Kinetics of aluminium deposition from aluminium chloride—alkali chloride melts

The kinetics of aluminium deposition from NaClAlCl₃ and NaClKClAlCl₃ melts (c_AlCl3 < 0.4 mol%) was studied by linear sweep voltammetry and potential step amperometry. The reduction of AlCl₃ on tungsten and aluminium electrodes was found to be diffusion controlled. The diffusion coefficients of AlCl₃ were: 3.5 × 10^?5 cm² s^?1 at 820°C in NaClAlCl₃, 2.7 × 10^?5cm²s^?1 at 825°C, and 2.1 × 10^?5cm²s^?1 at 705°C in KClNaClAlCl₃. The rate constant for AlCl₃ reduction at these conditions was found to be in the order of 0.2 cm s^?1, in good agreement with extrapolated literature data.     

Chronopotentiometric study of aluminium deposition from MClAlCl3 melts (MNa,K and Cs)

The kinetics of aluminium deposition from NaClAlCl₃, KClAlCl₃ and CsClAlCl₃ melts (C_AlCl3 = 3 × 10^?5 ?1 × 10^?3 mol cm^?3) was studied using the chronopotentiometric technique. It was found that in the system NaClAlCl₃ the reduction of Al(III) species is controlled by diffusion only, while in the system KClAlCl₃ and CsClAlCl₃, a chemical reaction precedes the reduction step. In the two latter systems the Al(III) in the complex anion AlCl^?₄ can be reduced also directly, if current density is sufficiently high.     

The electrodeposition of aluminium from xylene and ether-hydride electrolytes

A comparative study of aluminium electrodeposition from xylene and ether electrolytes is presented. The kinetic parameters of aluminium deposition from the tetrahydrofuran (THF) and xylene electrolytes are presented. In the case of the THF and diethylether solutions investigations of aluminium nucleation were also made. It was found that the rate of aluminium deposition was highest in the THF electrolyte composed of AlCl₃ and LiAlH₄ in a ratio 1:1. This electrolyte is recommended for deposition of aluminium protective coatings of thickness over 20 µm     

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.     

A promising method for electrodeposition of aluminium on stainless steel in ionic liquid

A promising method for aluminium deposition was proposed by using AlCl₃/[bmim]Cl (1‐butyl‐3‐methylimidazolium chloride) ionic liquid as electrolyte. By using this novel method, the volatile and flammable organic solvent systems and the high corrosive inorganic molten salts with high temperature can be substituted, and the deposit microstructure can be easily adjusted by changing the current density, temperature and electrolyte composition. The study was performed by means of galvano‐static electrolysis and the optimum operating conditions were determined based on the systematic studies of the effects of current density, temperature, molar ratio of AlCl₃ to [bmim]Cl, stirring speed and deposition time on the quality of deposited coatings. The electrical conductivities of electrolytes were also investigated as a function of temperature (298–358 K) and molar ratio of AlCl₃ to [bmim]Cl (from 0.1:1 to 2.0:1). Dense, bright and adherent aluminium coatings were obtained over a wide range of temperature (298–348 K), current densities (8–44 mA/cm²) and molar ratio (1.6:1–2.0:1). Results from the analysis of crystal structure show that all of the electrodeposits exhibit a preferred (200) crystallographic orientation. Temperature has significant influence on the crystallographic orientation and there does not appear to be an apparent impact of current density on it. Analyses of the chronoamperograms indicate that the deposition process of aluminium on stainless steel was controlled by three‐dimension nucleation with diffusion‐controlled growth and there is a conversion from progressive nucleation to instantaneous nucleation. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Vacuum infiltration of copper aluminate by liquid aluminium

This paper studies attained microstructures and reactive mechanisms involved in vacuum infiltration of copper aluminate preforms with liquid aluminium. At high temperatures, under vacuum, the inherent alumina film enveloping the metal is overcome, and aluminium is expected to reduce copper aluminate, rendering alumina and copper. Under this approach, copper aluminate toils as a controlled infiltration path for aluminium, resulting in reactive wetting and infiltration of the preforms.Ceramic preforms containing a mixture of Al₂O₃ and CuAl₂O₄ were infiltrated with aluminium under distinct vacuum levels and temperatures, and the resulting reaction and infiltration behaviour is discussed. Copper aluminates stability ranges depend on vacuum level and oxygen partial pressure, which determine both CuAl₂O₄ and CuAlO₂ ability for liquid aluminium infiltration. At 1100 °C and 0.76 atm vacuum level CuAl₂O₄ is stable, indicating pO₂ above 0.11 atm. Reactive infiltration is achieved via reaction between aluminium and CuAl₂O₄; however, fast formation of an alumina film blocking liquid aluminium wicking results in incipient infiltration. At 1000 °C and 3.8 × 10⁻⁷ atm vacuum level, CuAlO₂ decomposes to Cu and Al₂O₃ indicating a pO₂ below 6.0 × 10⁻⁷ atm; infiltration of the ceramic is hindered by the non-wetting behaviour of the resulting metal alloy. At 1000 °C and 1.9 × 10⁻⁶ atm vacuum level CuAlO₂ is stable, indicating pO₂ above 6.0 × 10⁻⁷ atm. Extensive infiltration is achieved via redox reaction between aluminium and CuAlO₂, rendering a microstructure characterised by uniform distribution of alumina particles amid an aluminium matrix.This work evidences that liquid aluminium infiltration upon copper aluminate-rich preforms is a feasible route to produce Al-matrix alumina-reinforced composites. The associated reduction reaction renders alumina, as fine particulate composite reinforcements, and copper, which dissolves in liquid aluminium contributing as a matrix strengthener.     

A new bath for the electrodeposition of aluminium. II. Kinetics and mechanism of the deposition and dissolution processes

The electrodeposition of aluminium was carried out successfully in AlCl₃-LiAlH₄/tetrahydrofuran-toluene solutions. The kinetic parameters controlling the cathodic deposition and anodic dissolution of aluminium were derived. The effect of the total concentration of aluminium and the molar ratio of AlCl₃ to LiAlH₄ on the current density-potential behaviour of the rotating disc aluminium electrode was also studied. A detailed mechanism for the electrode reactions was suggested.     

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.     

A new bath for the electrodeposition of aluminium. III. The electrochemical behaviour of the rotating aluminium disc electrode in pure AlCl3/THF-toluene solutions

The electrochemical behaviour of the rotating aluminium disc electrode in AlCl₃/tetrahydrofurantoluene solution was studied. The kinetic parameters controlling the electrode processes were investigated. The effect of the concentration of AlCl₃, the speed of rotation of the rotating disc electrode and the temperature on the cathodic and anodic current density was analysed. The results were compared with those for AlCl₃–LiAlH₄/THF-toluene solutions.     

A new bath for the electrodeposition of aluminium. I. conductivity measurements

The conductivity of mixed hydrides of aluminium chloride and lithium-aluminium hydride in a mixed solvent of tetrahydrofuran (THF) and toluene was measured with respect to the total concentration of aluminium and also to the molar ratio of LiAlH₄ to AlCl₃. The values obtained were compared with those of the THF-benzene mixed solvent and those of the NBS (National Bureau of Standards) bath –AlCl₃ and LiAlH₄ in dithyl ether. The results showed that a solution of AlCl₃ and LiAlH₄ with a molar ratio of 3:1, respectively, in THF-toluene (80 vol% toluene) with a total concentration of aluminium of about 1.0 moll^–1, has a suitable conductivity for the electrodeposition and dissolution of aluminium. In addition to its low price, the electrolytic bath obtained has low volatility and relatively good stability with respect to the other baths studied.     

Recovery of high surface area alumina from aluminium dross tailings

Aluminium dross tailings (ADT), a chemical waste from a factory producing aluminium in Egypt, was used for the recovery of the aluminium content in the form of alumina (Al₂O₃). Extraction of the aluminium was carried out via atmospheric‐ and high‐pressure leaching with caustic soda. Then, it was separated from the sodium aluminate solutions thus obtained, using six different precipitation methods. The precipitates were filtered, washed, dried and calcined at 600 °C to produce the test aluminas. The precipitates and the calcination products were analysed for their chemical and crystalline phase compositions. The calcination products (aluminas) were subjected to sorpometry for surface area determination, and to particle sizing. The results indicated that highly pure (SiO₂ < 0.53% and Na₂O < 0.14%), well‐defined crystalline γ‐aluminas of high surface areas (>200 m² g⁻¹) and uniform small crystallite sizes (60–70 Å) could be efficiently recovered from ADT. Optimal recovery conditions were examined and determined. © 2000 Society of Chemical Industry     

The electrodeposition of aluminium on brass from a molten aluminium chloride-sodium chloride bath

The electrodeposition of aluminium on brass (63/37) from a dry AlCl₃/NaCl melt (80wt.% purified AlCl₃, 20 wt.% NaCl) using static as well as rotating cathodes and a variety of conditions has been studied. The current efficiency of the process is very high and the electrodeposits are smooth, fine grained and silver white in appearance. Although at lower temperatures, there is a tendency for flakes to be formed locally these are easily removed and do not affect the overall quality of the electroplate.     

The kinetics of iron dissolution and passivation in solutions containing oxygen

The iron dissolution and passivation was studied in sulphate solution containing different oxygen concentrations and within the range 3 < pH < 6. The measurements were carried out cyclic voltammetrically and under steady-state polarization conditions using a rotating disc electrode of highly pure recrystallized iron. The obtained kinetic data can be explained mainly by the iron dissolution and passivation mechanism proposed previously for oxygen free solutions. This mechanism is characterized by the formation of different oxide intermediates [Fe(OH)_n]_ads, with n = 1,2,3 depending on the pH and the potential. Above a critical pH-value, the diffusion controlled oxygen reduction is the dominating cathodic process, and the anodic iron dissolution and passivation is inhibited by the time-dependent formation of three-dimensional porous oxide layers on the electrode surface. In neutral solutions the iron dissolution and passivation mechanism turns to that encountered in alkaline media.     

Alkali and earth alkaline metal contents in the aluminium cathode in aluminium electrolysis

The contents of sodium, lithium, calcium and magnesium in aluminium in contact with NaF–AlF₃-based melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950–1030 C. The experimental data were compared with a thermodynamic model. It was found that the addition of alumina or CaF₂ to the NaF–AlF₃ melts has only a minor effect on the equilibrium content of sodium in aluminium. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a smaller effect on the concentrations of lithium, calcium and magnesium in aluminium in industrial cells.     

Emf measurements in the NaClAlCl3 system—I. Activity of AlCl3 in basic melts at temperatures close to the liquidus line

Emf measurements in the formation cell Al|AlCl₃(l), NaCl(l)|Cl₂ were carried out at different mol fractions of AlCl₃ in the range 0 < x_AlCl3 < 0.5. The activity of AlCl₃ increases strongly as the equimolar composition is approached, indicating the presence of a fairly stable AlCl^?₄ complex ion in such melts. Liquidus temperatures at various melt compositions were determined from emf-temperature curves as well as from cryoscopic measurements.     

Fabrication of aluminium titanate castable

Abstract

A castable was made using an aggregate of aluminium titanate (AT) of composition 95%Al_1·57 Fe_0·43 TiO₅ , 2% corundum, and 3% aluminosilicate glass with 10% high alumina cement, deflocculant, and water. Cast blocks after firing at 1400°C for 6 h were used to determine physical and mechanical properties at ambient temperature. The castables had a crushing strength of 125 MPa, a bend strength of 14 MPa, and a Young's modulus of 9·6 GPa. They had a coefficient of thermal expansion (CTE) of 1·3 × 10^-6 K^-1 from ambient temperature to 1200°C. The low Young's modulus and CTE indicate that the castable should have excellent thermal shock resistance. The properties of the AT castable when compared with those of commercial high alumina castables showed it to have a similar strength, but much lower porosity (10%), Young—s modulus, and CTE. Phase analysis of the cast blocks showed that the major phase was iron aluminium titanate with minor phases (<10%) of corundum and a calcium aluminosilicate, which is probably a glass.     

Electrode kinetics of the aluminium deposition from tetrahydrofuran electrolytes

The electrolyte applied for the study of aluminium deposition consisted of AlCl₃ and LiAlH₄ in tetrahydrofuran or tetrahydrofuran—benzene mixture. It has been shown that the kinetic parameters such as the exchange current density and transfer coefficients depend to a great extent upon the molar ratio of both components. An electrode mechanism has been suggested in good agreement with the experimental results obtained.     

Modelling of anodic dissolution of pure aluminium in sodium chloride

A mathematical model for simulating a passive aluminium (Al) surface with a pit in which active electrochemical metal dissolution occurs has been developed. The model includes hydrolysis products of Al and the species obtained as a result of homogeneous reactions between chloride and Al³⁺ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in homogeneous reactions are treated explicitly using kinetic constants taken from the literature. The validity of assuming reaction equilibrium has been addressed. Solution potential values and species concentrations are predicted for different dissolution current densities. The acidity in the pit is explained by the hydrolysis of Al³⁺; an analytical expression for the pH values at the pit bottom for a given dissolution current density is presented.The model is applied to a real capillary geometry used in electrochemical microcell experiments. It was found that for r_cap/r_pit < 100, where r_cap and r_pit are the capillary end and pit radii, respectively, the insulating capillary wall affects the species concentrations and the solution potential. Moreover, for r_cap/r_pit < 20, the shape of the capillary, which might not be cylindrical, should be taken into account.     

The electrodeposition of aluminium on mild steel from a molten aluminium chloride-sodium chloride bath

The electrodeposition of aluminium on mild steel from a dry and HCl-free AlCl₃/NaCl melt (80 wt% purified AlCl₃, 20 wt% NaCl) has been studied. Electropolishing the substrate in the same electrolytic bath produced satisfactory electroplates with almost 100% current efficiency. The quality of the plate has been examined through photomicroscopy of the cross-section, profilometry, electron microscopy, and electron microprobe analysis of the surface of the electroplate. It has been suggested that the greyish colour of the plate is most probably due to the presence of iron as an impurity in the electroplated aluminium.