XPS-Studies on the Formation of Passive Layers on Sn and CuSn Alloys in 0.1 M KOH

Passive layers on tin, CuSn4 and CuSn19 were prepared in 0.1 N KOH (pH 12.5) under potentiostatic control with systematic variation of the relevant parameters, such as potential and time, and examined by X-ray photoelectron spectroscopy. XPS studies of potentiostatically polarised Sn-electrodes show the linear growth of the oxide layer thickness with increasing potential. Angular resolved-XPS measurements and XPS-depth-profiles allow the development of a model of the passive layer: In the passive potential range it consists mainly of SnO2 with a small contribution of SnO. Tin hydroxide was found only on top of the layer. Sputter profiles indicate a dominating SnO2 part which increases with the potential and the passivating time. SnO was detected underneath a layer of SnO2 with a thickness constant with potential.