Copper Oxidizes when exposed to the atmosphere due to the reaction with Oxygen. The reaction takes place when water, moisture condensation or rain, in which oxygen is dissolved is in contact with copper.
Copper exhibits good resistance to corrosion in urban, marine, and industrial atmospheres. The major factors that control the initial rate of attack on copper, and that cause copper to oxidize, are moisture, temperature, and the level of pollution.
Soon after exposure of copper to the atmosphere, due to the fact that copper oxidize, the bright copper surface takes on a dull tan tarnish.
After a few years this tarnish gradually changes to dark brown or black.
At a later stage the corrosion products of copper turn green due to the formation of copper sulfate, carbonate and chloride salts in varying concentrations.
The corrosion rate of several types of copper in a 20-yr test averaged 1 pm/yr (0.05 mpy) in an industrial atmosphere, 0.8 pm/yr (0.03 mpy) in a marine atmosphere, and 0.5 pm/ yr (0.02 mpy) in a rural atmosphere.
The basic reaction that cause copper to oxidize is the reaction of copper with the oxygen in the atmosphere:
2 Cu(s ) + O2(g ) --> 2 CuO(s )
After the first oxidation copper oxidize giving the following oxides:
Native copper Cu Red
Cuprite Cu2O Red
Chalcocite Cu2O Dark gray
Chalcopyrite FexCuyS Gold Metallic
Covellite CuS Blue
Bornite Cu5FeS4 63.3 Golden brown to copper red
Brochantite Cu4SO4(OH)6 green hydrated copper sulfate
malachite, Cu2CO3(OH)3 green hydrated copper carbonate
azurite Cu3(CO3)2(OH)2 blue hydrated copper carbonate
The formation of natural green patina as seen on copper roofs and statues requires a long time and several different methods have been developed to achieve the same results artificially using chemical reactions.
Copper has good resistance to corrosion by all types of freshwater, corrosion rates are from 5 to 25 microm/yr (0.2 to 1 mpy). Corrosion rates for water saturated with air and carbon dioxide are an order of magnitude greater than those for municipal or distilled water.
Copper also has good resistance to corrosion in seawater. Because of its outstanding resistance to fouling by marine organisms, it is widely used for sheathing on surfaces exposed to seawater.
Copper is sometimes used in seawater piping, but it is subject to corrosion-erosion where the flow velocity is greater than 0.9 to 1.2 m/s (3 to 4ft/s). These velocities are often attained at changes in pipe cross section or flow direction.
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