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 . 5H(2)O). 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 . 5H(2)O film via transport of Cu2+ from the film-electrolyte interface into the bulk solution.