Passivity of Tin and CuSn Alloys in Alkaline Media studied by X-ray Photoelectron Spectroscopy

Abstract

This chapter presents a study in which the formation and chemical composition of passivating layers on Sn and CuSn alloys are examined in 0.1 M KOH with electrochemical methods and X-ray Photoelectron Spectroscopy. The oxidation of the sputter cleaned samples were performed in an closed system without exposure to laboratory air. The passive layer on tin consisted of a Sn(II)-oxide /hydroxide film in contact with the electrolyte followed by an inner part of Sn(IV) and a Sn(II) fraction at the oxide/metal interface. Time dependent measurements show a growth of the inner SnO2 layer. The composition of the passive layers on CuSn4 and CuSn19 are significantly different. On CuSn4 alloys, the well known duplex structure of copper oxides Cu2O / CuO and Cu(OH)2 are formed with a small tin oxide fraction inside the layer, where the tin oxide content remains constant with time. In CuSn19, the layer composition is significantly different due to the increased influence of the less noble metal tin. In the passive potential range, a minor contribution of copper oxide is found in the outer part of the layer and tin is accumulated in the inner part of the protective oxide film, which contains mainly Sn(IV).