COPPER DISSOLUTION IN NAHCO3 AND NAHCO3+NACL AQUEOUS-SOLUTIONS AT PH 8

The anodic oxidation of copper in 0.001 to 0.1M NaHCO3 and NaHCO3 + NaCl aqueous solutions at pH 8 has been studied using a rotating disk electrode. The first oxidation product is a thin porous Cu(I) oxide film formed by a solid-state mechanism. In the solutions containing low NaHCO3 concentrations (< 0.05M), this film undergoes localized corrosion. However, for concentration of NaHCO3 > 0.05M, more positive anodic potentials, or longer exposure times, further oxidation of copper take place through the thin porous Cu(I) oxide film. This process results in the formation of a stable outer layer of the precipitated Cu(II) oxides and basic copper carbonate compounds which protect the electrode towards further corrosion. The presence of chloride ions in bicarbonate solution has a promoting effect on the breakdown of the passive film; they shift the corrosion and breakdown potentials towards more negative values. Bicarbonate has an inhibiting influence on the corrosion of copper, since it counters the aggressive attack of chloride ions.