Anodic dissolution of nickel in concentrated sulfuric acidic solutions

The anodic dissolution of nickel in concentrated sulfuric acidic solutions is characterized by two diffusion plateaux related to the active and transpassive regions. For the same speed of rotation of a rotating disc electrode, the two plateaux currents are almost identical. However, the less anodic one gives rise to a rough electrode surface while a polished surface is observed at the more anodic one. For nickel as-received, mass transport influences the current over the whole potential range. After heat treatment, only the current for the two plateaux were mass transport controlled. Electrochemical impedance diagrams show that the lowest frequency capacitive loop is influenced by mass transport. Electrohydrodynamic (EHD) impedance diagrams show two different regions. In the low frequency range, the results follow the theoretical curve corresponding to a uniformly accessible electrode with a very high Schmidt number around 10⁷. At high frequency, the EHD impedance may correspond to an interface covered by a gel layer formed from the products of the anodic dissolution.     

New electrodes for oxygen evolution in acidic solution

Conventional electrowinning of metals such as zine and copper from 1–1.5 kmol m^–3 H₂SO₄ electrolytes involves anodic oxygen evolution at Pb alloy/PbO₂ anodes operating at 200–800 A m^–2. The oxygen overpotential, estimated to be about 0.6 V, constitutes a significant proportion of the cell voltage (typically 2.5 V for Cu and 3.3 V for Zn). The objective of this work was to lower the anode overpotential and so decrease the process specific energy requirements, by devising new anode materials based on: (1) PTFE-bonded PbO₂ catalysts, supported on Pb–Ag alloys; (2) modification of the pore structure of porous electrodes to maximize the utilization of the available surface without the use of PTFE bonding. Both methods are shown to produce significant benefits in lowering oxygen overpotentials, though the latter technique has been tested only in alkaline electrolytes, as yet. However, with the former approach, substrate oxidation through the porous catalyst layer has been found to cause catalyst shedding after>60 h at 1 kA m^–2, though careful pre-oxidation of the anode substrate appears to extend the anode life.Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Anodic dissolution and oxygen evolution on binary and ternary iron-silicon alloys

Some aspects of the anodic behaviour of iron-silicon alloy anodes are reported. The metallurgy of various alloy compositions before and after anodic exposure is given. The surface-formed films have been studied using infrared, ESCA, and other methods. The steady-state potential-current-time behaviour has been investigated. Potential oscillations, not previously reported for this system, were found. The protective mechanism of the Si is explained in terms of a model based on relative atomic volumes, while the oscillations appear to be due to anodic film breakdown.     

The oxygen evolution reaction on platinum,iridium, ruthenium and their alloys at 80°C in acid solutions

Kinetic parametes were determined for the oxygen evolution reaction on 50–50 atom percent alloys of RuIr, RuPt, and IrPt and compared with results obtained using ruthenium, iridium, platinum, and RuO₂/TiO₂ electrodes. The potentiostatic studies were made on oxide covered electrodes at 80°C in both 1.0 M H₂SO₄ and 1.0 M CF₃SO₃H. Cyclic voltammetric studies showed that while these noble metals and alloys are about equally effective as electrocatalysts for the hydrogen evolution reaction, striking differences in activity are found for the oxygen evolution reaction. The order of electrocatalytic activity towards oxygen evolution in H₂SO₄ is Ru > RuIr alloy ～ RuO₂/TiO₂ ～ Ir > IrPt alloy > RuPt alloy > Pt. The type of acid used had very little effect on the kinetic parameters. The lower electrocatalytic activities when platinum is present is probably due to the formation of a platinum oxide film. The dual barrier model is used to interpret the results for the electrodes containing platinum. The best electrocatalysts for oxygen evolution in acid solutions consist of noble metals which form oxide films (RuO₂, IrO₂) possessing metallic characteristics.     

Preparation of ruthenium dioxide electrodes and their anodic polarization characteristics in acidic solutions

Anhydrous ruthenium dioxide electrodes were prepared by heating hydrous ruthenium dioxide at different temperatures and their anodic polarization characteristics in acidic solutions were investigated, in connection with their physical properties. The anodic polarization characteristics are greatly affected by the preparing temperature and are closely related to their physical properties, such as the crystallinity, the crystalline size and the resistivity. Generally, the electrode composed of poorly crystalline ruthenium dioxide shows the good anodic polarization characteristics. In conclusion, it was revealed that excellent anode material is obtainable by heating hydrous ruthenium dioxide in air at about 450°C. This electrode is quite stable under the anodic polarization in acidic solutions and has strong resistance against the anodic dissolution together with very low overvoltage for the oxygen evolution.     

The anodic evolution of oxygen on platinum oxide electrode in alkaline solutions

The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasil'shchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.     

Anodic, cathodic and cyclic voltammetric deposition of ruthenium oxides from aqueous RuCl3 solutions

Anodic, cathodic and cyclic voltammetric (CV) deposition of ruthenium oxides from aqueous RuCl₃ solutions have been investigated using stationary and rotating disk electrodes (RDE) in this work. The CV deposition behavior was examined using a RDE to differentiate the contribution of current from the reactions of ruthenium ions in the electrolyte and ruthenium oxides already adsorbed on the electrode. The results indicate that the CV growth of ruthenium oxides within the potential range of aqueous electrolyte decomposition is attributed to the anodic oxidation of ruthenium ions in the electrolyte. Cathodic deposition occurs only at potential negative than −0.30 V versus saturated calomel electrode (SCE) when H₂ evolves on the electrodes. Anodic deposition of ruthenium oxides can be obtained effectively in the potential range of ca. 0.9-1.1 V versus SCE, depending on the pH value of the electrolyte. The optimum anodic and cathodic deposition potential for maximum deposition efficiency is 1.0 and −0.9 V versus SCE, respectively, in the electrolyte solution of pH 2.     

Anodic behaviour of pyrite in acid solutions

In acid solution, pyrite gives reproducible anodic current3v?oltage curves with Tafel slopes 95 ± 10 mV. There is no influence of pyrite semiconducting properties on the kinetics of the anode process. This process, as determined by coulometry at constant potential and product analysis, is a combination of the reactions

and

The sulphate route dominates over the potential range accessible at ambient temperature, and the sulphate yield increases linearly with potential. Data from other sources suggest that sulphate yield at fixed potential may be independent of temperature. Evidence from studies of the electrochemical behaviour of sulphur and its compounds leads to the conclusion that elemental sulphur is not an intermediate in the sulphate route. A mechanism for sulphate formation involving adsorbed oxygen-containing intermediates is postulated and the kinetics analysed under Temkin adsorption conditions. The Tafel slope and pH dependence suggest that the second electron transfer step is rate-determining.     

Anodic oxidation of tellurium in NaOH solutions

The potential of tellurium anodes has been measured as a function of current density in 0.01-3 N NaOH at 30°C. The results indicate the oxidation of tellurium into tellurous acid. The initial discharge of hydroxyl ions governs the overall reaction rate within the concentration range 0.01-1 N. In concentrations above 1 N, the dual chemical-electrochemical formation of TeO is suggested to be rate-determining in the low current density range. At higher currents, hydroxyl-ion discharge again controls the reaction rate.

Accumulation of tellurous acid on the electrode surface at high anodic polarization leads to the interference of resistance overpotential, whose magnitude in dilute solutions is so large that the potential rapidly approaches 40 V. At such a potential, tellurium is probably oxidized to the trioxide.     

Anodic behaviour of arsenopyrite and cathodic reduction of ferrate(VI) and oxygen in alkaline solutions

This paper presents the results of an electrochemical study of the anodic characteristics of arsenopyrite in strongly alkaline solutions and of the cathodic reduction of ferrate(VI) and of dissolved oxygen at an arsenopyrite surface at potentials which are relevant to the oxidation reactions. Cyclic voltammetry at both arsenopyrite disc and arsenopyrite disc/platinum ring electrodes has shown that arsenic(III) is the main product of the anodic process at potentials in the region of the rest potential during oxidation by either ferrate(VI) or oxygen. Evidence for partial passivation of both the anodic and cathodic reactions has been obtained from potentiostatic current–time transients. The initial stage of oxidation by ferrate(VI) has been shown to be mass-transport controlled and this is also true of the oxidation by oxygen in dilute solutions of sodium hydroxide.     

Anodic behaviour of indium in sodium chloride solutions

Passive film growth, electrodissolution and pitting corrosion of indium electrodes in NaCl solutions in the pH range 2–12.6 were studied using potentiostatic and potentiodynamic techniques complemented with SEM. In the pH range between 10.5 and 12.6 the oxide formation should involve a mixed control, a surface reaction plus a diffusion process. On decreasing pH the process becomes more complex due to the formation of In(I). The effect of chloride ions upon the active and passive behaviour of indium has been determined. Chloride ions in neutral and alkaline solutions produce pitting at potential values more positive than those of the active—passive transition. Localized corrosion appears to be under a competitive surface mechanism involving the formation of the passive film and the nucleation and growth of an indium chloride salt layer.     

Anodic passivation of tin in slightly alkaline solutions

The anodic oxidation of tin in 0.2 M borate buffer solution has been studied by potentiodynamic, potentiostatic and galvanostatic techniques. It was found that although a large amount of charge is consumed during anodic oxidation, only a fraction of this charge is involved in film growth.

The initial stages of metal passivation under dynamic conditions are best described by Mueller's dissolution—precipitation model. Under potentiostatic conditions two different equations describe the kinetics of film formation. For anodic potentials lower than 1.6 V vs sce a logarithmic dependence holds between cathodic charge and time. At higher potentials, the growth rate is controlled by a nucleation process and the rate law is more complicated. Under galvanostatic conditions the logarithms of the growing time is proportional to the inverse of the cathodic charge. These results are discussed in terms of theories of film growth.     

羧甲基壳聚糖在酸性溶液中缓蚀性能

采用失重及电化学方法（EIS 和 Tafel曲线）研究了羧甲基壳聚糖（CM-chitosan）作为一种绿色缓蚀剂,在1 mol·L-1 HCl 和0.5 mol·L-1 H2SO4溶液中对碳钢的缓蚀作用。结果表明,CM-chitosan是一种良好的缓蚀剂,且在1 mol·L-1 HCl 溶液中的缓蚀作用要优于在0.5 mol·L-1 H2SO4溶液中的缓蚀作用。在HCl和H2SO4两种酸性溶液中,CM-chitosan在碳钢表面的吸附都遵循修正的Langmuir吸附等温式。     

纯Fe在酸性AlCl3-EMIC离子液体中的阳极行为

重点研究了纯铁（Fe）在2∶1酸性AlCl₃-氯化1-甲基-3-乙基咪唑（AlCl₃-EMIC）中的阳极行为。通过三电极体系测定了Fe在AlCl₃-EMIC中的阳极极化和不同电位下的电流-时间曲线,利用恒电位溶解计算了Fe溶解的价态,最后通过扫描电子显微镜（SEM）观察了不同电流密度下Fe的溶解形貌。结果表明：Fe表面的氧化膜会阻碍Fe的溶解,在去除氧化膜后,当电位达到-0.35 V时,Fe开始发生活性溶解,产物为Fe（Ⅱ）。Fe的阳极电流密度随电位的增加而增加,达到峰值后显著下降。小电流密度下（如2 mA·cm^-2）,Fe的溶解形貌随时间的延长,呈现先点状腐蚀再均匀腐蚀的特点;而大电流密度下（如15 mA·cm^-2）,Fe电极表面会生成一层固态物质,同时可见清晰的划痕,即不发生明显的腐蚀。     

Transient film formation on chalcopyrite in acidic solutions

The transient time-dependent region characteristic of the anodic passive film formation on chalcopyrite in sulphuric acid solutions was studied by galvanostatic measurements. The occurrence of two passivation subregions was observed. Both followed the Sato–Cohen (logarithmic) model for the growth of anodic passive films. The experimental electric field for the two passivated steps was approximately 10⁵ V cm^?1. The electric field for the first step, at lower potentials, decreased with increasing ionic strength, but was pH independent. In the second step, the electric field decreased with increasing pH. The Tafel relationship was followed for constant electrical charge passed. The transition points between first and second step had almost the same electrical charge for different current densities and the electrical charge of the transition point decreased with increasing pH. The effects of temperature and iron/copper ion additions were also studied.     

Hydrogen evolution on conducting polymer electrodes in acidic media

Hydrogen evolution reaction (HER) was studied on polyaniline (PAn), polypyrrole (PPy) and on aniline/pyrrole (PAn–PPy) copolymer in acidic solutions. The cathodic Tafel slopes (b_c) and exchange current densities (j₀) were calculated from Tafel curves obtained in solutions of X M H₂SO₄ (X = 0.1, 0.2, 0.3, 0.4 and 0.5 M). Activation energies (E_a) were determined. The E_a-values were found to be ca. 26 for PAn, 36.5 for PPy, 40.6 for PAn–PPy and 20.6 kJ mol⁻¹for Pt.     

Anodic behaviour of zinc in methanol solutions of lithium perchlorate

Anodic dissolution of poly- and single-crystals of zinc was performed in methanol solution of lithium perchlorate by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The mechanism of anodic dissolution of zinc in organic solvents occurs in two oxidation steps. It is the first step that surface anodic product is created. Stability of the surface product is much better in anhydrous organic environments than in aqueous media; because the product is stable, a barrier layer composed of is formed at low anodic overvoltage. The formation of the layer is much stronger on the low index than on the high-index plane (0 0 0 1), where the adsorption of anodic product is more difficult.     

Anodic characteristics of amorphorous palladium-base alloys in sodium chloride solutions

The anodic characteristics of a variety of amorphous palladium-base alloys were examined with a view to their use for the production of sodium hypochlorite by electrolysis of dilute sodium chloride solutions at ambient temperature. The corrosion resistance of palladium-metalloid alloys was obtained by alloying with platinum group metals and/or valve metals. Among these alloys, rhodium-containing alloys showed high electrocatalytic activities for chlorine evolution. Surface activation treatment was, however, necessary to obtain sufficiently high activities for chlorine evolution at low overpotentials. Surface-activated amorphous alloys possessed considerably higher current efficiency for chlorine evolution in comparison with currently used anodes.     

Dissolution kinetics of WO3 in acidic solutions

Potentiostatic polarization and rotating disk electrode techniques were used to obtain the rate constant for the dissolution of electrochemically-formed (at 1 V) WO₃ on tungsten (W) in acidic solutions. The corresponding rate constant for the chemical dissolution of WO₃(s) powder was found by measuring the dissolved tungsten concentration as a function of time and pH. The chemical dissolution experiments supported the view that the rate-determining step in the anodic reaction of W in acidic solution is the chemical dissolution of WO₃(s) formed on the metal surface. Zeta potential measurements gave the isoelectric point (iep) of the WO₃(s) powder as pH 1.5, a value that was somewhat smaller than the point of zero charge (pzc) of WO₃(s) formed on W metal (pH 2.5). This difference was attributed to the highly hydrated nature of the oxide film formed on W metal in aqueous systems.