Copper is the only commonly used metal, that in the normal electrochemical series, is more noble than hydrogen. The secret to copper's corrosion resistance is its ability to form a protective, stable surface refered to as a patina. (Pure copper is known by its often green "copper roof" look.) This protective layer can be improved upon by adding various alloy elements.
These compact and protective layers are made up in an atmosphere of oxide and heavily alkalined salts. The corrosion behaviour is influenced by the presence of oxygen or other oxidising materials. Depending on the environmental parameters, the material will either corrode further or will formation of a protective layer. Copper can be in danger of corrosion only when put in contact with acidic or oxidising material or when placed in a solution, such as oxygenated water, that prevents the formation of the protective patina layer.