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 s olution (pH = 9.1) at 0 V versus saturated sulfate electrode. The aim of this X-ray photoelectronspectroscopy (XPS) study was to elucidate t he relationship between the alloying tin content and the electronic co nductivity of the passive layers. It was found that metallic lead, lea d oxide and tin oxide were present in the passive film. XPS did not di stinguish SnO from SnO2 signals, but SnO2 was assumed to be present fo r reasons of its thermodynamic stability. The signals of tin, lead and oxygen (main components of the passive films) were depleted in unshif ted and shifted signals when the conductivity of the films (evaluated by electrochemical study) became very low. The shifted signals were as sumed to have originated from positively charged zones on the surface film due to the low conductivity. The shifting increased when the allo ying tin content decreased, that is when the film conductivity decreas ed. The concentration of tin (as compared with lead) in the passive fi lms was largely increased, up to 44, 28, 14, 13 and 3 wt.%, respective ly, for 2.5, 1.5, 1.3, 1 and 0.5 wt.% Sn alloys. Semi-quantitative ana lysis showed that one of the effects of alloying tin could be the thin ning of the passive films, as observed for 2.5 wt.% Sn alloy. The inne r layer of the passive films was found to be rich in conducting tin ox ide and metallic lead.