Anodic oxidation of copper in alkaline solutions: Part IV. Nature of the passivating film

Abstract

Potentiostatic and X-ray photoelectron spectroscopic (XPS) techniques have been used to study the passivation of copper electrodes in 1.0 mol dm −3 LiOH. For potentials <−80 mV (vs. SCE), a porous base layer of Cu 2O is present, and an upper layer of Cu(OH) 2 is formed by nucleation and growth from solution. The dissolved Cu 2+ ions necessary for Cu(OH) 2 precipitation are produced by metal dissolution in the pores of the Cu 2O layer. Under these conditions, the surface is only partially passivated, since metal dissolution can continue in the pores of the base layer. For potentials >−60 mV (vs. SCE), these pores, and eventually the whole surface, are covered by a layer of CuO identified by XPS. When this layer is formed, the extent of Cu(OH) 2 formation is drastically reduced. In addition the cupric ion dissolution rate is reduced, indicating a much higher degree of surface passivation.