X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF PASSIVE LAYERS FORMED ON LEAD-TIN ALLOYS

Lead-tin alloys were prepared with increasing tin content up to 2.5 wt.% Sn. These alloys were passivated in de-aerated sodium tetraborate solution (pH = 9.1) at 0 V versus saturated sulfate electrode. The aim of this X-ray photoelectronspectroscopy (XPS) study was to elucidate the relationship between the alloying tin content and the electronic conductivity of the passive layers. It was found that metallic lead, lead oxide and tin oxide were present in the passive film. XPS did not distinguish SnO from SnO2 signals, but SnO2 was assumed to be present for reasons of its thermodynamic stability. The signals of tin, lead and oxygen (main components of the passive films) were depleted in unshifted and shifted signals when the conductivity of the films (evaluated by electrochemical study) became very low. The shifted signals were assumed to have originated from positively charged zones on the surface film due to the low conductivity. The shifting increased when the alloying tin content decreased, that is when the film conductivity decreased. The concentration of tin (as compared with lead) in the passive films was largely increased, up to 44, 28, 14, 13 and 3 wt.%, respectively, for 2.5, 1.5, 1.3, 1 and 0.5 wt.% Sn alloys. Semi-quantitative analysis showed that one of the effects of alloying tin could be the thinning of the passive films, as observed for 2.5 wt.% Sn alloy. The inner layer of the passive films was found to be rich in conducting tin oxide and metallic lead.