Alexey D. Davydov

Capacitance and photocurrent study of electronic properties of anodic oxide films on Nb and Ta. Evaluation of the ionized donor concentration profile in Nb2O5 film

Electronic properties of electrochemically formed oxide films on Nb were studied by photocurrent and differential capacitance measurements in 0.025 M KH2PO4+0.025 M Na2HPO4 electrolyte, pH 6.9. Oxide films of n-type conductivity were formed galvanostatically for final potentials ranging from 4 to 230 V. Measurements were performed in two potential regions, which correspond to formation of a depleted layer of variable thickness at relatively low potentials, and to complete depletion of oxide films of electronic charge carriers at higher potentials. In the first potential region the behavior of both capacitance and photocurrent, was governed by a build up of a depleted layer of potential dependent thickness. In the second, high potential, region, which extends up to the oxide film formation potential, the photocurrent and capacitance of oxide films in most features followed the trends typical of dielectric films containing defects and traps. The photocurrent and capacitance measurements on presumably dielectric oxide films formed on Ta were staged for comparison. The capacitance–potential measurements performed in the first potential region enabled us to construct the ionized donor concentration profile across the Nb2O5 film width. The limitations on the use of the C–E profiling method for electrochemically formed oxide layers are considered.

Limiting current of electrochemical deposition of copper from copper sulfate and sulfuric acid solution on a vertical electrode under conditions of natural convection

Abstract The transport problem is solved and a new equation is derived for calculation of the limiting current of copper deposition on a plane vertical cathode in CuSO4+H2SO4 (in excess) solution under the conditions of natural convection. In contrast to the earlier published studies, no ionic transport numbers were used for consideration of the contribution of migration transfer. The derived equation allows us to determine quantitatively the effect on the limiting current not only of the mass coefficient of supporting electrolyte, but also of the diffusion coefficients of its ions.

Oscillatory convective instability in electrochemical systems

Abstract Oscillatory instability induced by buoyancy forces in an electrochemical cell with plane horizontal electrodes is considered. The space between the electrodes is filled with solution containing two types of single-charged cations and one type of single-charged anion. Under conditions of transport limitation of the electrochemical reaction rate, low-frequency convective–concentration oscillations may arise in the system, which are the internal standing waves in the concentration stratified liquid. The conditions for the development of the waves, their frequency and space period are found. A possibility for using internal standing waves for the construction of low-frequency electrochemical resonators is discussed.

Electrochemical dissolution of tungsten under pulsed conditions

Limiting diffusion currents for the anodic dissolution of a tungsten rotating disc electrode in alkaline solution under rectangular current pulses have been determined experimentally. These currents were calculated by use of approaches developed by Ib1, Cheh, and Chin. Experimental and calculated results were compared. It is shown that the use of bipolar voltage pulses enhanced the anodic dissolution of tungsten in alkaline medium.

High-rate electrochemical dissolution of Ni–Cu alloys in nitrate electrolyte

High-rate anodic dissolution of nickel, copper, and two alloys Ni30Cu70 and Ni65Cu32 in NaNO3 solution was studied, using a flow-channel electrochemical cell. In all experiments, the initial interelectrode distance was 208 μm and the electrolyte velocity was 15 m s−1. The dependence of the current efficiency and surface brightening on the current density was determined. Voltage transients at various current densities were measured and voltammograms were constructed. Compared to nickel and copper, the alloys showed intermediate behaviour, especially at j > j1. The shape of the voltage transients and the occurrence of surface brightening were more suitable to detect the existence of a limiting current region than voltammograms, especially for Ni. Using the voltammograms and literature data, anodic and cathodic potentials and the voltage drop in the interelectrode gap at a given current density were estimated for j < j1.

Electrochemical Behavior of Nickel-Aluminum Alloys in Sodium Chloride Solutions

The anodic and cathodic reactions involved in the corrosion process on several nickel-aluminum alloys including two intermetallic compounds NiAl and Ni3Al in the NaCl solutions are studied. A procedure of pretreatment of test specimens and measuring the anodic and cathodic voltammograms is developed. It enabled us to obtain reproducible results including Tafel portions of voltammograms. The corrosion potentials Ecorr and corrosion currents icorr are determined by the coordinates of the intersection of anodic and cathodic Tafel plots. The dependences of Ecorr and icorr on the alloy composition (the content of nickel in the binary nickel-aluminum alloys), on the concentration of NaCl, and рН of unbuffered NaCl solutions with the additions of HCl or NaOH are determined. The anodic behavior of the alloys in a wide potential range is studied using the potentiodynamic method and the method of stepwise raising anodic potential with an exposure of electrode at each potential for a certain time. The dependences of pitting potential on the concentration of solution are determined for two intermetallic compounds.

Effective numbers of electrons as a characteristic of adsorption processes on perfectly polarizable electrode hemosorbents

An investigation of electroless electrochemical detoxification processes occurring on porous hemosorbent electrodes was performed. Cupric ions, isopropanol, t-butanol, and free hemoglobin were used as model toxicants. The elementary act of electrochemical adsorption detoxification was shown to nearly always be accompanied by partial transfer of charge. It was concluded that the method of experimental determination of effective numbers of electrons that correspond to the elementary act of detoxification is a robust tool for the selection and evaluation of relative effectiveness of hemosorbent electrodes.

Experimental Study of Propagation Stage of Pitting Corrosion of 20Cr13 Steel

The initiation and development of pitting corrosion of steel 20Cr13 in the NaCl solutions with various concentrations, temperatures, and pH values are studied under the potentiostatic conditions and at the free-corrosion potential. The pitting and repassivation potentials are determined using the method of cycling voltammetry. In spite of the fact that thus determined pitting potential is more positive than the corrosion potential (the open-circuit potential Eo.c.), the long-term experiments, which were performed at the free-corrosion potential, showed that pitting corrosion takes place without imposing a potential using an external power source. It is concluded that the probability of pitting corrosion of steel should be determined by comparing the corrosion potential (the open-circuit potential) with the repassivation potential Erp. Steel 20Cr13 is prone to the pitting corrosion, because Erp is more negative than Eo.c.. In the potentiostatic experiments, the variation of the depth and diameter of pits and their number with the time and the effect of temperature and electrode rotation on the pit propagation are studied. The results, which were obtained at the free-corrosion potential, are much less reproducible. In this case, in contrast to the potentiostatic conditions, the pit depth increased only slightly and the pit width increased to a larger extent. The effect of concentration, pH value, and temperature of NaCl solutions on the pit propagation is considered. It is concluded that the data on the development of pitting corrosion under the potentiostatic conditions can be hardly extended to the conditions of free corrosion potential.