Copper alloys are known for their fine lustre and high resistance to corrosion, but they will tarnish under certain conditions. A surface oxidation of the metals normally occurs under the influence of the atmosphere. At elevated temperatures this process may result in colored so-called temper films. The presence of hydrogen sulfide in the atmosphere may accelerate the reaction so that tarnishing occurs even at room temperature.
When copper alloys are exposed to an outside atmosphere a black film will soon form on the surface. After some years, however, the film usually will change to a pleasing green patina.
Ugly dark stains, “water stains,” are formed when copper-base material is left in contact with stationary water for a lengthy period. Such water damage may be caused by rain or condensed moisture in transit or in storage. A similar staining is caused by hygroscopic surface impurities. Typical examples are chlorides coming from tap water that has evaporated on the metal surface, solder residues, hand perspiration, sulfuric acid and dust.
Copper alloys can be protected from corrosion by means of passivating inhibitors, slushing compounds, lacquers or packaging. 6.3.6
