L. A. Beketaeva

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.

Nanosize catalysts based on carbon materials promoted by cobalt tetra(para-methoxyphenyl) porphyrin pyropolymer for oxygen electroreduction

We present the results of electrochemical and structural investigations of several carbon materials: carbon blacks AD 100 and XC 72, ultradisperse diamond (UDD), multiwalled nanotubes (MWNT), various types of filament-like carbon materials (CFC series), and similar carbon materials promoted with cobalt tetra(para-methoxyphenyl) porphyrin (CoTMPhP) pyropolymer (PP). The electrochemical studies were performed at room temperature in 0.5 M H2SO4 by using a rotating disk electrode (RDE), a rotating ring-disk electrode (RRDE) (a thin layer of test material was applied onto the disk electrode), and a floating electrode. Structural characterization of initial and promoted carbon materials involved the determination of specific surface area by the BET method and by the polarization capacitance from cyclic voltammograms, and the particle morphology and dimensions by the transmission electron microscopy (TEM) method. The study of kinetics and mechanism of oxygen electroreduction on carbon materials promoted with CoTMPhP PP showed that the catalysts based on carbon materials of CFC and UDD series possess high specific activity in this reaction and high selectivity with respect to oxygen reduction to water. These catalysts are superior to the catalysts, in which carbon blacks AD 100 and XC 72 are used as the supports, in the specific activity.

Study of degradation of membrane-electrode assemblies of hydrogen-oxygen (air) fuel cell under the conditions of life tests and voltage cycling

The degradation processes of HiSPEC 9100 (60% Pt/C) and 13100 (70% Pt/C) cathodic monoplatinum catalysts, which were tested under the model conditions and in the composition of membrane-electrode assemblies (MEA) of hydrogen-air and hydrogen-oxygen fuel cells, are studied. It is shown that, in all cases, the main reason for a decrease in the catalyst activity was a decrease in its surface area, which was caused by the coarsening of platinum nanoparticles, irreversible oxidation of a fraction of active centers, and the destruction of the catalyst due to the carbon support oxidation. The results of electrochemical measurements are supplemented with the structural investigations by the methods of transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that the degradation processes of MEA in the accelerated stress tests (AST) are similar to those in the long-term life tests. With respect to a decrease in the catalyst active surface area, the application of 2500 cycles in the voltage range of 0.6 to 1.2 V in the AST is equivalent to the life tests for 1010 h. During the fuel cell operation, the destruction of polymer electrolyte proceeds along with the catalyst degradation. This leads to a decrease in the ion-exchange capacity of the membrane and ionomer in the composition of cathode active layer.

Estimation of corrosion current by the analysis of polarization curves: Electrochemical kinetics mode

It is shown that the corrosion parameters, which were calculated by the computer analysis of a portion of pre-Tafel region of experimental polarization curve, enable us to obtain the complete calculated polarization curve containing the pre-Tafel and Tafel regions, which coincides with the complete experimental curve.

Electrochemical behavior of stainless steel in aerated NaCl solutions by electrochemical impedance and rotating disk electrode methods

Electrochemical behavior of stainless steel in 0.5 M NaCl solutions is studied. It is shown that, for this steel, under a considerable cathodic polarization of electrode, the oxygen electroreduction proceeds via a four-electron mechanism. The corrosion current is estimated. From the impedance spectra, elements of an equivalent circuit that describes the electrode behavior under the ac conditions are calculated. A mechanism of the steel passivation in the electrolyte is proposed.

Dependence of corrosion current on the composition of titanium-nickel alloy in NaCl solution

The corrosion current density and corrosion potentials of titanium-nickel alloys are determined as a function of a ratio between the alloy components in 0.5 M NaCl solution. For the alloy range from the alloy containing approximately 5 wt % nickel to TiNi intermetallic compound, the corrosion current of the alloys is determined predominantly by titanium, which exhibits a low corrosion rate (below 1 μA/cm2). A certain decrease in the corrosion current as compared with the corrosion current of titanium is observed with increasing content of nickel in the alloy. This is explained by an increase in the polarization of cathodic reaction. At a higher content of nickel, the corrosion behavior of the alloys is determined by this component; the corrosion current increases with increasing nickel content in the alloy.

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 current density by the rate of cathodic depolarizer consumption

The method of determination of metal corrosion rate, which does not require polarization of test specimen by an external power source, is described. The method involves the measurement of H+ ions consumption rate in the cathodic reactions at a constant pH value of solution, which is maintained by the compensating acid additives. The method application is demonstrated by two examples: the corrosion of iron in the weakly acidic sulfate and chloride solutions. Thus obtained results are compared with the data, which were determined by the methods of Tafel extrapolation and analysis of pre-Tafel portion of polarization curve.

Determination of corrosion current density on bulk nickel and nickel powder by the rate of cathodic depolarizer consumption

The method of “compensating additives” is used to determine the corrosion current of nickel and nickel powder in 0.5 M NaCl solution containing 0.01 M HCl. Comparing the corrosion currents measured on bulk nickel and nickel powder, the specific electrochemically active surface area of nickel powder is estimated.

Corrosion behavior of aluminum in 1 M HCl solution

The corrosion behavior of pure (99.999) aluminum in 1 M HCl solution is studied. The regularities of local gas evolution on the surface of test specimen at the open-circuit potential are determined. A number of sites, where hydrogen gas evolves, varies with the time passing through a maximum. The sizes of bubbles prior to their detachment from the specimen surface are determined. The time dependences of gas bubble radius in the course of the bubble growth are obtained. From the experimental results, it is concluded that, at the sites of hydrogen gas evolution, the cathodic reaction prevails, whereas the anodic reaction (aluminum etching) proceeds at the rest specimen surface area. No pits form at the sites of hydrogen evolution during the experiments (up to 5 h). The quantitative analysis of the cathodic polarization curve enabled us to estimate the rate (the corrosion current density) of almost general corrosion after the decay of local gas evolution. The long-term experiments (for 2 months) showed that the pitting corrosion of pure aluminum takes place in 1 M HCl.