Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze

In humid atmospheres, copper and bronze passivate, with the formation of an oxidized layer (patina). Patinas may also be created by chemical patination procedures, which are frequently used to achieve special visual effects. However, unless these patinas are effectively protected, corrosion may be initiated when they are exposed to a polluted atmosphere. In the present study the stability of the green nitrate and the green chloride type of patinas, and of electrochemically formed patina, was investigated, as well as suitable protection methods. Two inhibitors, dissolved in different solutions, were used: benzotriazole and the less hazardous imidazole type of inhibitor, and waxes. The dissolving of untreated and pre-treated surfaces, in test solutions representing urban rain (pH 5.0), was investigated by means of potentiodynamic techniques. The microstructure of the patinas and the corrosion products was investigated by SEM/EDX. It was found that both investigated inhibitors inhibited the corrosion of electrochemically formed patina, and of the green chloride type of patina, but were ineffective in the case of the green nitrate type of patina. The difference between the performance of the investigated inhibitors when brushed onto patinated bronze, and when the bronze is immersed in a solution containing the dissolved inhibitor, was determined.