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 electrodeposition of aluminium from molten aluminium chloride/sodium chloride

The electrodeposition of aluminium from an AlCl₃/NaCl melt (80 wt % purified A1C1₃, 20 wt % NaCl at 175°C) has been studied and conditions established for the production of good quality electroplate on steel. To obtain a coherent, dendrite-free plate at current densities up to 16 mA/cm², the presence of not less than about 0.07 wt % hydrogen chloride was found to be essential. Under these conditions the current efficiency was 85%. Continued plating resulted in a progressive increase in current efficiency up to practically 100% and a progressive deterioration in plate quality. Addition of more HCl at this stage restored the original conditions. These observations explain inconsistencies in previously reported values for current efficiency and maximum current density. The mechanism of the action of HCl was not established but it is suggested that it may have an action akin to chemical polishing by dissolving off the high spots on the already deposited aluminium. Alternatively, the electrolytic reduction of hydrogen preferentially on active sites may block these off and so prevent the build-up of aluminium dendrites.     

Passivation of the aluminium cathode during deposition of aluminium from aluminium chloride-sodium chloride melts

The kinetics of aluminium deposition from NaCl-AlCl₃ melts (c_AlCl3 < 10 mol%) contained in alumina crucibles was studied by linear sweep voltammetry and potential step amperometry at temperatures around 820°C. At low concentrations (c_AlCl3 < 0.4 mol%) the reduction of AlCl₃ on liquid aluminium has been found to be diffusion controlled. At higher concentrations a passivation of the aluminium electrode was observed during the deposition reaction. The passivation appears to be caused by precipitation of alumina from supersaturated melt in the diffusion layer at the aluminium cathode.     

有机熔盐电镀铝的研究进展

在有机熔盐电镀铝体系的构成、物理和电化学性质的基础上,对卤化烷基吡啶类、卤化烷基咪唑啉类、氯化烷芳基铵盐类电镀铝及铝合金的研究现状进行了阐述,并对这3种有机熔盐电镀铝体系的优缺点进行了归纳总结。最后,指出了有机熔盐电镀铝体系的应用前景和发展方向。     

High-rate plating of aluminium from the bath containing aluminium chloride and lithium aluminium hydride in tetrahydrofuran

An electrolyte for the high-rate plating of aluminium from the tetrahydrofuran solutions of aluminium chloride and lithium aluminium hydride has been developed. A smooth and coherent deposit of aluminium has been obtained at the current density of 18 A dm^–2 without stirring, whereas the conventional diethyl ether solvent bath allows good plating up to 5 A dm^–2 under the same condition. The current densities applicable are increased with an increase in the molar fraction of aluminium chloride in tetrahydrofuran. The conductivity of the plating solution was measured at various molar ratios of aluminium chloride to lithium aluminium hydride. A plateau region of the conductivity curve plotted against the molar ratio is consistent with the composition of the plating bath giving a good plating. The plateau region is enlarged with an increase in total aluminium concentration.     

Black nickel electrodeposition from a modified Watts bath

Black nickel coatings were electrodeposited on to steel substrates from a Watts bath containing potassium nitrate. The best operating conditions necessary to produce smooth and highly adherent black nickel were found to be NiSO₄ · 6H₂O 0.63 M, NiCl₂ · 6H₂O 0.09 M, H₃BO₃ 0.3 M and KNO₃ 0.2 M at pH of 4.6, i=0.5 A dm⁻², T=25 °C and t=10 min. The modified Watts bath has a throwing power (TP) of 61%, which is higher than that reported, not only for nickel, but also for many other metals electrodeposited from different baths. The potentiostatic current–time transients indicate instantaneous nucleation. X-ray diffraction (XRD) analysis shows that the black nickel deposit is pure metallic nickel with Ni(111) preferred orientation.     

Phenol electropolymerization on phosphated mild steel via zinc electrodeposition

Phenol electropolymerization to polyoxyphenylene coatings on phosphated steel and phosphated galvanized steel, normally impossible under the conditions allowing effective coating formation on either steel or zinc, is activated by cathodic deposition of zinc. A critical zinc amount has been found to be necessary to suppress electrochemical processes competitive with electropolymerization and induce coating formation with nearly 100% efficiency. SEM analysis showed that this critical amount of zinc corresponded to the formation of a low number of zinc nuclei emerging at the surface of the phosphate layer, on which, however, polyoxyphenylene was formed as a continuous coating. A.c. impedance tests showed that initial barrier properties are worse for coatings grown on zinc-plated phosphated samples than for those grown on steel from the same solutions. However, the long term corrosion resistance is much better in the former case, the improvement being largely associated with zinc cathodic protection     

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.     

Zinc-nickel alloy electrodeposition from uncomplexed acid bath

Zinc-nickel alloys, as protective coatings, are well known and a new citrate bath has been investigated. It was observed that boric acid favoured nickel deposition playing the dual role of specific adsorption and catalysis. The voltammetric behaviour of the alloy deposition in the presence of boric acid is described. The effect of Ni/Zn ratio, and potential scan range on the cathodic and anodic portions of the voltammograms are discussed. The presence of two cathodic peaks at high Ni/Zn ratios, which disappeared at lower ratios was identified.     

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     

Influence of tartaric acid on zinc electrodeposition from sulphate bath

The effect of tartaric acid on zinc electrodeposition from sulphate plating bath was investigated by electrochemical impedance spectroscopy (EIS), stationary polarization curves, X-ray diffraction and SEM imagery. The study shows that it is possible to obtain homogeneous, compact and dendrites-free zinc deposits from sulphate solutions containing tartaric acid. For various hydrodynamic methods (rotating disc electrode and static vertical or horizontal electrode), the results indicate that, the presence of only small quantity of tartaric acid, may induce significant changes on deposition rates and deposit quality.     

Influence of bath temperature and bath composition on Co-Ag electrodeposition

A study of the best conditions to prepare smooth heterogeneous Co-Ag films with low amounts of S from a thiourea-based electrolytic bath has been performed. Using a 0.01 M AgClO₄ + 0.1 M Co(ClO₄)₂ + 0.1 M thiourea + 0.1 M sodium gluconate + 0.3 M H₃BO₃ + 0.1 M NaClO₄ bath, low temperature (10 °C) allowed obtaining compact and smooth deposits containing 2 wt.% sulphur. Decreasing thiourea content 0.06 M and increasing gluconate concentration up to 0.3 M, better deposits (more compact with lower sulphur content (1.2 wt.%)) were obtained. A clear influence of the species present in the bath on the film quality was observed: while gluconate favoured film cohesion, boric acid hindered hydrogen adsorption. For all films, fcc-Ag, hcp-Co and hcp-CoAg₃ phases were always detected by XRD, TEM and electron diffraction, their proportions varying with the electrodeposition conditions. Magnetic measurements revealed that the increase in the CoAg₃ led to an increase in the film coercivity. GMR values were only measured at cryogenic temperatures, they being higher for the deposits with the lowest sulphur content revealing that sulphur exerts a negative effect on magnetoresistance.     

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     

Pulsed electrodeposition of microcrystalline chromium from trivalent Cr-DMF bath

Pulsed electrodeposition (PED) with square wave has successfully been applied to deposit microcrystalline chromium from Cr-dimethylformamide (DMF) bath. The influence of the duty cycle, on-time, off-time, frequency, and pulse peak current on thickness, current efficiency, and hardness were investigated. Based on the analysis of the microstructure, the corrosion behavior of both direct-current deposited (DCD) and pulse-current deposited (PED) chromium in 3.5% NaCl solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results indicated that both pulsed electrodeposits and direct-current deposits have high charge transfer resistance R _ct and very low I _corr compared with mild-steel substrate.     

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.     

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.     

Electrochemical synthesis of hafnium carbide powder in molten chloride bath and its densification

Nanocrystalline powder of hafnium-rich-HfC has been successfully synthesised by the electro-deoxidation of HfO₂–carbon precursors in molten chloride. The progress of the solid state reduction was monitored ex situ by analysing partially reduced samples using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It has been shown that the reduction started by converting HfO₂ to CaHfO₃ and an oxycarbide phase of the form HfC_xO_2(1?x). The CaHfO₃ phase then also reduced to give HfC_xO_2(1?x), which subsequently reduced to HfC by ionising oxygen. The morphological analysis indicated almost no growth in the grain size occurred during the course of the electro-deoxidation. This investigation showed some loss of carbon during the electro-deoxidation resulted in metallic rich HfC. The synthesised powder exhibited better sinterability than the commercial HfC powder. Using the synthesised powder, fully dense monolithic HfC ceramics were produced by pressureless sintering at 1973 K with average grain size of about 3 μm.     

Coal gasification using a molten iron bath

A coal gasification process using a molten iron bath as reactor has been developed by Sumitomo Metals. Pulverized coal is blown onto this molten iron together with oxygen and steam as gasification agents. Tests using a pilot plant having a capacity of 60 t (coal) day^?1, has shown that a sulphur-free carbon monoxide and hydrogen-rich gas can be generated at high coal conversion efficiency.