Because of the presence of HYDROGEN SULPHIDE..
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Due to presence of Oxygen, Copper oxide forms and the color of copper material changes to red and brown.

Due to presence of sulphur in air, copper sulphide is formed, it is blue in color.

Due to the presence of  carbon dioxide in air,  copper carbonate (hydrated) is formed, it is greenish. 

Also due to the presence of sulphur dioxide , and water vapour in air, copper sulphate (hydrated) forms as a layer on the surface of copper material.  It is green in color.

All these can be called as  oxidation of copper and corrosion of copper.

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