V. S. Shaldaev

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.

Development of pitting corrosion of stainless steel 403 in sodium chloride solutions

Pitting corrosion of stainless steel 403 in the NaCl solutions is studied. It is shown that the experimental results obtained under the potentiostatic conditions do not provide sufficient information on the behavior of steel under the conditions of free corrosion. The corrosion experiments take long time, their results exhibit poor reproducibility; however, they are necessary for obtaining reliable data on the corrosion behavior of steel. The development of corrosion process is reflected in the variation of corrosion potential with time. The effect of concentration and temperature of NaCl solutions on the development of pitting corrosion is studied. The peculiarities of the distribution of pits over the test specimen surface are revealed. In 1 M NaCl solution at 20°C, few pits arise. Some of them repassivate; some pits grow, initially, in diameter and depth and, then, almost only in depth. The examples of through pit formation on the specimens 4 mm thick are presented.

Development of pitting corrosion on 20Kh13 steel

The nucleation and propagation of corrosion pits on 20Kh13 steel in the NaCl solutions with various concentrations, temperatures, and pH values are studied under the potentiostatic conditions and at the free-corrosion potential. In the potentiostatic experiments, the variations of the depth and diameter of pits and their number with the time are determined. It is shown that the state of metal surface in a certain area adjacent to the active pits changes. The dimensions of the area are estimated. The results, which were obtained at the free-corrosion potential, were much less reproducible. At the free-corrosion potential, in contrast to the potentiostatic conditions, the pit depth increased only slightly and the pit width increased to a larger extent. It is shown that the pH value of NaCl solutions has a pronounced effect on the development of pitting corrosion on 20Kh13 steel.

Determination of Corrosion Rate of Molybdenum, Rhenium and Their Alloys in Sodium Chloride Solution by the Method of Tafel Extrapolation

Based on the measured voltammograms, the corrosion potentials and corrosion current densities of Mo, Re, and several their alloys in the NaCl solutions were determined by the method of Tafel extrapolation. The dependences of Ecorr and icorr on the alloy composition, including the corresponding values for the metals (alloy components), were plotted. The obtained values of icorr and the corrosion rates in the linear units were compared with the literature data.

Pitting on the 20Kh13 Steel in Chloride Solutions

The dependences of the pitting potential, determined by potentiodynamic method, on the NaCl solution concentration, pH, and temperature are found. The repassivation potential and the induction time for the pitting under different conditions are determined. The corrosion potential and kinetic parameters of the steel dissolution in pits are measured. The corrosion current is evaluated, leading to the determination of the steel dissolution rate in pits at the corrosion potential.

The effect of electrode rotation on pitting passivity breakdown of metal in sodium chloride solution

The effect of electrode rotation on the initiation and development of pitting passivity breakdown of two steels and nickel in the chloride solutions is studied. The effect of rotation is most pronounced for the development of pitting on steels in the neutral unbuffered NaCl solution. The electrode rotation has almost no effect on the pitting development in the nickel/NaCl + NaOH solution sistem. The discussion of experimental results is presented.

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.

Limiting Current of Tungsten Electrochemical Dissolution under Conditions of Natural Convection: Vertical Plane Electrode

A problem of ionic transfer in the electrochemical dissolution of a plane vertical tungsten anode in alkaline solution under the conditions of natural convection is solved. An equation is obtained, which enables one to calculate the limiting current of tungsten dissolution under these conditions. The limiting currents of tungsten dissolution in KOH solutions of various concentrations are calculated and measured. The calculated limiting current are 20% higher than measured ones. The introduction of an empirica numerical coefficient into the equation for the limiting current enables us to perform sufficiently accurate calculations.

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.

Effect of Natural Convection on the Anodic Dissolution of Horizontal Tungsten Electrodes

The data on the limiting currents of anodic dissolution of horizontal tungsten electrodes measured in alkaline solutions under the conditions of natural convection are presented. It is shown that conditions determining natural convection (downward or upward position of the electrodes active surface, solution concentration, etc.) affect the limiting current, the current variation with time, and the origination of convective instabilities of various types.