On the theory of anodic levelling: behaviour of macroprofiles

The rate of anodic levelling of macroprofiles under mass-transport control is studied analytically and by the finite element method (FEM) using the Nernst diffusion-layer approach. It is found that the rate of anodic levelling of macroprofiles is always slower than that of microprofiles and depends on the geometrical parameters ε₀, δ, λ representing, respectively, the profile height, the diffusion-layer thickness and the profile wavelength. On sinusoidal profiles when ε₀/δ → ∞, the rate of levelling tends towards zero.     

Anodic levelling of model profiles with pulsating current

The rate of anodic levelling with pulsating current is being investigated with triangular model profiles made of nickel using a flow channel cell. Observed results are compared to theoretical calculations of the rate of anodic levelling of macroprofiles and of microprofiles under d.c. conditions. It follows that the rate of anodic levelling is smaller above the pulse-limiting current density than that below it, and is smaller than that predicted for an ideal macroprofile. With pulsating current the rate of levelling of macroprofiles did not depend on profile orientation with respect to electrolyte flow.     

On the theory of anodic levelling: FEM simulation of the influence of profile shape and cell geometry

The Finite Element Method (FEM) has been applied to study the influence of profile shape and cell geometry on the rate of anodic levelling under conditions of negligible electrode polarization. The precision of the numerical computations is assessed by comparison with analytical solutions for limiting cases. It is shown that at intermediate cell geometries the rate of levelling of triangular and sinusoidal profiles depends on two dimensionless parameters expressing the ratio of profile height to profile wavelength and to interelectrode distance, respectively. Levelling of spaced grooves or recesses is slower than levelling of regular sinusoidal or triangular profiles of similar geometry.     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.     

An ellipsometric study of anodic passivation of nickel in borate buffer solution

Ellipsometric and electrochemical measurements of the anodic passivation and secondary passivation films on nickel in borate buffer solution at pH 8·42 were made. The reference bare surface, obtained by cathodic reduction of electropolished nickel, gives the optical constants N₃ = 1·65?3·81 i for nickel. In the passivity region, where a little dissolution of nickel occurs, the surface film has the optical constants N₂ = 2·7?0·25 i and the film thickness as estimated by ellipsometry increases linearly with rise of the potential, from 4 to 13 Å, which is in fairly good agreement with the coulometrically estimated film thickness. In the oxygen evolution region, secondary passivation occurs with an abrupt increase in both optical constants and thickness of the surface film.The anodic passivation of nickel is compared with that of iron in terms of thickness of the surface film as a function of potential.     

Interaction between nickel hydroxy phthalocyanine derivatives with p-chlorophenol: Linking electrochemistry experiments with theory

In this work the interaction between peripherally (β) substituted nickel tetrahydroxyphthalocyanines (β-NiPc(OH)₄ and β-Ni(O)Pc(OH)₄) with p-chlorophenol is theoretically rationalised by performing calculations at B3LYP/6-31G(d) level. Density functional theory (DFT) and molecular orbital theory are used to calculate the condensed Fukui function for phthalocyanine derivatives and p-chlorophenol, in order to determine the reactive sites involved when p-chlorophenol is oxidized, and to compare theoretically predicted reactivity to experimentally determined electrocatalytic activity. Electrocatalytic activities of adsorbed NiPc derivatives: ads-α-NiPc(OH)₈-OPGE (OPGE = ordinary poly graphite electrode), ads-α-NiPc(OH)₄-OPGE and ads-β-NiPc(OH)₄-OPGE are compared with those of the polymerized counterparts: poly-α-Ni(O)Pc(OH)₈-OPGE, poly-α-NiPc(OH)₄-OPGE and poly-β-NiPc(OH)₄-OPGE, respectively.     

A general model of the anodic behaviour of nickel in acidic media

An experimental study of the anodic behaviour of nickel in acidic media is carried out by steady state polarization curves and faradaïc impedance measurements as a function of various parameters related to the electrolyte: pH, concentration and the nature of the anions. We propose several models of the electrode reaction path based on heterogenous chemical kinetic concepts. By a convenient numerical fitting method, we calculate a set of kinetic parameters which are afterwards put into the theoretical equations for dc and the impedance. By this procedure we are able to choose a general model accounting for the behaviour of nickel in a wide range of experimental conditions: active dissolution, passivity, secondary passivity and the oxidation of OH^? to molecular oxygen.     

On the theory of the ising model of the macromolecular solution

The partition function of a macromolecular binary solution is transposed in the partition function in Ising's model. On the basis of this model the macrocanonical function is explained by the presence of the interaction forces, in the one-dimensional as well as in the two-dimensional cases. The chemical potential of the macromolecular solution is calculated in both cases and compared with the results obtained by Flory and Huggins.     

The anodic characteristics of the massive β-MnO2 doped with noble metals in sodium chloride solution

The anodic characteristics of the massive β-MnO₂ doped with a slight amount of noble metals were investigated in NaCl aqueous solution. The doping of noble metal, especially Pd, decreases the chlorine overvoltage enormously. By the kinetic considerations, it was clarified that noble metal sites dispersed on the oxide surface serve effectively as an active site for the chlorine evolution reaction. Furthermore, it was suggested that new effective catalysts for the chlorine evolution might be developed by using such noble metal-doped β-MnO₂ systems.     

Voltammetry and anodic stability of a hydrous oxide film on a nickel electrode in alkaline solution

A hydrous oxide film on a nickel electrode was formed electrolytically by square-wave cycling in 1 mol dm-³ NaOH solution. The reproducibility of a second scan in a voltammetric experiment depended on holding the potential at a negative value (–1.1 V vs SCE) and on the positive potential limit. The hydrous oxide film, with a charge enhancement factor of 19, exhibited two stages of oxide reduction. Coulometric data showed that the majority of the oxide (80-90%) was reduced in Ni(III)/ Ni(ii) transition almost reversibly at potentials where the higher oxide was formed while, depending on the potential of formation, about 10–20% of the NiOOH was irreversibly reduced in the potential region of the hydrogen evolution reaction. An anodic stability test in the oxygen evolution reaction was carried out at 0.1 A cm^–2 The hydrous oxide layer was stable over 14 days of electrolysis with an unchanged charge enhancement factor. The electrocatalytic activity of the electrode, however, expressed through the overpotential at the same current density, was not maintained.     

Influence of some amino acids and vitamin C on the anodic dissolution of tin in sodium chloride solution

The safety of canned foodstuffs should be evaluated not only by recognition of functional foods but also by the presence of toxicants in the human diet. In this study, the anodic dissolution of tin in 3.5% NaCl solution with and without 25–100 mM glycine, serine, methionine, vitamin C and some of their binary mixtures was studied by means of potentiodynamic and impedance techniques. The corroded tin surface was examined by SEM. The results indicated that the passive behaviour of tin is greatly improved by the presence of 50–100 mM glycine and methionine while such improvement is achieved only at 100 mM serine. Both cysteine and vitamin C showed aggressive action. The influence of the glycine/methionine mixture was found to be similar to that of the individual components, whereas the presence of cysteine in the cysteine/methionine mixture annuled the inhibiting action of methionine. The impedance results revealed that the dissolution reaction of tin in NaCl solution with and without glycine, serine and methionine is controlled by the charge transfer process while in the presence of cysteine and vitamin C, the mixed charge transfer and diffusion control is dominant. A rough, bare tin surface was observed in the presence of cysteine and vitamin C while a clean surface covered by corrosion products, in some zones, was observed in NaCl solution containing methionine. © 2001 Society of Chemical Industry     

Ionization equilibria in solution of nickel(II) chloride in N,N-dimethylformamide

Visible absorption spectra and the molar conductance curve at NiCl₂ dissolved in dimethylformamide (DMF) have been determined at 25°C. Absorption spectra of ternary system Ni(ClO₄)₂ + Et₄NCl + DMF and NiCl₂ + chlorobenzene + DMF have also been determined at 25°C. It has been found that at lower concentrations of NiCl₂ monochloride nickel complex NiCl(DMF)⁺₅ and outer-sphere ion-pair {NiCl(DMF)⁺₅Cl⁻}⁰ coexist in the solution. The presence of pseudotetrahedral complex NiCl₃DMF⁻ and the existence of coordinative disproportionation equilibrium have been established in the more concentrated solutions of NiCl₂. The formation constant of outer-sphere ion-pair equal to (1.33 ± 0.2) × 10² has been derived from the conductometric data. The equilibrium constant of disproportionation reaction equal to (3.89 ± 0.2) × 10² has been determined on the basis of the absorption spectra of NiCl₂ in DMF.     

Processing of oxidized nickel ores with ammonium chloride

A method for processing nickel ores with ammonium chloride is considered. The results of thermodynamic calculations for the reactions of the oxide components of nickel ore with ammonium chloride are given. Thermogravimetric and kinetic investigations were performed. The technological sequence of the separation of oxidized nickel ores into individual oxides was developed.     

Electrical behavior of polycarbonate doped with nickel chloride

The dc electrical conductivity (σ) and permittivity (ε′) of polycarbonate (PC) discs doped with different concentrations of NiCl₂ are studied. Both (σ) and (ε′) increase with increasing the concentration of NiCl₂, with a maximum value at 30 wt %. The activation energies for pure and doped PC are 0.49 and 0.53 eV, respectively. The increase in the electrical conductivity with increasing NiCl₂ concentration is attributed to the formation of chargetransfer complexes (CTC); the increase in ε′ may be due to the rise in the interfacial polarization which results from the increase in boundaries between PC and NiCl₂ phases. The currents in PC discs, both pure and doped with NiCl₂, in the high-field region are attributed to space charge limited conduction. The time dependence of conduction current before and after reversal of applied voltage is also investigated. Some parameters such as the density of mobile ions and their drift mobilities are estimated. © 1995 John Wiley & Sons, Inc.     

Kinetics and mechanism of the anodic dissolution of nickel in HCl-dimethylsulphoxide solutions

The anodic dissolution of nickel in HCl-DMSO solutions containing different supporting electrolytes has been studied between 20–40°C. The electrodissolution and electrodeposition are predominantly activated electrode processes. At high anodic potentials and in the presence of perchlorate ions at high concentration passivation sets in.A probable reaction pathway is postulated which explains most of the experimental findings. Passivation corresponds to a salt precipitation-dissolution process at the electrode vicinity.     

The anodic oxidation of superimposed metallic layers: theory

The anodic oxidation of one metal superimposed upon another is governed by several factors, the most important being the resistivity difference between the two oxides. When the oxide of the superimposed metal is the less resistive, the metal order is conserved, the oxygen order is conserved, and the final voltage under constant current conditions is just the sum of the component potentials across the two oxide layers. When the superimposed oxide is the more resistive, however, fingers of the substrate oxide force their way through it after the manner of the Rayleigh-Taylor effect in superimposed liquid layers; the metal order is then partially inverted, the oxygen order likewise, and the final voltage is less than the sum of the components. These phenomena, however, will be modified by the effects of transport number, relative metal migration rate, oxide structure and Pilling-Bedworth ratio; the nature and possible interactions of these factors are discussed. Experimental evidence on the anodization of superimposed layers is then reviewed, and all observations can be accounted for.     

The anodic polarization characteristics of the graphite in alkaline solution

The anodic behaviour of a graphite electrode in alkaline solution was studied by means of the linear potential sweep method, the controlled potential electrolysis and the colourimetry. Linear sweep polarization curves indicated that the surface of graphite electrode is oxidized in the anodic region from ?0.3V.The numbers of electron participating in the oxidation of a carbon atom was calculated to be 1, 2, and 2 at 0.2, 0.3, and 0.4V, respectively, based on the quantity of electricity which was measured by the controlled potential electrolysis and the numbers of carbon atom on the surface of the graphite electrode, which was calculated with the double layer capacity.The structure of oxide layer formed on the graphite electrode was examined by the colourimetry with HgBr₂ and NaOEt. It was found that the oxide layer on the electrode contained carbonyl groups.     

Evaluation of electroactive intermediate states in anodic O2 evolution at chemically formed nickel oxide: Comparison with behaviour at nickel metal anodes

The behaviour of the kinetically involved intermediate states arising in the electrocatalysis of anodic oxygen evolution at chemically formed, high-area nickel oxide (NiO·OH) films on nickel metal as substrate is examined by means of analysis of potential (V) decay transients, following interruption of anodic polarization currents at various overpotentials. The potential decay behaviour is treated in terms of the dependence ofV(t) on log (time,t), and of ln (–dV/dt) as f[V(t)]. The pseudocapacitance associated with the potential-dependence of the coverage or surface density of the overpotential-deposited species involved as intermediates in the reaction at the oxide electrode surface is evaluated jointly from the potential decay and Tafel polarization behaviour, following procedures developed recently.In anodic O₂ evolution on oxide surfaces, such as NiO·OH, the intermediate states in the kinetics of the reaction are to be identified as OH or O species coupled with potential-dependent Ni(III) and Ni(IV) oxidation states of nickel, and the surface density of these states can be evaluated experimentally.The results obtained for anodic O₂ evolution on the chemically formed nickel oxide films are compared with the behaviour at anodically formed thin oxide films on nickel metal.     

The anodic dissolution of nickel—1: Perchlorate and fluoride electrolytes

A potentiostatic sweep technique has been used to study the anodic dissolution of nickel in acidic perchlorate, acetate and fluoride solutions. At slow potential sweep rates a prepassive film exists throughout the anodic region in perchlorate and acetate electrolytes. By the use of fast sweeps, or by the addition of F^?, film formation and growth is sufficiently reduced to reveal a linear anodic. Tafel region. The rate of active dissolution, which is independent of [H⁺] and [F^?], obeys the following rate law; i = 2Fka_w exp [βFE/RT], with β = 0·53.The following mechanism is proposed for active dissolution, with the first step rate-determining: (1) Ni + H₂O → NiOH_ads + H⁺ + e^?, (2) NiOH_ads → NiOH⁺ + e^?, (3) NiOH⁺ + H⁺ ? Ni²⁺ + H₂O. Prepassivation is thought to occur from the intermediate NiOH_ads through a solid state mechanism.