engleski

Stability of Chemically and Electrochemically Formed Patina and Their Versatile Protection

Copper and its high copper alloys-bronze tend to passivate in humid air forming an oxide layer (patina), which protects copper and its alloys from further corrosion process. The artists use special patination procedures for visual effects. At this point, there is a need to delay or stop the upcoming corrosion process if the patina is not stabile due to several possible conditions. In this case the surface is brushed, sprayed or wiped with natural, synthetic or microcrystalline waxes that are sometimes doped with organic inhibitors, such as benzotriazole. The aim of the present study is to test the stability of electrochemical and chemically formed patina, together with many possible ways of protection such bronzes. Green-to-blue colour patina was chosen to be tested. Electrochemically formed patina was obtained by potentiostatic polarization during 55h. Basic green chemically formed patina was applied by using copper nitrate solution with a hot dip technique. Final finishes that were tested were: benzotriazole and healthly less hazardous imidazole type inhibitor, waxes and laquers with added inhibitors. Some organic mixtures, such as Polaroid B44, common finish for bronze sculptures are tested. The mechanism of prevention will be compared. The mechanism of aging of bronze is determined in a test solution of acid rain (0.2 g/L Na2SO4, 0.2 g NaNO3 and 0.2 g/L NaHCO3 acidified to pH5). A classic electrochemical approach is used to study corrosion behaviour of bronze with a similar compositions as statue. The untreated and pre-treated surfaces are investigated with different electrochemical techniques: corrosion potential, linear polarization, potentiodynamic measurements and electrochemical impedance spectroscopy. The structures of the patina and corrosion products after electrochemical tests are characterized by scanning electron microscopy (SEM). The use of different ways of protecting unprotected and pre-patinated surfaces is compared in order to efficiently protect bronze artefacts.

bronze; patina; protection methods

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