In situ redox conductivity, XPS and impedance spectroscopy studies of passive layers formed on lead-tin alloys

Lead-tin alloys (up to 2.5 wt.% Sn) have been passivated in deaerated sodium tetraborate solution (pH=9.1). The electronic conductivity of the passive films, evaluated by the exchange current density of a redox system, increases sharply when the alloying tin content is increased from 0.8 to 1.5 wt.% Sn. X-ray photoelectron spectroscopy (XPS) signals for lead, tin and oxygen in the passive films are shifted to higher binding energies when the passive films are non-conducting. The conductivity of the passive films is related to a marked enrichment of tin oxide. An impedance spectroscopy study has shown that the polarization resistance of the passivated electrodes increases in tetraborate solution when the alloying tin content is increased. The same alloys are passivated in sulfuric acid solutions and develop an oxide film under the sulfate layer. Results show that without tin, the oxide layer behaves as a semiconductor. With tin, the composite lead-tin oxide is electronically conducting and non-ionically conducting in pH=9.1 solution, but in sulfuric acid tin oxide is unstable and increases the ionic conductivity of the passive layers.