Electrochemical behaviour of Al, Al–Sn, Al–Zn and Al–Zn–Sn alloys in chloride solutions containing indium ions

The electrochemical behaviour of pure aluminium and three of its alloys were investigated in 0.6m NaCl in the presence and absence of In3+ ions. The study comprised polarization and potentiostatic current–time measurements complemented by SEM–EDAX investigation. In 0.6m NaCl the corrosion resistance of the alloys decreases in the following order: Al < Al–Sn < Al–ZnAl–Zn–Sn. The addition of In3+ ions to the test electrolyte revealed activation of pure Al which increases with increase of In3+ concentration. Similar results were obtained for the binary Al–Zn and the ternary Al–Zn–Sn alloys, while Al–Zn alloy displayed a higher activation effect with In3+. It is also concluded that the existence of Zn either as an alloying element or present as a cation in the electrolyte leads to an enhanced activity of aluminium in presence of In3+ ions. Deactivation is observed in the case of Al–Sn alloy on addition of In3+ because tin retards the diffusion pathway of In to the bulk alloy, in addition to the presence of iron as an impurity in the alloy.     

Corrosion rates of lead based anodes for zinc electrowinning at high current densities

The corrosion rates of anodes made from various lead/silver alloys have been determined during electrolysis in sulphuric acid solution, pure and containing additives, using current densities in the range 2500 to 10 000 A m^–2. An increase in acid concentration, and in some cases temperature, caused an increase in the corrosion rate. In the absence of manganese in the bath, the corrosion rate was effectively independent of current density in the range studied, whereas in the presence of manganese, the corrosion rate decreased with decreasing current density. The corrosion rates of various calcium, tin and thallium alloys of lead were also determined. The presence of chloride ions in the electrolyte increases the corrosion rate, whereas potassium ions and strontium carbonate have a negligible effect. Pre-treating silver/lead anodes with a solution of acidic potassium fluoride at 500 A m^–2 prior to testing markedly decreased the corrosion rate in the presence of manganese, but increased the corrosion rate with manganese absent. The effect of zinc on the corrosion rate in synthetic electrolyte solutions, with and without manganese present, has also been determined for silver/lead alloys at 10 000 A m^–2. At zinc levels over 1 M, the corrosion rate increased with and without added manganese. As the work has been undertaken in an attempt to improve the electrowinning of zinc, an electrolyte based on acidified industrial solution has also been tested. The rates observed were similar to those obtained for synthetic zinc-containing solutions.     

Aluminium alloys in sulphuric acid Part I: Electrochemical behaviour of rotating and stationary disc electrodes

Anodic oxidation of various aluminium alloys was investigated by means of rotating disc electrodes in 3 M H₂SO₄ as a function of Cl^–, F^–, Zn²⁺ and In³⁺ concentration. Al-In, Al-Zn/In and Al-Zn/Sn alloys yielded current-potential curves at the lowest overpotentials and faradaic efficiencies for anodic oxidation of up to 98% at currents 50 mA cm^–2. While these alloys were electrochemically active in the presence of chloride as the only additive in sulphuric acid, binary aluminium alloys with Ce, Ga, La, Nd, Sn, Ta, Te, Ti or Tl were only active when Cl^–, Zn²⁺ and In³⁺ species were added to the electrolyte. With the exception of Al-Ga, binary alloys displayed high faradaic efficiencies of up to 95%. Fluoride additives resulted in current-potential curves at even more negative potentials than those with chlorides. In contrast to Cl^–, fluoride ions are consumed during the aluminium oxidation process due to complexation with Al(III).     

Characterisation of different grades of commercially pure aluminium as prospective galvanic anodes in saline and alkaline battery electrolyte

The effect of calcium chloride and sodium chloride on the electrochemical properties of various grades of aluminium namely 2s, 3s, 26s and 57s in alkaline citrate solution has been examined by studying the self corrosion, open circuit potential, anodic polarization and anode efficiency. It has been found that among the different grades of aluminium the 57s grade is found to be the most promising galvanic anode material in the 4N NaOH containing 20% wt/vol. of sodium citrate and 2.5% wt/vol. of CaCl₂·2H₂O. Further, it has been found that the above alkaline citrate electrolyte required for aluminium air batteries can be prepared from tap water, saline water and even sea water. The presence of chloride ions in the electrolyte, even up to a concentration of 2.5% wt/vol. as CaCl₂·2H₂O, is not found to alter the electrochemical properties of the different grades of aluminium as anode materials, though 57s is found to be the best.     

Wear of graphite anodes during electrolysis of add sulphate solutions

Graphite electrodes were prepared by mixing calcined coke and coal tar pitch. They were pressed under 250 kg cm^–2 and heat treated up to 2800° C. Rectangles measuring 70 mm x 40 mm x 8 mm were anodically polarized under galvanostatic and potentiostatic conditions. Electrolyses were conducted at 10–50 mA cm^–2 for periods ranging from 10–120 hours in Na₂SO₄ solutions acidified with sulphuric acid to various pH values. The wear of graphite anodes increased with decreasing bath temperature, increasing acid concentration, decreasing pH of the electrolyte and increasing current density. A model is suggested which assumes that corrosion takes place via the formation of a lamellar crystal compound with the formula (C ₈ ⁰ O)(OH)₃HSO ₄ ^– ·2H₂SO₄.The compound is unstable at higher temperatures when corrosion is effected by oxidation of graphite by atomic oxygen. The formation of the carbon ions was found to be a necessary precondition for the formation of the complex.     

Growth of TiO2-based nanotubes on Ti–6Al–4V alloy

The possibility of tailoring titania nanotube films on Ti–6Al–4V alloy is investigated using electrolytes based on NH₄H₂PO₄ with the addition of different concentrations of NH₄F. Several morphologies from high aspect ratio nanotubes to barrier layers are achieved by the control of the electrolyte composition, regarding its pH and fluoride concentrations. The morphology and composition of the anodic layers were evaluated by scanning and transmission electron microscopy, Rutherford backscattering spectroscopy (RBS) and Wavelength dispersive X-ray fluorescence spectroscopy.The formation efficiency and the fluoride ions content in the nanotubes depend on the F concentration in the electrolyte. The higher concentration of fluoride ions in the electrolyte promotes an increase of the F incorporated in the nanotubes, about 12 at.% but, reduces the nanotube formation efficiency. However, no significant presence of phosphorus was detected into the films by means of the above-mentioned analytical techniques.     

A bipolar electrode cell for aluminium electrorefining

Electrorefining of aluminium was carried out at 750 °C using bipolar electrode cells with centre holes 2, 10 or 20 mm in diameter. Through the centre holes liquid electrorefined aluminium rises to the electrolyte surface. The bipolar electrode cell consists of graphite cathodes, Al–Cu–Fe–Mn or Al–Cu–Fe–Zn alloy anodes and a BaCl₂–NaCl–AlF₃–NaF electrolytic melt. The centre hole size of more 20 mm in diameter is required to continuously float up the aluminium electrodeposited onto the electrolyte surface, while the current efficiency of the cell decreases with increase of the centre hole size, from 97% at 2 mm diameter to 92% at 20 mm diameter. Aluminium of 99.97% purity precipitates at the cathode. Iron, manganese and zinc included in the alloy as impurities are hardly deposited and the concentrations of these elements in the deposit are 100, 80 and 170 ppm, respectively. In this process aluminium can be produced with an energy consumption of about 4.9 × 10³ kWh(t-Al)^–1, which is one-third smaller than that of the Gadeau process.     

Electrochemical Behaviour of Al, Al—In and Al–Ga–In Alloys in Chloride Solutions Containing Zinc Ions

The electrochemical behaviour of Al, Al—In and Al–Ga–In alloys in 0.6 m NaCl solutions with and without Zn²⁺ was investigated. The study was performed by means of open circuit potential, potentiodynamic polarization, potentiostatic current-time and electrochemical impedance spectroscopy measurements as well as by SEM-EDAX examination. It was found that the Al—In alloy exhibits the highest negative open circuit potential in 0.6 m NaCl and the corrosion resistance of the tested electrodes decreases in the following order: Al > Al–Ga–In > Al—In. The greater activity of the Al—In alloy was interpreted on the basis of the autocatalytic attack by indium. The potentiostatic current–time measurements in Zn²⁺ containing electrolyte at potentials above the pitting potential revealed that Zn²⁺ has an insignificant influence on the Al electrode, while it enhances the corrosion of the Al–Ga–In alloy and improves the attack morphology of the Al—In alloy. Furthermore, the impedance spectra recorded under open circuit conditions showed a decrease in the polarization resistance of Al—In and Al–Ga–In alloys in presence of Zn²⁺ indicating the activating effect of Zn²⁺ ions.     

Influence of different anions on the behaviour of aluminium in aqueous solutions

The influence of chloride, sulfate and perchlorate anions on the behaviour of native oxide layers on aluminium is investigated using electrochemical techniques. Due to its influence on the open circuit potential and the cathodic side of the polarization curve the oxygen concentration has been carefully controlled. Two kinds of attack on a commercially pure aluminium (99.5 wt %) have been observed. In all the investigated 0.5 M Cl^–, 0.5 M ClO^– ₄ and 0.5 M SO^2– ₄ aqueous solutions the metal is corroded around the iron and silicon containing precipitates, but only in Cl^– and ClO^– ₄ solutions is crystallographic pitting observed. Comparison with high purity aluminium (99.99 wt %) shows that pitting corrosion is not influenced by the presence of impurities in the aluminium alloys, but by the presence of anions in solution. The pH and/or oxygen concentration determine whether or not the pitting potential coincides with the corrosion potential.     

Potential dependent selective dissolution of Ti–6Al–4V and laser treated Ti–6Al–4V in acid/chloride media

The corrosion properties of Ti–6Al–4V and laser surface melted (LSM) Ti–6Al–4V samples were investigated in 0.05 M H₂SO₄/0.05 M NaCl solution. Laser surface treatment was found to increase the corrosion potential and decrease the corrosion rates of the alloy. The current–potential profile of the LSM was found to be generally noisy below 0.5 V, indicating an unstable surface, which undergoes continuous dissolution and repassivation. However, above 0.5 V the LSM specimen exhibited higher corrosion current compared to the untreated alloy. Inductively coupled plasma (ICP) analysis of metals in solution was carried out after controlled potential electrolysis. Generally, the aluminium percentage was found to be the highest in solution compared to titanium and vanadium. The aluminium percentage in solution reached 94% compared to titanium and vanadium upon polarization in the passive region at 1.01 V. SEM showed that some local and shallow pitting to occur in both the untreated and LSM alloy. EDS results showed that aluminium composition of the electrolysed alloy surface is lower than the original material composition, and decreased from 6% in the original alloy to 0.18% after two hours of electrolysis of the LSM specimen.     

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 effects of 20 kHz and 500 kHz ultrasound on the corrosion of zinc precoated steels in [Cl−] [SO2−4] [HCO−3] [H2O2] electrolytes

The effects of 20 and 500 kHz ultrasound on the corrosion of precoated steels were studied by analysing the behaviour of a zinc coated steel electrode in the corrosion electrolyte [Cl^–] [SO^2– ₄] [HCO^– ₃] [H₂O₂]. The electrolyte was subjected to 20 kHz ultrasound, 500 kHz ultrasound and silent conditions. Zinc plated specimens were exposed to those solutions and growth of the corrosion products was studied by scanning electron microscopy, X-ray microanalysis and grazing incidence X-ray diffraction. Mass transfer measurements were taken on platinum macro- and microelectrodes; they highlighted a specific effect of ultrasound on the growth of zinc corrosion products depending on frequency. Ultrasound greatly influenced corrosion rates; however, the reaction sequence appeared unchanged by the use of an ultrasonic field compared to silent conditions.     

Catalytic wet peroxide oxidation of phenol over Al–Cu or Al–Fe modified clays

Al–Fe or Al–Cu modified clays were prepared from two natural montmorillonites and employed in the phenol oxidation with hydrogen peroxide in water. The samples were efficient in phenol elimination under mild experimental conditions (atmospheric pressure, 293 K and small quantities of hydrogen peroxide) without considerable leaching of the metal ions. The clays modified with Fe achieved high conversions of phenol and TOC thus showing to be very selective towards the formation of CO₂ and H₂O.     

Electrochemical and surface properties of aluminium in citric acid solutions

The electrochemical and surface properties of aluminium in 0.05 M citric acid solutions of pH 2–8 were studied by means of open circuit potential (OCP), potentiodynamic polarization and potentiostatic current–time transient measurements. The OCP reached a steady-state value very slowly, probably due to the slow rate of detachment of surface complexes into the solution. Aluminium exhibits passive behaviour in citric acid solutions of pH 4–8. Tafel slopes b _c were characteristic for hydrogen evolution on aluminium covered by an oxide film. The corrosion kinetic parameters E _corr, i _corr and b _a suggest that surface processes are involved in the dissolution kinetics, especially in the pH range 3–6. Current–time transient measurements confirm that, in citric acid solutions of pH 3–6, the dissolution is controlled by surface processes, that is, by the rate of detachment of surface complexes, while in solutions of pH 2, 7 and 8 dissolution is under mass-transport control. The addition of fluoride ions to citric acid changes the controlling steps of the dissolution process. Citrate and fluoride ions compete for adsorption sites at the oxide surface, and adsorption of these ions is a competitive and reversible adsorption.     

High speed zinc electrowinning system using a hydrogen anode and a rotating aluminium disc cathode

The performance of a novel high speed zinc electrowinning system using a hydrogen anode and an aluminium rotating disc cathode (1 m diam.) was investigated under various experimental conditions. This new type of zinc electrowinning system was continuously operated at a current density of 70 A dm^–2, which is twelve times higher than that usually employed. Current efficiency is 90% at 50 A dm^–2 in an electrolyte containing 60 g dm^–3 Zn + 160 g dm^–3 H₂SO₄, the zinc purity being at least 99.999%. The energy usage of the system is 1650 kWh per tonne of zinc, 380 m³ of H₂ gas being required.     

Comparative study of the potentiodynamic behaviour of aluminium at low potentials in barrier layer and in pore-forming electrolytes

The potentiodynamic behaviour of a 99.9995% aluminium electrode in several barrier layer and pore-forming electrolytes at a temperature of 25° C, with sweep rates in the range 1–200 mV s^–1 and potentials between –2.00 and 3.00 V (versus SCE), has been studied. In the anodic sweep the potential of zero current,V _j=0, depends on the pH of the electrolyte and corresponds to a corrosion potential (the cathodic and anodic reactions coexist). In the 0.5 M H₃BO₃-borax solution (pH=6.3) and afterV _j=0, the current density increases in a convex form, approaching a steady-state value. For this electrolyte and the potential ranges studied, the anodic charges corresponding to the anodic sweep are independent of the sweep rate and are associated with the formation of a barrier layer oxide. In the neutral 0.50 M propanedioic acid solution (pH=7.0) and the potential ranges studied, the anodic charges are much greater than those corresponding to the H₃BO₃-borax solution, and metal corrosion combined with a solvent effect of the electrolyte is found. The greatest anodic charges are found for the H₂SO₄ solution (pH=1.0); the anodic current increases rapidly in the potential ranges studied due to pore initiation. The cathodic and anodic behaviour of aluminium depend strongly on the electrolyte employed, basically through the effect of the electrolyte on the oxide film and through the pH of the solution.     

Effects of cobalt, temperature and certain impurities upon cobalt electrowinning from sulfate solutions

The effects of cobalt concentration, temperature and the presence of zinc, copper and iron ions in the electrolyte on current efficiency and cathodic quality were investigated by cyclic voltammetry and galvanostatic methods during cobalt electrowinning. The results showed that high cathodic efficiency of cobalt deposition was obtained from solutions containing cobalt concentration in the range 30–60 g l^–1. Current efficiency increased from 94% to 97% with increase in cobalt concentration to 60 g l^–1 at 20 °C. It was also found that increase in temperature to 50 °C enhanced the cobalt deposition reaction, along with the rate of hydrogen evolution, resulting in little change in current efficiency. The presence of foreign cations in the electrolyte not only adversely affects current efficiency but also promotes cracking and peeling.     

Corrosion Behaviour of Ti–6Al–4V Alloy in Concentrated Hydrochloric and Sulphuric Acids

Open-circuit potential, polarization and electrochemical impedance spectroscopy (EIS) measurements were used to investigate the corrosion behaviour of Ti–6Al–4V alloy in H₂SO₄ and HCl solutions. The corrosion rate of the alloy was found to increase with increasing acid concentration. The corrosion behaviour of the alloy was compared to that of pure titanium in both acids. The results showed that the alloy is more corrosion resistant than pure titanium in the investigated solutions. The apparent activation energies of the corrosion process for titanium are lower than those of the alloy in the same solutions, which reflects the higher corrosion resistance of the alloy.     

Anodic behaviour of titanium in concentrated sulphuric acid solutions. Influence of some oxidizing inhibitors

The anodic polarisation behaviour of the titanium electrode and the rate of corrosion are measured in aqueous sulphuric acid solutions as a function of the electrolyte concentration. Increasing acid concentration increases the critical current for passivity. This current passes through a maximum (4.88 mA cm^–2) at 25 N and then decreases again. Increasing temperature increases the critical current for passivity. The effect of oxidizing agents such as K₂Cr₂O₇, KMnO₄, KIO₃, Na₂MoO₄, NaClO₃, HNO₃ and TiCl₄ on the polarisation of titanium are also investigated. K₂Cr₂O₇, HNO₃, TiCl₄ are the most powerful corrosion inhibitors at room temperature and also at 80° C in 10 N H₂SO₄, while in more concentrated sulphuric acid solution (25 N), corrosion inhibition is observed only by HNO₃ for a very short period.     

The effect of thiourea on zinc electrowinning from industrial acid sulphate electrolyte

The effect of thiourea, with and without glue and antimony additions, on the current efficiency (CE) and polarization behaviour of zinc deposition and on the morphology and preferred orientation of the zinc deposits electrowon (at 430 A m^–2 and 35°C) from industrial acid sulphate electrolyte (55 g l^–1 Zn and 150 g l^–1 H₂SO₄) has been determined. Increasing concentrations of thiourea in the electrolyte decreased the CE for zinc deposition; the additional presence of antimony did not significantly alter the decrease in CE but the presence of glue resulted in a further substantial decrease in CE. Thiourea changed the zinc deposit morphology and orientation, and also altered the shape of the zinc deposition cyclic voltammogram.