The mass-transfer-limited dissolution of copper in acidic and binary cupric-sulfate solutions is investigated. The dissolution produces a dull surface with a coarse microtexture. This system has an unusually large limiting-current plateau, where a salt film precipitates. The film is characterized with steady polarization measurements as well as AC-impedance and flow-modulation spectroscopy. A low-field, film-conduction mechanism explains best the high-frequency region of the AC-impedance spectrum. The electrochemical behavior of the system is compared to other high-rate metal-dissolution systems. Shortcomings of present salt-film models are discussed.