Why does copper turn green over time

(ORDO NEWS) — Copper products acquire a characteristic green color over time. Why do they deteriorate so much and can this be avoided?

For copper, the appearance of a green tint is quite normal and even good. Unlike rust on iron, the formation of a patina only strengthens this metal.

Copper is one of the few metals whose color differs from the standard grey-white. Its natural color is peach or pinkish white, with a bright metallic sheen.

However, over time, the color of this metal changes beyond recognition and becomes green.

Copper acquires a greenish hue during the oxidation process that occurs when it comes into contact with air.

Under conditions of high humidity, oxidation occurs faster, which leads to the formation of a thin bluish-green outer layer on the surface of the metal, called patina.

How patina is formed

The color of the patina is due to the minerals of which it is composed and which are formed on the surface of the metal during oxidation.

One of them, brochantite, is green, another, malachite, is emerald blue, and the third, azurite, is blue. The patina can be compared to the rust on iron, which also forms during oxidation.

However, unlike rust, patina does not destroy the metal, but, on the contrary, contributes to its protection.

Copper oxide is a strong substance that adheres very tightly to the base metal and prevents further deterioration.

The thicker the patina layer becomes, the more it resists corrosion of the metal underneath. Perhaps the most famous example of copper oxidation is the Statue of Liberty, built in 1886.

Despite the fact that the statue is over 100 years old, the layer of patina on it still does not exceed 0.13 millimeters.

What other metals turn green?

Metal alloys containing copper are also subject to oxidation and patination. Over time, brass, which is made from copper and zinc, and bronze, made from copper and tin, also take on a greenish tint.


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