When the lead-acid battery positive plate is made of pure Pb or of low-antimony non-Sn lead alloys, during low current polarization or stay at an open circuit the positive plates are passivated. This passivation is due to the formation of an uninterrupted corrosion layer of tet-PbO with high resistance. It was established that Sn keeps down the passivation phenomena in the plate. In the present paper, through SLV and photoelectrochemical investigations, it was found that Sn facilitates the process of oxidation of PbO to PbOn (1 < n < 2) and lowers the potential at which this reaction starts. This phenomenon is explained using a model of the oxidation of PbO to PbOn based on the semiconductive properties of PbO. Sn creates hole conductivity in the PbO layer allowing the oxidation reaction to proceed at the PbO/solution interface. The oxygen which has diffused into the oxide oxidizes PbO to PbOn. Above a certain n value the oxide conductivity becomes equal to that of PbO/sub 2/. The corrosion layer resistance decreases dramatically thus suppressing the passivation phenomena on the positive plate grid.