Re: Why does copper tarnish to form patina?

Patina is the result of atmospheric corrosion on copper. It is generally green in colour when well established, but may be initially black before becoming green. The green colouration comes from the copper oxides that form.

Because copper appears low in the reactivity scale of metals, the formation of patina takes an extended period of time, usually 8 - 10 years. Compare this with a matter of hours for the formation of rust (Iron Oxide) on a cleaned steel surface. The formation of patina also slows down further oxidation once it is established, and this is why there are buildings throughout Europe for example with copper sheeted roofs some hundreds of years old.

It is now common to treat newly installed copper roofing with proprietary treatments that actually accelerate the formation of patina, in order that the aesthetics are improved, and the protective properties enhanced immediately.