Evaluation of the protective properties of natural and artificial patinas on copper. Part I. Patinas formed by immersion

The variable character of the ambient parameters determine different passivating properties of the corrosion products formed during outdoor exposure. Three year tests, at least, are necessary in the atmosphere where the patina should be naturally formed, to have long term information about the protectiveness it can develop, its stabilisation rate and the morphology of the attack suffered by the metal. Patinas formed in laboratory, on the contrary, can have well defined chemical composition and morphology, allowing accelerated formation through uniform attack, with controllable corrosion rate of the metal. The protective properties of the corrosion products of copper forming light blue, green, purple and brown artificial patinas and of patina naturally formed during outdoor exposure were evaluated through different characterisation techniques. Potentiokinetic polarisation, scanning electron microscopy (SEM) and EDX surface analysis were applied on the weathered metal samples. The composition of the corrosion products was determined through X ray diffraction and IR spectroscopies. Comparison of the results allowed the classification of the green patina, among the artificial and the naturally formed by outdoor exposure, as the best, from the protective properties point of view. The other 3 presented different failures, consisting of poor adherence, high porosity and exfoliation of the underlying metal.