Alkaline Zincate Solutions Research Articles

The Behavior of the Amalgamated Zinc Electrode in Supersaturated Alkaline Zincate Solutions

The behavior of the amalgamated zinc electrode in unsaturated and supersaturated zincate solutions has been studied by measuring equilibrium potential. Excess zinc oxide can be dissolved in the KOH solutions to form supersaturated zincate solutions. The electrode potential was measured as a function of zincate concentration in the zincate solutions containing constant free hydroxyl ion. It has been found that the equilibrium potential of the amalgamated zinc electrode first increased and then decreased with the increase of zincate concentration from unsaturated solutions to supersaturated solutions. The influence of zincate concentration and its activity coefficient on the equilibrium potential of the amalgamated zinc electrode were discussed. © 2001 The Electrochemical Society. All rights reserved.

Double Zincate Pretreatment of Sputter-Deposited Al Films

The characteristics of double zincate pretreatment of thin Al films deposited on glass plates using magnetron sputtering and ion-beam sputtering were investigated. Traces of Zn deposition and immersion potential as well as surface observations using scanning electron microscopy and atomic force microscopy showed that continuous dissolution of an Al film during the double zincate pretreatment occurred in the case of a magnetron sputter deposited film, resulting in Al film failure from the substrate. On the other hand, the substitution reaction of Al dissolution and Zn deposition occurring on the ion-beam sputter-deposited film ceased during the first and the second zincate treatment processes. The difference between the behaviors of the double zincate treatments for the two kinds of sputter-deposited films is related to the film structure. A magnetron sputter-deposited film has a columnar structure, resulting in higher susceptibility to the dissolution reaction in a concentrated alkaline zincate solution. On the other hand, an ion-beam sputter-deposited film has a fine microcrystalline structure with a low density of defects, resulting in lower susceptibility to the dissolution reaction. © 2001 The Electrochemical Society. All rights reserved.

Zinc electrodeposition from alkaline zincate solution by pulsating overpotentials

It is well known that smooth zinc deposits cannot be obtained from alkaline zincate using constant overpotential and current rate. During prolonged metal deposition, spongy and dendritic deposits are formed. It has been shown that the deposits are less agglomerated in the case of square-wave pulsating overpotentials regime than the ones obtained in case of constant overpotential regime. This is explained in a semiquantitative way by two phenomena: selective anodic dissolution during overpotentials ?off? period and decreasing diffusion control. These effects is more pronounced at higher pause-to-pulse ratio. Increasing the pause-to-pulse ratio causes a reduction of the ratio between diffusion and activation overpotential, resulting in a more compact deposit. Confirmation of the proposed semiquantitative mathematical model was obtained by zinc electrodeposition onto a copper wire from a 0.1 M zincate solution in 1.0 M KOH at room temperature.

Photo-induced dissolution of zinc in alkaline solutions

The passive behaviour of zinc in an alkaline pH 13.0 solution under dark and illumination conditions has been studied using dc polarization and ac impedance techniques. Illumination with polychromatic light in the wavelength region 200–900 nm caused significant dissolution of anodically formed passive layers on zinc. This was evident from increased anodic currents, a ten-fold decrease in the charge transfer resistance and an increase in the capacitance of the passive layer. The photo-induced dissolution effect was only slightly evident under open-circuit conditions, where a precipitated or adsorbed layer forms on the electrode. These results are explained in terms of the n-type semi-conducting properties of ZnO.

Characterization of the Passive State on Zinc

Despite intensive investigations, the nature of the passive state is one of the most complex and unresolved subjects in the electrochemistry of zinc in alkaline solutions. In this paper, we explore the electrochemistry of the passive state on zinc in 0.1 M sodium borate/1 M sodium hydroxide solution of pH 10.5. During the course of passivation, several characteristic features in the anodic region are observed, including a wide passive range extending over >2 V and a secondary passivation phenomenon that occurs at high anodic potentials. The steady‐state current in the passive state is found to be independent of the applied voltage, which is consistent with the barrier layer being an interstitial zinc conductor or an oxygen vacancy conductor (or both) with interstitial zinc being the most likely defect. This model is also consistent with the well‐known n‐type character of the passive film on zinc.

Passivation of zinc in alkaline solution effected by chromates and CrO3–graphite system

Electrochemical properties of porous zinc electrode in alkaline solution have been modified by the additive of chromium compounds such as chromates and CrO3–graphite system. Insertion of chromates inside the zinc electrode caused the limited solubility of the anodic dissolution products and their retention in the electrode interior. Different behaviour of the passivated zinc electrode was observed depending on cation of the used chromate. The improvement of cyclic performance of zinc has also been noted after the electrode is admixed with CrO3–graphite system containing intercalated/incorporated oxide. Electrochemical properties of zinc electrode strongly depend on the way of chromium insertion to active material of anode. Anodic and cathodic charges have been measured during potentiodynamic cycles for the observation of passivating effect.

Electrowinning of Nonnoble Metals with Simultaneous Hydrogen Evolution at Flow‐Through Porous Electrodes: III. Time Effects

The electrowinning of zinc from a recirculating alkaline zincate solution at a flow‐through porous electrode was investigated. Experimental results were obtained to test the predictions of a mathematical model. The effects of electrolyte flow rate, cell current, and electrode thickness on the concentration‐time relations and coulombic efficiency‐time relations were studied. The experimental results show that the highest recovery rate was obtained at both high flow rates and cell currents. There is, however, a practical limit for increasing both the cell current and the electrolyte flow rate. Reasonable agreement between the model predictions and experimental results was obtained. A case study is presented to show how the model equations are used to predict the relationship between time, cell current, flow rate, and coulombs passed which are required to recover 95% of the zinc content of the electrolyte.

アルミニウム合金のジンケイト前処理におよぼす試料表面調整の効果

Surface pretreatment effects of Al and Al-Mg alloy on Zn deposition in the zincate process were evaluated using SEM and XPS. Al surface dissolution in a concentrated NaOH solution followed by zincate treatment produced rapid Zn particles nucleation with uniform size due to Al surface oxide film thinning. The anodic oxidation pretreatment of Al used jointly with electropolishing followed by zincate treatment also resulted in deposition of uniform Zn particles. Anodic oxidation pretreatment without electropolishing, however, resulted in nonuniform Zn deposition indicating that defects in the oxide film could be active sites for Zn nucleation. Al-Mg alloys require both alkaline and acid cleaning in surface pretreatment because Al dissolves in concentrated alkaline solution, and Mg dissolves in acid solution. Magnesium or its enriched compounds on the surface during zincate process in an alkaline zincate solution inhibits Al dissolution and Zn deposition. XPS showed that Zn2+ remains in the Al oxide layer, which partially charged after Zn stripping in a HNO3 solution. The role of the first zincate process in double zincate treatment is discussed based on these results.

Oxidation of Zinc in Alkaline Solutions Studied by Electrochemical Impedance Spectroscopy

Electrochemical dissolution and passivation reactions of zinc have been studied in 1.0 M KOH solutions by electrochemical impedance spectroscopy. Equivalent circuits have been worked out by simulating the impedance data and using the results to model the dissolution and passivation reactions. A Tafel plot constructed from the charge‐transfer resistances provides an exchange current of 0.11 A/cm2 and an α value of 0.36 for zinc oxidation. The maximum rate of zinc oxidation is observed at about −1.30 V vs. the Hg/HgO reference electrode as judged from the charge‐tranfer resistance minimum obtained from impedance measurements. A negative polarization resistance with a reverse semicircle on the Nyquist plot illustrates the transition process from an active to passive potential region at −1.10 V. At high anodic overpotentials, the zinc electrode behaved as a semiconductor electrode due to a compact ZnO passive film formed on the electrode surface.

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.

Cathodic processes on zinc in alkaline zincate solutions

The charging process of alkaline zinc secondary batteries is accompanied by the formation of hydrogen. Both the current and the coulombic efficiency, and in part the energy efficiency, are dictated by the production of hydrogen. Gasometric methods are used to determine the rates of hydrogen evolution and a potentiodynamic polarization method is employed to study the behaviour of zinc in alkaline solutions. It is found that an increase in ZnO concentration and a decrease in KOH concentration lower the rate of hydrogen evolution.

A Rotating Disc Electrode Study of Zinc Electro-deposition from Alkaline Zincate Solutions

SummaryThe method of the rotating disc electrode was used for the potentiodynamic investigation of zinc deposition on steel from alkaline zincate electrolytes. The effect of two types of additives -basic and brighteners, as well as their combinations on the cathodic characteristics of zinc electrode position was established.The measurements under controlled hydrodynamic conditions show that zinc deposition from electrolytes without additives is always mass transport limited and that deposits are powdery. In the presence of additives the deposition is kinetically controlled and smooth compact zinc is formed.The inhibition influence of the additives on the electrochemical reduction of zinc was also estimated. The data were correlated with Hull cell experiments.

無電解ニッケルめっきによるアルミニウム上へのマイクロバンプ形成

Chip bumping for flip chip and tape automated bonding (TAB) assembly has become increasingly important in microelectronic devices. Several methods using vapor deposition followed by electrolytic deposition to form bumps on a spattered aluminum electrode have been reported. The metallization process can be simplified by use of the electroless plating process. Generally the zincate process is used for electroless nickel plating on aluminum substrate. However, the process can not be applied to the metallization of fine circuits, pads or microbumps with photoresists because photoresist are attacked by the highly alkaline zincate solution. Accordingly, electroless nickel plating on aluminum substrate by methods other than the zincate process have been investigated. As a result, straight-walled bumps of up to approximately 20μm in height and between 20μm to 80μm in cubic area can be formed without damege to the photoresist by nickel displacement followed by electroless plating. In addition, nickel bumps can be more uniformly fabricated on an aluminum substrate initially accelerated with DMAB and followed by electroless nickel plating.

Applications of Porous Flow‐Through Electrodes: V . Electrowinning of Zinc from Flowing Alkaline Zincates at Packed‐Bed Electrodes

The electrowinning of Zn was achieved from flowing alkaline zincate solutions at packed‐bed electrodes. The current efficiency of the process is affected by the simultaneous hydrogen evolution, the zincate concentration, flow rate, conductivity, and temperature of the electrolyte. The process is mass‐transfer controlled. The limiting current is directly proportional to the concentration and flow rate of the zincate solution. The maximum current efficiency coincides with the limiting current of the Zn electrowinning reaction. While increases in the zincate concentration and/or flow rate increased the current efficiency, similar increases in the temperature and/or electrolyte conductivity had adverse effects. This result is interpreted in the light of existing electrochemical theory. While the current efficiency indicates the percentage of the total current consumed in electrowinning, we also defined the conversion efficiency which is a measure of the ability of the packed bed electrode to electrodeposit the zincate ions as they flow through the electrode. This conversion efficiency, as well as the limiting current, depend on the mechanism of mass transfer within the packed bed. Although we measured Zn limiting currents of a few hundred milliampere per square centimeter at only modest electrolyte flow rates, the conversion efficiency was low in view of the relatively large size of the particles of the bed. The results agree with predictions based on a dimensionless group which combines the structural and transport properties of the system. Conditions are defined where one can achieve higher conversion efficiencies.

Potentiostatic passivation of zinc in alkaline solutions

The potentiostatic passivation of upward-facing horizontal zinc electrodes in quiescent 0.4–4 M KOH solutions has been studied according to different models and by SEM, coulometry and ICP. An initial flat maximum was found for all the potentiostatic transients except in 0.4 M KOH. The greatest part of the transients consisted in the initial maximum followed by a current plateau and a sigmoidal sharp decrease. The remaining curves were gradual modifications of the latter. The first part of all the transients could be interpreted according to a diffusion-controlled growth of a porous film coupled to metal dissolution from the electrode surface. In this model, the metal is dissolved as its ions and part of them react with the anions already present in the solution to form the anodic film, the rate of electrocrystallization being determined by the instantaneous concentration of the metal ions at the electrode surface. However, the second phenomenon appearing in practically all the potentiostatic transients, i.e., the current plateau and further sigmoidal sharp decrease must be explained by a model of 2D ohmic-controlled film growth.

Ionic Mass Transfer of Zinc in Alkaline Solutions with Simultaneous Hydrogen Evolution

Sherwood numbers were determined for mass transfer of zinc in alkaline solutions with coevolution of hydrogen. The separation between the zinc and the hydrogen current was performed using polarization curves. The separation between Sherwood numbers due to natural and bubble convection was obtained by an additive law since hydrogen evolution took place only at current densities higher than the zinc limiting current density. It is shown that the contribution to mass transfer by bubbles, relative to natural convection, increases with both zinc concentration and hydrogen current density. Under industrial conditions of high zinc concentration (0.9 mmol/cm3 ) and high current densities (−0.24 A/cm2), the effect of the hydrogen bubbles is predominant even if the hydrogen current efficiency is low (3.3%), the Sherwood numbers being, respectively, 424 and 115. The data obtained in this work and others give support to the dimensionless correlation

On the Electrowinning of Zinc from Alkaline Zincate Solutions

The cathode current efficiency for the electrowinning of zinc was determined under different conditions of alkali concentration (2–8M), current density (10–40 mA · cm−2), and temperature (30–75°C). Some remarkably high values of current efficiency were observed especially at 75°C. A possible mechanism involving the discharge of monovalent complex ions is suggested. The effect of two tetra‐alkyl ammonium salts TAS(a) and TAS(b) was investigated. The tetra‐alkyl ammonium hydroxide, TAS(a) was found more suitable than the tetra‐alkyl ammonium ethoxide, TAS (b). The morphology of zinc deposits was inspected by the scanning electronmicroscope; these were mainly spongy deposits. The cell voltage was measured and the energy consumption was calculated and was found to decrease with the increase of temperature. The total energy kWh/kg of deposited zinc from an additive‐free solution at 75°C was 1.253 kWh/kg. In the presence of 0.37 mg/l TAS(a), a further reduction was observed. The energy consumption amounted to 1.180 kWh/kg at the same temperature. The current density for maximum efficiency was 20 mA/cm2.

Different commercial grades of aluminium as galvanic anodes in alkaline zincate solutions

The self corrosion and electrochemical properties, such as open circuit potentials, polarisation characteristics and anode efficiencies, of different grades of aluminiumviz., 2S, 3S, 26S and 57S were examined in 4 M NaOH, containing 0.01 to 0.6 M zinc oxide. From these studies, 3S and 57S aluminiums were found to be the most suitable anode materials, among the different grades of aluminium and 4 M NaOH solution containing 0.6 M zinc oxide was found to be the best electrolyte.

The Passivating Films of Zinc in Alkaline Solution

The Passivating Films of Zinc in Alkaline Solution

Mechanism of Cathodic Potential Oscillations of the Zinc Electrode in Alkaline Solutions

A detailed investigation of the mechanism of cathodic potential oscillations of zinc in alkaline solutions is described and analyzed here. The oscillation period is divided into three successive times, each corresponding to a particular phenomenon, which are defined as: the depletion, supersaturation, and recovery time. They are studied experimentally using chronopotentiometric recordings over a large range of conditions (−0.0055 to −0.17 A/cm2 and 0.15–0.9 mmol/cm3 in 8.4 mmol/cm3 aqueous at room temperature) and the results are analyzed theoretically. The nature and relative importance of the different overpotentials are determined and discussed during each one of the three times occurring in each period. The controlling mechanism for both the zinc reaction and zinc complex ion transport is thus deduced and discussed.