CHARACTERIZATION BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY OF PASSIVELAYERS 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 sodi um 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 compo und, PbOx. By impedance spectroscopy measurements, it was found that t he polarization resistance of the passivated electrodes increased when the alloying tin content increased. Pure lead and Pb-0.5 wt.% Sn allo y exhibited a semi-conducting behaviour, while surface films formed on tin-rich alloys were found more electronically conducting. These feat ures were related to the important enrichment of the passive layers wi th corrosion-resistant SnO2. The same alloys were passivated in sulfur ic 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 (g reater than or equal to 1 wt.%), the composite lead-tin oxide was elec tronic conducting and non-ionic conducting in a solution of pH=9.1, bu t in sulfuric acid tin oxide was unstable and increased the ionic cond uctivity of the passive layers.