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 borate buffer pH 9.3 and examined with x-ray photoelectron spectroscopy (XPS). The electrochemical preparation of the sputter-cleaned samples and their transfer to the ultrahigh vacuum was performed in a closed system under protection of purified argon. The XPS 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(II), 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 formed at low potentials. At high potentials a secondary passive layer is formed consisting of Co(II)/Co(III) mixed oxides. The oxidation to Co(III) is related to a significant increase of the layer thickness from 1 to 4 nm. Time-resolved XPS investigations show that the primary passive film grows within <1 s, whereas the Co(III) formation of the secondary passive film requires at least 300 s.