A kinetic and electrochemical study of the zincate immersion process for aluminium

To plate aluminium, its surface is often first coated with a thin layer of zinc which is formed by immersion in an alkaline zincate solution. This paper describes a kinetic and electrochemical study of the zincate immersion reaction. Using an aluminium sample in the form of a rotating disc, the effects of varying the zinc concentration (0.01–0.5 m), disc rotation speed (66–1380 rpm), temperature (5–72°C), and sodium hydroxide concentration (1.5–9.0 m) on the kinetics were investigated. It was found that the reaction was usually first order. When the zincate concentration was 0.1 m, the reaction was chemically controlled with an activation energy of 35 ± 7 kJ mol^–1. At high zincate concentrations, high disc rotation speeds and low sodium hydroxide concentrations, a thin film of zinc metal was formed on the aluminium surface, blocking the subsequent reaction. It was found that the most compact and adherent zinc films were formed at high zincate concentrations. This finding is consistent with industrial practice. These results are explained using mixed potential measurements and Evans' diagrams.     

The estimation of the coulombic efficiency of zinc electrodeposition by measurement of current efficiencies at a rotating ring disc electrode

The rotating ring disc electrode (RRDE) offers a simple method for the determination of the current efficiency (CE) of zinc electrodeposition in acidic zinc sulphate electrolytes. The hydrogen evolved during zinc deposition can be detected at the ring. This allows estimation of the current due to hydrogen formation at the disc and hence CE for electrodeposition of zinc can be calculated. Investigations of the conditions under which reliable measurements of CE can be obtained are described. A correlation between CE, determined using the RRDE, and the coulombic efficiency (QE), determined by weighing the zinc deposit obtained after 2 h electrodeposition at constant current, is established for a range of electrolyte compositions. It is suggested that measurement of CE at a Pt–Zn RRDE provides an efficient means of estimating QE for zinc electrolytes.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Kinetics and mechanism of the formation of doped skeletal copper catalysts: the effect of zincate compared to undoped and chromate-doped systems

Addition of zincate to the leach liquor for the preparation of skeletal copper increases the copper surface area; however it does not stabilize the structure against rearrangement. The leaching kinetics have been studied using a rotating disc electrode (RDE) at 269–293 K in 2–8 M NaOH and 0.0005–0.1 M Na₂ZnO₂. Zincate ions precipitate as zinc oxide, due to the local consumption of hydroxide ions near the leach front as the aluminium dissolves. This oxide hinders the aluminium dissolution, slowing the leaching rate. It also hinders copper dissolution/redeposition and prevents copper diffusion, thus reducing the structural rearrangement significantly, and causing the formation of a much finer copper structure with increased surface area. The zinc oxide redissolves as the leach front passes, releasing the copper to rearrange once more, thereby allowing the surface area to decrease with time. The activation energy for leaching was found to be 84 ± 6 kJ mol^–1.     

Surface activation of manganese oxide electrode for oxygen evolution from seawater

Utilizing the fact that the equilibrium potential of oxygen evolution is lower than that of chlorine evolution, oxygen evolution in seawater electrolysis was enhanced by decreasing the polarization potential under galvanostatic conditions through increasing the effective surface area of manganese oxide electrodes. Electrodes were prepared by a thermal decomposition method. IrO2-coated titanium (IrO2/Ti electrode) was used as the substrate on which manganese oxide was coated (MnOX/IrO2/Ti electrode). Subsequently, oxide mixtures of manganese and zinc were coated (MnOX–ZnO/MnOX/IrO2/Ti electrode). The effective surface area of the MnOX–ZnO/MnOX/IrO2/Ti electrodes was increased by selective dissolution of zinc (leaching) into hot 6M KOH. The oxygen evolution efficiency of the MnOX/IrO2/Ti electrode was 68–70%. Leaching of zinc from the MnOX–ZnO/MnOX/IrO2/Ti electrodes with 25mol% or less zinc led to a significant increase in the oxygen evolution efficiency. The maximum efficiency attained was 86% after leaching of zinc from the MnOX–25mol%ZnO/MnOX/IrO2/Ti electrode. However, large amounts of zinc addition, such as 40mol% or more are detrimental because of a decrease in the oxygen evolution efficiency. This is due to the formation of a double oxide, ZnMnO3, which is hardly dissolved in hot 6M KOH.     

The electrodeposition and dissolution of zinc and amalgamated zinc in alkaline solutions

The reaction mechanism of zinc and amalgamated zinc was investigated with the galvanostatic transient technique in the concentration range 1.5–10 M KOH. The Tafel slopes of the zinc electrode were 40 mV anodically and 120 mV cathodically. The cathodic reaction order in zincate was found to be +1. From the Tafel slopes and the dependence of the exchange current density on the activity of the KOH, the reaction orders in OH^? were calculated, yielding values of 2.3± 0.8 in the anodic and ?0.8 ± 0.2 in the cathodic direction. These results are consistent with the suggested mechanism of Bockris et al.[16] for the zinc electrode. The Tafel slope in cathodic direction of the amalgamated zinc electrode was a function of the KOH concentration (120 mV at KOH concentrations up to 3 M; about 60 mV in 10 M KOH); the anodic Tafel slope was 30 mV over the whole concentration range. These results and measurements at constant ionic strength suggest a mechanism which involves the participation of water. The difference in behaviour of the zinc electrode and the amalgamated zinc electrode is probably caused by changes in the adsorption characteristics due to amalgamation.     

Study of the performance of secondary alkaline pasted zinc electrodes

Calcium zincate was prepared by a chemical coprecipitation method and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical performance of pasted zinc electrodes with bismuth and calcium additives was investigated by the charge–discharge method. The addition of metallic bismuth powder improves the discharge performance of zinc electrodes due to the formation of an electronic conduction matrix. The calcium-containing zinc electrodes showed higher discharge capacity, less shape change and longer cycle lifetime. Moreover, zinc electrodes using calcium zincate as active material show better electrochemical performance than those with the physical mixture of zinc oxide and calcium hydroxide.     

Reverse pulse plating of copper from acid electrolyte: A rotating ring disc electrode study

Coulombic or cathode efficiencies (CE) were determined for the reverse pulse plating of copper from CuSO₄/H₂SO₄ electrolyte for a variety of pulse conditions. The CE was seen to decrease as the magnitude of the current on the anodic pulse increased. This may be explained by an increase in Cu⁺ intermediates near the electrode surface and was verified by polarization data obtained from a rotating ring disc electrode (RRDE). The influence of certain additives on the CE during reverse pulse plating and on the polarization curves was also examined. When polyethylene glycol and Cl^– (0.86mm) were added to the electrolyte, the CE was observed to drop significantly for a particular set of pulse parameters. The polarization curves at the RRDE suggested that the copper-electrolyte interface was blocked by an adsorbed layer over a wide potential range. The results are explained in terms of a model in which Cl^– ions are concentrated near the electrode surface within the adsorbed polyethylene glycol layer and this is supported by observed rotational dependencies for the RRDE.     

Structural and electrochemical characterization of mechanochemically synthesized calcium zincate as rechargeable anodic materials

Hydrated calcium zincate was synthesized by mechanical ball milling of ZnO and Ca(OH)₂ in water at room temperature. The structural and electrochemical properties of this material used as rechargeable anodic material were examined by microelectrode voltammetry, charge–discharge measurements and structural analysis. The results showed that during mechanical milling, ZnO, Ca(OH)₂ and H₂O reacted rapidly to form Ca[Zn(OH)₃]₂ · 2H₂O which was subsequently transformed to a stable structure CaZn₂(OH)₆ · 2H₂O. Since this composite oxide has lower solubility in KOH solution (<35 wt %) and better electrochemical reversibility than ZnO-based negative materials, the zinc anodes using this material can overcome the problems of shape changes and dendritc formation, and therefore exhibit improved cycling life.     

Understanding aluminum behaviour in aqueous alkaline solution using coupled techniques: Part I. Rotating ring-disk study

Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements have been undertaken to study the behaviour of pure aluminum electrodes in alkaline media. The measurements did consist of linear sweep voltammetry from anodic to cathodic potentials on 4N, 5N or 5N5-aluminum samples in 4 M aqueous potassium hydroxide solution. In the potential range studied (−0.7 V versus NHE to −2.5 V versus NHE) the aluminum undergoes oxidation/dissolution into aluminates anions at high electrode potential while it yields strong hydrogen evolution at low potentials. Thanks to the RRDE technique, we show that hydrogen starts to evolve from the aluminum electrode even above the open circuit potential. Also, the oxidation state of superficial aluminum varies according to the electrode potential: whereas non-conducting aluminum oxides are present above the open-circuit potential hindering hydrogen evolution reaction (HER), they tend to disappear below the ocp, due to the strong hydrogen evolution, following the probable porous oxide layer blow up induced by the hydrogen bubbles formation. In consequence at very low potential, HER occurs on bare aluminum, HER kinetics being much faster than on oxide-covered aluminum.     

Determination of the oxygen reduction products on ASTM A516 steel during cathodic protection

The electrochemistry of steel in aerobic and anaerobic aqueous alkaline solutions was studied with or without forced convection to investigate the cathodic processes occurring on steel exposed by defects in polymer coated steel pipe. The results are relevant to the mechanistic understanding of the effect of cathodic protection on the disbonding of fusion bonded epoxy (FBE) coatings on steel. Moderate (pH9.8) and strongly (pH14) alkaline aqueous solutions were used to simulate the water layers at the cathodically polarized steel surface on the soil-side of buried pipe. A rotating gold ring and steel disc electrode (RRDE) in alkaline aqueous electrolyte equilibrated with 1atm oxygen over solution was used to measure the rotation rate dependent current for the electroreduction of oxygen, O2, on an ASTM A516 steel disc and the resulting peroxide generation, which was determined by monitoring the oxidation current on the gold ring. An appreciable fraction of the oxygen reduction current on the steel disk gave rise to peroxide generation over a wide range of potentials, from –0.2 to –0.9V vs SCE in 1M KOH. The observation of peroxide generation is noteworthy, because oxidizing agents, such as peroxide and its decomposition products, superoxide and hydroxy radical, can degrade the polymers used for coating pipelines. As result, oxidative degradation of polymer or interfacial compounds may be a cause of the accelerated disbonding observed for protective coatings on steel pipelines under cathodic protection.     

Feasibility study to probe the growth of chromate based conversion on Al by means of rotating ring disc electrode (RRDE) measurements

The use of a rotating ring disc electrode (RRDE) is introduced in this paper to study the formation of chromate-phosphate layers on rolled aluminium surfaces. A RRDE was developed containing an Al disc, which was converted with a chromate phosphate film, and a carbon ring, where the Cr⁶⁺ reduction rate was measured in situ. The principle of the method is that Cr⁶⁺ is transported from the bulk of the solution towards the disc and the ring electrode. At the disc electrode, a fraction of Cr⁶⁺ is consumed during the conversion of the Al disc electrode. This leads to a decrease in ring current, which is directly related to the amount of Cr⁶⁺ being reduced at the disc electrode. For proper operation of the RDDE system, an electrode with optimised geometrical characteristics is used. In order to be sure that the measured current on the ring is in relation with the Cr⁶⁺ reduction rate occurring at the aluminium disc, a number of requirements need to be fulfilled. After showing that the RDDE system is indeed able to monitor in situ the Cr⁶⁺ content during conversion, the amount of Cr⁶⁺ reduced at the disc electrode is calculated from the recorded ring current time curves. The method allows (i) to follow the growth of a conversion layer quantitatively in situ and (ii) to study the influence of important process parameters such as fluoride content, chromate content of the conversion bath and influence of mass transport.     

A zinc–air cell employing a porous zinc electrode fabricated from zinc–graphite-natural biodegradable polymer paste

Porous zinc anodes have been fabricated from a mixture of zinc and graphite powder using gelatinized agar solution as the binding agent. Agar is a biodegradable polysaccharide polymer extracted from marine algae. The graphite content and the agar solution concentration were varied to find the best electrode composition. Zinc–air cells were fabricated using the porous zinc anode, a commercially available air cathode sheet and KOH electrolyte in the form of elastic jelly granules. The electrode performance was evaluated from the zinc–air cell galvanostatic discharge capability. In the cell design, a thin agar layer was introduced between the electrode-gelled electrolyte interfaces, resulting in substantially improved cell discharge performance. The inclusion of particulate graphite into the electrode did not enhance the electrode performance due to the formation of a graphite-rich layer, which obscured the electrode porosity. A zinc–air cell employing the optimized porous zinc electrode demonstrated a capacity of 2066 mA h and specific energy density of 443 Wh kg^–1.     

State of hydrogen electrochemically stored using nanoporous carbons as negative electrode materials in an aqueous medium

Nanoporous carbons were used as negative electrode material in aqueous KOH medium to store hydrogen by electrodecomposition of water at atmospheric pressure. The storage capacity by this process is approximately one order of magnitude higher than in the gas phase at ambient conditions. By considering the particularities of the electrochemical characteristics, this paper gives information on the mechanism and on the kind of bond between hydrogen and the carbon host. For most experiments, a self-standing porous carbon cloth electrode has been used in order to avoid any side effect which could be due to additives. After galvanostatic hydrogen charging, the carbon material was analyzed by galvanostatic discharge and temperature-programmed desorption in order to determine the nature of the carbon-hydrogen bond and the amount of hydrogen sorbed. The activation energy for hydrogen desorption was estimated to be 110 kJ/mol, that confirms a weak chemical character of the hydrogen-carbon bond. Although the bond is stronger than in the case of physisorption, the fraction of hydrogen irreversibly trapped is low compared to the reversible fraction. Finally, we show that the reversible capacity can be significantly enhanced by increasing the temperature to 60 °C during the electrochemical reduction of water. The well-defined plateau during the oxidation step demonstrates high potentialities of this technique for electrochemical energy storage in nanoporous carbons using an aqueous medium.     

Observation of ‘inverted peak’ during molecular oxygen reduction at Au electrode in alkaline media

A well-defined inverted peak, i.e. a cathodic peak current during the positive going potential sweep was observed at the lead (Pb) modified polycrystalline Au electrode during oxygen reduction reaction in O₂-saturated 0.1 M KOH solution when it contained an appropriate amount of potassium iodide (KI). To explain the observed phenomena, a binary-adlayer of Pb and iodide formed onto the Au electrode surface as a result of iodide-induced adsorption of anodically stripped Pb²⁺ during the anodic scan of the voltammetric measurements and reduction of electrogenerated hydrogen peroxide favored by the enhanced electrostatic attraction of the anionic molecules (HO₂⁻) to the partially positively polarized Pb in the binary-adlayer are proposed as the probable origin of the appearance of the inverted peak. Effect of the electrochemical operating factors such as the rate and direction of potential scan, etc. on the inverted peak is investigated and the results are compared with those for the usual electrochemical processes. Experimental measurements are performed on the basis of cyclic voltammetric and chronoamperometric techniques to support the phenomena.     

Influence of alloying additives on the performance of commercial grade aluminium as galvanic anode in alkaline zincate solution for use in primary alkaline batteries

The self-corrosion of different grades of commercial aluminium such as 2S, 3S, 26S and 57S in 4 M NaOH containing 0.6 M ZnO has been determined by weight loss measurements. It is found that 26S and 57S aluminium exhibit negligible corrosion rates in the range 0.05–0.06 mg cm^–2 min^–1, which can be attributed to the formation of a zincate coating on the aluminium surface. The influence of zincating on the performance of binary and ternary alloys of 26S and 57S aluminium obtained by incorporating alloying elements such as zinc, indium, thallium, gallium and tin as galvanic anode in 4 M NaOH containing 0.6 M ZnO has been examined by studying self corrosion, steady state open circuit potential, galvanostatic polarization and anode efficiency. It is found that zincated ternary alloys of 26S and 57S aluminium containing zinc and indium can serve as good galvanic anodes in alkaline medium. AC impedance measurements and X-ray diffraction studies have been carried out to understand the nature of the film formed on the aluminium surface.     

The influence of sorbitol on zinc film deposition, zinc dissolution process and morphology of deposits obtained from alkaline bath

A novel cyanide-free zinc deposition bath was developed in which sorbitol was added at various concentrations. Voltammetric studies indicated that the reduction process is influenced thermodynamically and kinetically by the sorbitol concentration. Also, two cathodic processes were observed, one (wave) associated with the hydrogen evolution reaction (HER) on 1010 steel, the other (peak) with zinc bulk reduction, simultaneous to the HER. Furthermore, the plating-process kinetics was controlled by mass transport. The presence of sorbitol in the bath led to formation of light-grey zinc films, even during the HER, without cracks and dendrites. Plating current efficiency decreased from ∼ ∼62% to 43% on increasing the sorbitol concentration in the plating bath. In the presence of 0.1 M and/or sorbitol concentrations higher than 0.2 M, Zn electrode dissolution was inhibited. However, a small dissolution of zinc electrode was observed with 0.05 M sorbitol in alkaline solution without zincate. SEM micrographs showed that the 1010 steel substrate was fully covered by Zn film and that the sorbitol affected the morphology of zinc films, acting as a grain refiner.     

Effect of brighteners on hydrogen evolution during zinc electroplating from zincate electrolytes

Hydrogen evolution during zinc electrodeposition on a steel substrate from zincate electrolytes containing different additives was studied using various experimental techniques.The hydrogen evolution reaction is limited by the electron transfer step. Hydrogen evolution is most intensive during the first seconds from the beginning of electrodeposition due to the lower overpotential of hydrogen on steel as compared with that on zinc. The evolved hydrogen is dissipated in three ways. Most is dissipated to the atmosphere via gas bubbles at a constant rate. Some is dispersed in the electrolyte some diffuses into the steel substrate, predominantly at the commencement of deposition. The additives affect both the total amount of evolved hydrogen and its distribution. The highest amount of hydrogen is evolved in the presence of the anisaldehyde bisulphite containing composite additive. The highest amount of hydrogen included in the substrate and remaining in the electrolyte corresponds to the use of the Na–N-benzylnicotinate containing additive. In this case blistering is observed.     

In situ activation of nickel cathodes by sodium molybdate during alkaline water electrolysis at constant current

The nickel cathode undergoes significant deactivation at 100 mA cm^–2 in 30 wt% KOH at 70°C containing KOH metallic impurities. The time dependence of the potential reveals two distinct regions: the first due to hydrogen absorption, while the Tafel parameters, charge transfer resistance and double-layer capacitance remain fairly constant; the second due to the electrodeposition of metallic impurities as manifested by SEM pictures, EDX analysis and cyclic voltammograms. The resulting deactivation is due mainly to the increase in the Tafel slope, although the electrode impedance reveals a significant increase in electrode roughness. The hydrogen discharge at 100 mA cm^–2 in 30 wt% KOH containing 4mm sodium molybdate largely attenuates the deactivation process owing to hydrogen absorption and the deposition of impurities. Thein situ activation is ascribed to the formation of a spongy molybdenum-base deposit on the nickel cathode during the first day of water electrolysis.     

The influence of hydrogen- and cation-underpotential deposition on oxide-mediated Pt dissolution in proton-exchange membrane fuel cells

In order to fully understand the influence of a lower potential limit on platinum dissolution and the likely mechanism for mass and surface-area loss under potential cycling conditions, the dissolution of a Pt catalyst in a N₂-saturated 0.5 M H₂SO₄ solution was examined using an electrochemical quartz nanobalance (EQCN) flow cell, a rotating ring-disk electrode (RRDE) and inductively coupled plasma mass spectroscopy (ICP-MS). Due to the observation that cycling to a lower potential limit, which coincides with the hydrogen under-potential (H_UPD) region, results in a decrease in the dissolution rate, cations capable of interfering with the hydrogen UPD process (Zn²⁺, Li⁺, Na⁺, K⁺, and Cd²⁺) were introduced to the solution. Larger rates of mass loss were observed in the presence of these cations during the cycling process in the UPD region, despite apparently negligible effects on the behavior with more positive lower potential limits or on oxide formation and stripping. It was found that the quantity of soluble Pt species produced during the electrochemical reduction of PtO₂ was proportional to the charge associated with oxide stripping at the disk electrode during the RRDE experiment.