Passivity of cobalt in borate buffer at pH 9.3 studied by x-ray photoelectron spectroscopy

passive layers on cobalt were prepared under potentiostatic control in bera te buffer pH 9.3 and examined with x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their tr ansfer to the ultrahigh vacuum was performed in a closed system under prote ction of purified argon. The ICPS signals were evaluated quantitatively on the basis of standard spectra, which yields the composition of the passive layer, i.e. the contribution of the cobalt and oxygen species Co(0), Co(IT) , Co(III), OH-, O2- and H2O. Depending on the potential, cobalt forms two different passive layers, The primary passive film consists of Co(II) oxide and Co(II) hydroxide and is f ormed at low potentials. At high potentials a secondary passive layer is fo rmed consisting of Co(II)/Co(III) mixed oxides. The oxidation to Co(TIT) is related to a significant increase of the layer thickness from 1 to 4 nm, T ime-resolved XPS investigations show that the primary passive film grows wi thin <1 s, whereas the Co(III) formation of the secondary passive film requ ires at least 300 s, Copyright (C) 2000 John Wiley & Sons, Ltd.