Electrochemical behaviour of copper in aqueous moderate alkaline media, containing sodium carbonate and bicarbonate, and sodium perchlorate

Abstract

The voltammetric polarization of Cu specimens in Na₂CO₃, NaHCO₃ and NaClO4 solutions (8-12pH range) has been investigated. Voltammetry data were complemented with SEM and electron microprobe analysis. Results are found to be in agreement with the passivation model developed for Cu in plain NaOH solutions. For the latter the process can be described in terms of two steps, namely, at low potentials the initial formation of a Cu₂O thin layer followed by the growth of a massive Cu₂O layer, and at higher potentials the appearance of a CuO-Cu(OH)₂ layer. These processes are accompanied by the formation of soluble Cu species. Beyond a certain potential which increases with the solution pH, copper pitting takes place. This model can be extended to Cu in carbonate/bicarbonate containing solutions by considering that Cu carbonates precipitate as long as soluble ionic Cu species are produced, without interfering appreciably with the formation of Cu oxides. The appearance of copper carbonate species is enhanced when pitting corrosion sets in. The precipitation of Cu carbonates occurs principally around pits. Cu pitting, although it is observed for all solutions, becomes more noticeable at the lowest pH values. At a constant pH, the density of pits increases in the order NaClO₄ > NaHCO₃ > Na₂CO₃. The influence of the electrolyte composition on Cu pitting is closely related to the blockage capability for pit nucleation and growth of the corresponding copper salts. Passivation in the Cu₂O-Cu(OH)₂ region hinders pitting corrosion.