Hydrated structures in the anodic layer formed on lead electrodes in H₂SO₄ solution

Abstract

SEM and TEM observations of the corrosion layer obtained during the potentiostatic oxidation of lead electrodes in H₂SO₄ solution have shown that, at potentials above 1.00 V vs Hg/Hg₂SO₄, a lead dioxide layer is formed with crystal and gel-like (hydrated) structures. The crystal zones of the corrosion layer contain α- and β-PbO₂ crystals. Applying controlled thermal degradation it has been established that hydrated zones (denoted as PbO(OH)₂) comprise about 10% of the corrosion layer. For comparison, the lead dioxide active mass of the lead-acid battery is hydrated over 30%. On prolonged polarization of the lead dioxide electrode at 1.50 V, the basic electrochemical reaction that takes place is oxygen evolution. It has been suggested that this reaction occurs mainly at the interface crystal/gel-like zones. On opening the circuit, the electrode potential reaches the equilibrium potential for the PbO₂/PbSO₄ system within a rather long period. This potential decay is related to the diffusion of oxygen through the bulk of the corrosion layer (probably through its hydrated zones) to the solution and to the metal. A suggestion is made that hydrated zones are also involved in the oxygen reaction.