Study of patina formation on bronze specimens

In this study, bronze samples (94.059% Cu; 0.023% Zn; 0.077% Pb and 5.801% Sn, w/w) were exposed for 8 months to a marine atmosphere (splash zone) at Barra of Tijuca Beach, Rio de Janeiro. X-ray diffraction analysis (XRD) was used to identify the composition of the corrosion products formed on the samples. The electrochemical properties were analyzed using potential and electrochemical impedance measurements. These analyses and measurements were obtained from periodically removed samples, and compared to those exposed to laboratory tests (total and alternating immersion) in 10−2 mol L−1 NaCl solution. The corrosion potential results, allied to XRD analysis, showed that the chemical nature of the films obtained on the surface of the field test samples was very similar to that obtained in the alternating immersion test. The main products identified were cuprite and atacamite. However, the film formed during the total immersion (TI) test was primarily composed of cuprite with nantokite traces, which was not observed in either the field or in alternating tests. The electrochemical impedance measurements obtained in laboratory and field test samples showed the formation of films with a porous structure. The thermodynamic evaluation was also consistent with the corrosion products formed.