The aim of this work was to study the corrosion of bare copper and of copper coated with an artificial patina layer developed in sulfate solution when exposed to solutions containing aggressive specimens normally found in the atmosphere of the city of São Paulo, namely Cl-, SO42- and NH4+, added as NaCl, Na2SO4 and (NH4)2SO4, respectively, and also to investigate the stability of the patina layer in these media. The electrochemical behavior was characterized by means of cyclic voltammetry and electrochemical impedance spectroscopy, while Raman spectroscopy, X-ray diffraction and SEM/EDS were employed to gather information about the chemical composition and microstructure of the samples. The electrochemical characterization was performed using classical electrodes with large exposed areas, a cavity microelectrode and an electrochemical microcell. Microstructural and chemical characterization revealed a bi-layered structure for the artificial patina with a porous outer layer composed mainly by brochantite and an inner more compact copper oxide layer, which is in accordance with the microstructure of sulfate patinas found in outdoor exposures. The results of the electrochemical tests with the classical electrodes revealed that the electrode behavior was extremely dependent on the electrolyte composition and that the solution containing ammonium ions is the more aggressive both to the bare and to the patinated sample. The experiments with the microcell evidenced the existence of domains with different electrochemical activities on the patinated sample surface, while the use of the cavity microelectrode showed that the copper patina is electrochemically active in the three electrolytes.
