Processes on the lead dioxide electrode in H2SO4 solution
In the lead dioxide potential region, i.e. over +0,95 V (vs. Hg/Hg2SO4 ref. Ed.), two types of processes take place on the electrode surface: processes related to the oxidation of lead to PbO2 and oxygen evolution processes. Studying the structure and phase composition of the anodic layer it has been found to comprise non-stoichiometric PbOn, α-and ß-PbO2 and hydrated lead dioxide PbO(OH)2. The corrosion of lead proceeds through the following elementary reactions that take place under the action of oxygen:
The relation between the rates of these reactions determines the type of phases formed in the anodic layer. These rates are affected by both the applied potential and the type and amount of grid alloying additives used. Thus Ag and Tl slow down considerably the corrosion rate (reaction 1), Sb accelerates this reaction, whereas Sn has almost no effect on the rate of reaction 1 but increases the rate of reaction 2, and partially that of reaction 3. As the specific electronic resistance of lead oxides depends on their stoichiometric coefficient, the changes in phase composition of the anodic layer under the action of these alloying additives affects its electroconductivity. Thus Sn and Sb increase this conductivity (Sb- or Sn- free effect).
It has been established that about 10% of the lead dioxide layer formed on a pure lead electrode comprise hydrated gel zones PbO(OH)2. On oxidation of Pb-Sb alloys the degree of hydration of the anodic layer reaches up to 30%. The hydrated (gel) zones present an elastic element in the structure of the anodic layer, which absorbs (dissipates) the mechanical stresses created as a result of corrosion and thus reduce or prevent altogether cracking of the anodic layer. This phenomenon influences the energetic performance of lead-acid battery positive plates.
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T. Rogachev, “Processes related to the anodic corrosion of lead and lead alloys on their polarization in the lead dioxide potential region”
B. Monahov, “Influence of antimony on the electrochemical properties of the lead electrode”
M. Bojinov, “Electrochemical behaviour of antimony in sulfuric acid solutions”
Keywords: lead dioxide electrode, α-PbO2, ß-PbO2, hydrated lead dioxide PbO(OH)2, Pb electrode corrosion, elementary reactions of lead corrosion, hydrated gel zones in PbO2, influence of Ag, Sb, S2 on PbO2 electrode