Most copper compounds are either in monovalent or divalent (binary). In some exceptional cases, such as in K3[CuF6] it occurs in trivalent and, in the exceptional case of Cs2(CuF6), in tetravalent.
Copper is the only metal in common use which has a standard potential higher than hydrogen.
Unlike acids, the corrosion resistance of copper depends not only on the type and concentration, but also on the amount of oxygen present and the oxidizing agent. In non-oxidizing acids, which do not contain dissolved oxygen, copper is stable.
Alkaline aqueous solutions of the hydroxides and carbonates of alkali and alkaline earth metals - with the exception of NH3 - have only a small reaction with copper.
Copper is stable in the air, including salty sea air . Its surface is first covered first with a dark brown, almost black protective layer, which, over time, changes to the familiar green patina such as that seen on an old copper roof. Patina is a mixture of cuprous salts (sulfate, carbonate, and, near sea air, choloride), the composition of which is determined by the concentration of materials in the air. Moist ammonia and hydrogen sulfide vapors do cause corrosion on copper surfaces.
Copper is resistant to water and drinking water, both warm and cold. Therefore, it is the perfect material for water pipes. Copper's excellence resistance to corrosion is made possible by its protective layer.