxidation of water to oxygen become predominant in the final stages of the electrolyses.     

The electrolysis of Al2S3 in AlCl3-MgCl2-NaCl-KCl melts

The electrolysis of Al₂S₃ has been investigated in AlCl₃-MgCl₂-NaCl-KCl melts. Experimental results obtained at 1023 K show that dissolved Al₂S₃ can be electrolysed to give aluminium and elemental sulphur. It has been found that the limiting current density for the electrolysis of Al₂S₃ in these melts is that at the anode and it increases with increasing Al₂S₃ concentration. It appears that within the current density range 0.2 to 1.2 A cm^–2, the cathodic current efficiency of the electrolysis of 5 wt % Al₂S₃ does not vary significantly with the current density. Within that current density range, current efficiencies of about 75 to 85% are obtained.     

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.     

Double-layer capacitance and polarization potential of baked carbon anodes in cryolite-alumina melts

Baked carbon anodes with varying apparent densities and baking temperatures were tested in Na₃AlF₆–Al₂O_3(sat) melts at 1010° C. The double-layer capacitance (C _dl) was used as an indicator of the wetted surface area. For unpolarized anodes,C _dl increased with increasing time of immersion and reached a constant level after 1.5–2h. The values decreased with increasing polarization potential in the range 1–1.5 V positive to aluminium. TheC _dl of polished samples increased markedly during electrolysis, particularly at low current densities. No clear correlation was found betweenC _dl and apparent density. Semi-logarithmic plots of potential versus current could be divided into three segments. The lower two were linear, the ranges and slopes being 0.01–0.1 A cm^–2, 0.20–0.44 V per decade and 0.1–0.5 A cm^–2, 0.18–0.24 V per decade, respectively. At higher current densities the curves bent upwards. The current density corresponding to an overpotential of 0.5 V increased slightly with increasing apparent density, whereas the ohmic voltage drop. at constant current density decreased. The current densities were corrected for differences in wetted surface area on the basis of theC _dl data. The change in baking temperature from 970 to 1100°C had no appreciable effect on the overpotential, whereas samples baked at 1250°C showed a somewhat lower overpotential.