Electrochemical study of passive layer formation on lead-base alloys immersed in 5.31 M H₂SO₄ solution

The process for the anodic formation of PbSO₄ films was studied over lead and two tin-containing lead-alloys electrodes with high- and low-tin contents, in 5.31 M sulfuric acid aqueous solution, using the potential step technique. It was found that the mechanism governing the formation of the lead sulfate anodic film was common for the overall electrodic process and that it takes place comprising three different contributions to the total current [J(t)]. The individual contributions defined were J_g the current density contribution due to 2D nucleation and growth of an insoluble PbSO₄ conducting adlayer, J_d the current density involved during electrodissolution of the lead electrode in the zones which were not covered yet by the lead sulfate film, and J_f was the current due to the growth of the passive layer induced by electrodissolution of lead metal and transport of the resulting Pb(II) ions through the PbSO₄ film. However, each stage contributed to the overall process in a different manner depending upon the Pb:Sn ratio. The results from voltammetry indicated that it is possible to establish the potential at which the lead sulfate film begins to form and that this was also a function of the ratio. The oxidation potential resulted most positive for the low-tin-containing alloy electrode. For all cases, the applied potential needed to form the passive layer influenced the relative importance of each contribution to the overall process.