Artificial patina layers are often used to give final appearance and also to restore damaged old copper and bronze surfaces. The main inconvenient of this process is that it frequently requires surface heating or total immersion of the metallic object in the patina solution, which is sometimes impossible to accomplish, mainly with big outdoor exposed objects or ancient artefacts. In the present investigation the corrosion behaviour in NaCl solution of bare copper and bronze was compared with the response exhibited by samples of these metals covered with two different artificial nitrate-based patinas obtained at room temperature by dabbing a soaked cotton swab above their surfaces. The electrochemical techniques used to assess the response of the samples were electrochemical impedance spectroscopy (EIS) and anodic polarization curves. The anodic polarization responses have shown that, for short immersion times, the presence of the patina layer does not change the corrosion mechanism of the samples, which seems, as proposed in the literature, to be dominated by the diffusion of a soluble Cu complex to the bulk of the solution. However, EIS diagrams have evidenced differences between the responses of the bare and patina-treated samples. While in the formers the diagrams exhibited a low frequency diffusion controlled phenomenon, in the latter the response is dependent of the structure of the patina layers. Moreover, EIS response have indicated differences between the corrosion behaviour of the samples treated with the different patina solutions, which were not evidenced by the anodic polarization curves, but which are in accordance with the microstructural features revealed by SEM images.
artificial patinas; copper; bronze; EIS; SEM; electrochemical behaviour
