Copper and Copper Alloys

In 1958 exposure tests were started in Sweden to gain knowledge of the natural patination and the corrosion behavior of copper and copper alloys when used outdoors. The investigation covered 36 alloys in sheet or rod form: 5 coppers, 20 brasses, 5 phosphor bronzes, 1 silicon bronze, 1 aluminum bronze, 1 cadmium bronze, 2 nickel-silvers, and 1 free-cutting phosphor bronze. Specimens were exposed in rural, marine, and urban atmospheres. After two- and seven-years' exposure, specimens were examined.

During the first years of atmospheric exposure the copper and copper alloys acquired a dark surface coating consisting mainly of copper oxide (Cu₂O). In urban and marine atmospheres, signs of green patina appeared on copper after about six to seven years; the basic copper salt causing the green color being sulfate in the urban, chloride and sulfate in the marine atmosphere. The average penetration as calculated from the weight loss during 7-years' exposure was: in rural atmosphere 0.2 to 0.6 μm per year, in marine atmosphere 0.6 to 1.1 μm per year, in urban atmosphere 0.9 to 2.2 μm per year. The corrosion rate decreased with the time of exposure. The losses in mechanical properties were in most cases of negligible importance.

The dezincification of brass was of significant degree only in β-brass and in certain (α + β)-brasses when exposed to the urban or the marine atmosphere. Dezincification was observed also in α-brass, even with as high copper content as 92 per cent, although the depth of attack was not very great. It should also be mentioned that arsenic was consistently effective as a dezincification inhibitor for α-brass only in the marine atmosphere. This indicates that the mechanism of dezincification is different in the presence and in the absence of chloride.

The tests described here are continued and further examinations will be carried out after 20 years of exposure.