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 (i_L) behavior was observed under E-pH conditions that were consistent with formation of a film of copper sulfate pentahydrate (CuSO₄·5H₂O). Comparison of experimental i_L 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 i_L 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.