Combined anode processes in sulfuric acid solutions

Abstract

Etching of carbon steel products in sulfuric acid solutions is, on the one hand, a common process in mechanical engineering and, on the other hand, harmful to the environment. As the concentration of H2SO4 decreases and the solution becomes saturated with Fe2SO4, the etching rate of the oxide film on the steel surface decreases. Therefore, while reducing the H2SO4 concentration to 25-30 g/l etching process is stopped, and replacing the etching solution. In this case, the concentration of Fe2SO4 reaches 400 g/l. During etching, two processes are observed: dissolution of oxides and dissolution of iron, which is located under the layer of oxides. These two processes can run simultaneously. The study of electrode processes in sulfuric acid solutions containing inorganic and organic impurities is the basis for the development of technological parameters for the regeneration of spent sulfate solutions. For the study used a little wear anode materials - platinum and lead oxide (IV). To determine the kinetic regularities of the anode process in selected anode materials, voltammetry was used with the construction of the obtained dependences in Tafel coordinates. The polarization dependences consist of two rectilinear sections with an inflection at logia ≈ -1,8 (ia, A · cm-2). The slope of the first section does not depend on the H2SO4 concentration and is 120 mV. The slope of the second section, for solutions with an H2SO4 concentration of 0,05 ... 0,37 mol/dm-3, is 60 mV, and for a concentration of 2,5 mol/dm-3 - 71 mV. A change in the concentration of sulfuric acid has practically no effect on the polarization of the anode. At a concentration of 5,0 mol·dm-3, in the area of high current density (≥ 1500 A∙m-2), the potential of the anode exceeds the PZCh for lead dioxide. This promotes the adsorption of sulfate ions on the surface of the composite anode and the onset of active oxygen formation. Under these conditions, the evolution of hydrogen peroxide is observed on the platinum anode. The polarization dependences consist of two straight sections with different slopes. For the entire range of sulfuric acid concentrations, at low current density portion is observed with a slope of 120 mV. Concentration of sulfuric acid at this site does not affect the kinetics of the process that corresponds to the literature data for platinum. The zero order in pH and independence from the concentration of sulfuric acid for this site indicates that the most probable mechanism of oxygen evolution is water rarefaction. The obtained values of the effective activation energy of the oxygen evolution process on the LOTC are close to the results obtained on platinum – 41,8 kJ mol-1 at Ea = 1,95 V. The value of the effective activation energy indicates the electrochemical nature of polarization, and its decrease with an increase in the anodic potential indicates a decrease in the strength of the bond between oxygen and the LOTC surface.