Behavior of Copper in Acidic Sulfate Solution: Comparison with Acidic Chloride

The anodic polarization behavior of copper in a 0.1 M sulfuric acid (H2SO4) + 1 M sodium sulfate (Na2SO4) solution (pH = 2.0) was studied at room temperature under quiescent and stirred conditions. The behavior was compared with aqueous equilibria via construction of a potential-vs-pH (E-pH) diagram for the copper-sulfate-water (Cu−SO42−−H2O) system. Interpretation of the behavior was aided by comparison with aqueous equilibria and polarization studies of copper in a 0.2 M hydrochloric acid (HCl) + 1 M sodium chloride (NaCl) solution (pH = 0.8). The initial anodic dissolution region in the acidic sulfate solution exhibited Tafel behavior with a slope consistent with formation of cupric ions (Cu2+) whose rate of formation was charge-transfer controlled. At higher potentials, limiting current density (iL) behavior was observed under E-pH conditions that were consistent with formation of a film of copper sulfate pentahydrate (CuSO4·5H2O). Comparison of experimental iL values with those predicted by mass transport-controlled processes, using estimates of the diffusion layer thickness obtained from the mass transfer-influenced region of apparent Tafel behavior in the acidic chloride solution, were in sufficient agreement to indicate iL was controlled by the rate of dissolution of the CuSO4·5H2O film via transport of Cu2+ from the film-electrolyte interface into the bulk solution.