When Does Copper Turn Green?
Although copper is highly resistant to the chemical action of the atmosphere and of sea water, it turns green if exposed to them for a long time. The color is caused by the formation of a thin coating of green basic copper carbonate known as patina or verdigris. The latter name comes from the old French vert de Grece, (green of Greece), but the reason for it is unknown. This beautiful green is often seen on copper roofs or statues, especially if they are near the sea.
The patina actually protects the copper below the surface from further corrosion, making it a good water-proofing material for roofs (which is why the roofs of so many old buildings are bright green). In fact, the weathering and oxidation of the Statue of Liberty’s copper skin has amounted to just .005 of an inch over the last century, according to the Copper Development Association.
Copper was the first metal man learned to use. Five thousand years ago, when men discovered deposits of pure copper in what are now Iraq and Cyprus, they found that this fairly soft metal could be easily melted, cast in moulds and hammered into tools, weapons and ornaments. About half the copper produced today is used by the electrical industry. Pure copper is the best cheap conductor of electricity and can be drawn into threads one-thousandth of an inch thick.
Copper corrodes at negligible rates in unpolluted air, water and deaerated non-oxidizing acids. However, it is susceptible to more rapid attack in oxidizing acids, oxidizing heavy-metal salts, sulfur, ammonia, and some sulfur and ammonia compounds.
Pitting failure occurs in cold-water copper lines that conduct aggressive well waters. Copper is susceptible to crevice corrosion attack.
Certain conditions can cause copper to corrode when it is exposed to particular soils, including:
- Abnormally aggressive soils
- Localized and long-line-type concentration cells created by differences in soil composition
- Action of stray direct currents (DC) flowing in the ground
- Faulty design and workmanship
- Certain conditions created by alternating currents (AC)
- Thermogalvanic effects
- Galvanic action involving dissimilar materials
Coupling of copper with aluminum or copper with steel can lead to severe galvanic corrosion. Cyanides are also very corrosive to copper pipe.
The good resistance of copper pipe alloys to corrosion by seawater depends partly upon the inherent cathodic nobility of the metal, but it also depends on the ability to form protective films. High-velocity and turbulent flow conditions can remove these films and permit local, rapid corrosion.
There are two types of copper corrosion:
- Uniform – Identified by the presence of a relatively uniform layer of copper corrosion byproducts across the inner surface of a pipe wall. It is typically associated with elevated copper levels at the taps.
- Non-uniform – Isolated development of corrosion cells across the inner surface of a pipe wall. Excessive pitting corrosion can lead to pinhole leaks in the pipe. This can result in water damage and mold growth.