Corrosion Mechanisms and Products of Copper in Aqueous Solutions at Various pH Values

The corrosion behavior of copper in aqueous solutions of different pH values was investigated using electrochemical and surface analysis methods. It was shown that the corrosion mechanism changed with pH and was associated with morphology of the surf ace films formed. In solution of pH 3, the copper surface was covered with porous corrosion products of cuprous oxide (Cu₂O). Corrosion was controlled predominately by diffusion in solution. In solutions of pH 4 to pH 5, formation of cubic Cu₂O on the copper surface provided a diffusion barrier to copper dissolution. The anodic process was controlled by a mixed diffusion of copper ions in oxide films and in solution. In solutions of pH 6 to pH 9, the oxide films (Cu₂O) became more protective. Diffusion in the oxide films became a rate-determining step of anodic dissolution. In pH 10 solution, a thin, compact Cu₂O film formed, and spontaneous passivation was observed. At pH 12 and pH 13, analysis by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) demonstrated that the copper surface was covered mainly with a protective monoclinic cupric oxide (CuO) layer. Alternating current impedance measurement results suggested the corrosion process possibly was controlled by diffusion in the oxide films.