Characterization of anodic deposits formed on Pb-Ag electrodes during electrolysis in mimic zinc electrowinning solutions with different concentrations of Mn(II)

This paper studies the influence of Mn(II) ion concentration on the characteristics of the films formed during water oxidation on Pb-Ag (0.5%). Tests were performed in an electrolytic bath whose composition was similar to that used in industrial electrowinning of zinc: 180 g/L H₂SO₄, 75 g/L Zn²⁺, with amounts of Mn(II) ranging from 2 to 12 g/L. A constant oxidation current density of 300 A/m² was applied for 96 h to Pb-Ag electrodes in order to form the anodic films. These films are composed of manganese oxides, and lead sulfates and oxides, determined by SEM-EDX analysis. In order to associate the corresponding reactions involved with each corrosion product, the anodic films were also potentiostatically formed as a function of time, and characterized by polarization curves obtained immediately after the film formation. It is shown that α-MnO₂, an electrocatalytic phase that favors both the reaction of oxygen evolution and Mn(II) oxidation, forms within the first 10 min, even at relatively low concentrations of Mn (2 g/L). After 1 h, the layer thickness increases and the catalytic effect tends towards its own inhibition; likewise, as the amount of Mn increases, the layers become more cracked and brittle, causing electrolyte penetration through the cracks of oxide layers, which results in the activation of the Pb-Ag surface anode.