Characterization by electrochemical impedance spectroscopy of passive layers formed on lead-tin alloys, in tetraborate and sulfuric acid solutions

Lead-tin alloys (up to 2.5 wt.% Sn) were passivated in de-aerated sodium tetraborate solution (pH=9.1). Voltammetry measurements showed that alloying tin had the effect of inhibiting the oxidation of lead into PbO or PbO2, but did not hinder the formation of an intermediate compound, PbOx. By impedance spectroscopy measurements, it was found that the polarization resistance of the passivated electrodes increased when the alloying tin content increased. Pure lead and Pb-0.5 wt.% Sn alloy exhibited a semi-conducting behaviour, while surface films formed on tin-rich alloys were found more electronically conducting. These features were related to the important enrichment of the passive layers with corrosion-resistant SnO2. The same alloys were passivated in sulfuric acid solutions in experimental conditions to develop an oxide film under the sulfate layer. It was found that for pure lead and Pb-0.5wt.%Sn, the oxide layer behaved as a semi-conductor. With alloying tin (≥1 wt.%), the composite lead-tin oxide was electronic conducting and non-ionic conducting in a solution of pH=9.1, but in sulfuric acid tin oxide was unstable and increased the ionic conductivity of the passive layers.