Active/Passive Behavior of Copper in Strong Sulfuric Acid

A combination of thermodynamic analyses and potentiodynamic polarization tests have been used to study the anod-ic behavior of Cu in strong H2S04 solutions in the concentration range 1—10 M. The studies were supplemented by chem-ical analyses of surface films. It was found that concentration-dependent changes in the activity of water played a major role in determining the anodic behavior and relative stability of corrosion product films. The anodic Tafel slope decreased from —41 to —31 mV with increasing acid concentration. The onset of limiting current and active—passive behaviors at higher potentials was determined by the formation of films of hydrated copper sulfate, CuSO4xH2O, and not by forma-tion of oxides. Limiting current behavior was observed in 1 M solutions, where the degree of hydration was x =5. Well-developed passivity occurred in 10 M solutions where x = 1. The results are relevant to industrial electrorefining opera-tions for Cu and indicate that chloride contamination, if present in sufficient amounts, could cause the premature onset of limiting current behavior (anode "passivity") during refining