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 passiv ating properties of the corrosion products formed during outdoor exposure. Three year tests, at least, are necessary in the atmosphere where the patin a should be naturally formed, to have long term information about the prote ctiveness 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 accel erated formation through uniform attack, with controllable corrosion rate o f the metal. The protective properties of the corrosion products of copper forming light blue, green, purple and brown artificial patinas and of patina naturally f ormed during outdoor exposure were evaluated through different characterisa tion 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 di ffraction and IR spectroscopies. Comparison of the results allowed the classification of the green patina, a mong the artificial and the naturally formed by outdoor exposure, as the be st, from the protective properties point of view. The other 3 presented dif ferent failures, consisting of poor adherence, high porosity and exfoliatio n of the underlying metal. (C) 1999 Elsevier Science Ltd. All rights reserv ed.