Mechanism of the action of Ag and As on the anodic corrosion of lead and oxygen evolution at the Pb/PbO( 2− x)/H 2O/O 2/H 2SO 4 electrode system

Abstract

When a Pb electrode, immersed in H 2SO 4 solution, is polarized anodically in the PbO 2 potential range the Pb/PbO( 2− x )/H 2O/O 2/H 2SO 4 electrode system is established. Oxygen is evolved at the oxide—solution interface. The oxygen atoms formed as intermediates diffuse into the anodic layer and oxidize the metal. Through a solid-state reaction, the metal is oxidized first to tet-PbO and then to PbO 2. By studying the changes in the rate—potential relations of the above reactions, as well as the phase and chemical composition of the anodic layer, it was possible to elucidate the effect of Ag and As on these processes. The additives were introduced into the electrode system either by alloying with lead or by dissolving them in the H 2SO 4 solution. When added to the solution, both Ag and As lower the overvoltage of the oxygen evolution reaction. They have practically no effect on the corrosion reaction under galvanostatic polarization conditions. If alloyed in the metal, Ag reduces the oxidation rate of Pb significantly, while As enhances it. Both additives lower the stoichiometric number of the anodic oxide layer, ie they retard the oxidation of PbO to PbO 2. The results of these investigations were used to develop further the model of the mechanism of the reactions proceeding during the anodic oxidation of lead in H 2SO 4 solutions.