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Why Does Copper turn green? 


Why Does Copper turn green?



Copper turns green 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 turn green, 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.

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.               

see also:

Copper pipe Corrosion

Why does copper oxidize

What is corrosion?

What is Rust?

Useful documents on Copper Turns Green:

Clear Protective Coatings for Copper and Copper Alloys - Copper and copper alloys such as brass and bronze are well known for their strength, corrosion resistance and attractive colours. The surface can be
preserved at any stage by the application of a clear protective coating.

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