Tarnishing of copper. Comparison of copper components at different ages - at left is a brand new piece, at centre one that has been exposed to air for a few months and at right a components showing distinct corrosion. Copper reacts slowly with oxygen in the atmosphere to form a layer of brownish black copper (II) oxide. This prevents further oxidation. In the presence of carbon dioxide the copper can react to form basic copper carbonate (Cu2(OH)2CO3), with a characteristic green-blue colour.