IN-SITU CONDUCTIVITY STUDY OF THE CORROSION LAYERS ON LEAD-TIN ALLOY IN SULFURIC-ACID

Cyclic polarization of lead and lead-tin alloys in 4.5 M H2SO4 with an d without the ceric-cerous redox couple, from -1500 to +1500 mV/Hg-Hg2 SO4 sat. K2SO4, showed that during polarization in the anodic directio n up to -200 mV, the PbSO4 film was electronically conducting. When Pb O was formed under the sulfate layer, the duplex passive film was nonc onducting and an anodic passive current was observed up to the formati on potential of PbO2. During reverse polarization from +1500 mV, after PbO2 reduction, the passive film on lead was shown to be nonconductin g. When the tin content in the alloy was higher than 1.0 weight percen t (w/o), the conductivity of this passive film was largely increased. It was shown, by electrochemical impedance spectroscopy that alloying with tin decreases the polarization resistance and reduces the thickne ss of the PbO layer. After alloy oxidation at +1500 mV, chronopotentio metric curves revealed the presence of PbO2, PbOx, and PbO. PbOx can b e gradually reduced at decreasing potentials below +800 mV because of its non-stoichiometric nature. Two steps were demonstrated in the redu ction of PbO2. Alloying tin at more than 1.5 w/o appears to favor the growth of PbOx and PbO2.