Structure and composition of electrochemically prepared oxide layers on Co in alkaline solutions studied by XPS

Passive layers on cobalt were prepared in 0.1 N NaOH (pH 13) under potentiostatic control with systematic variation of the relevant parameters, such as time and potential, and examined by 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. Evaluation of the XPS signals occurs on the basis of well-characterized standards. The results are compared with those obtained in borate buffer (pH 9.3), which have been published previously in this journal. Depending on the potential, cobalt forms two different passive layers. In 0.1 N NaOH (pH 13) the potential range of primary passivity goes from -0.4 to 0.3 V-SHE. The passive layer consists of a pure Co(OH)(2) film that grows from 10 nm after 5 min of passivation up to 37 nm within 90 min. The layer forms most probably according to a dissolution precipitation mechanism. For E > 0.0 V-SHE a thinner layer of only 4 nm (E = 0.2 V-SHE for 5 min) is formed. In the range of secondary passivity at E = 0.3-0.8 V-SHE Co(III) dominates the layer composition with contributions of CO3O4, CoOOH and Co2O3. After 5 min of passivation the layer thickness in this potential range increases slightly with increasing potential from 4 up to 7 nm. After 90 min the thickness reaches 9 nm, independent of the potential. Copyright (C) 2002 John Wiley Sons, Ltd.