Does Copper Rust or Corrode?

Copper is a metal used extensively in electrical, plumbing and architectural applications. It is an ideal material for many applications involving conducting heat or electricity, but can it stand up to corrosion? We take a look at the corrosion rates of copper and other metals.

Does Copper Rust?

Rust is the general term used to describe steel corrosion, where iron is transformed into iron oxide, so copper does not rust in the general sense, but it can corrode.

Rust is a form of corrosion but is not the only type, other materials such as copper, aluminum, and brass can also corrode, in the right environments.

How Does Copper Corrode?

We know the corrosion of steel – AKA rusting – is due to the iron in the steel being exposed to oxygen and water, a new substance forms called Iron Oxide. However, Copper and Copper alloys do not contain iron, so how do they corrode?

As with steel, oxygen is the cause of copper corrosion, it reacts with the copper on the outer surface of the metal to form copper oxide. This process is much slower than the corrosion of steel, and gives the outer layer of the copper a blue/green hue.

Do All Metals Corrode?

So we know how copper corrodes, but do all metals corrode?

Mild Steel

Most of us will have seen steel corrosion at some point, the telltale orange dusty, or flaky surface can be seen in all sorts of places from car fenders, to bridge structures and more. If left without intervention, especially in a moist environment, corrosion can transform an entire steel component to rust.

Steel corrodes so readily that any component to be used for a prolonged period, in almost any climate, will need protection in some form, be that painting, plating or oiling.

Stainless Steel

The name stainless is derived from its ability to resist corrosion, compared to steel. Whilst its chemical composition is somewhat similar in that it contains iron and carbon, it also contains chromium.

The chromium present in the material still reacts with the oxygen in the air, but stops once a thin layer has been formed, preventing further oxidation. There are certain circumstances where stainless steel can corrode, such as in highly acidic or alkaline environments, depending on the grade used.

You may see stainless steel with the typical rust-colored tarnishing usually present on mild steel, this can be caused by a few different things. For one, not all grades are fully corrosion resistant, there are ferritic grades with low levels of chromium and high levels of iron that can still rust.

Another cause can be contamination, even a scratch caused by a piece of mild steel can embed enough ferrous steel into the stainless to cause visible rusting, this can also be caused by grinding or welding sparks contacting the material.

Aluminum

In a similar way to stainless steel, aluminum does not corrode as readily as steel, and as aluminum alloys contain no iron it certainly does not rust. However, aluminum oxide is formed on the outside of the metal and again prevents further corrosion.

Aluminum oxide presents challenges when manufacturing parts from aluminum. Due to its high melting point (2,000°C vs 600°C of aluminum), welding requires an alternating current, in contrast to the direct current used for steel or stainless steel.

Brass

Brass is an alloy primarily made of copper and zinc. Like copper, it does not rust in the traditional sense, but it can experience tarnishing due to copper oxide forming on the outer layer. This process is generally even slower than the oxidation of copper, due to the lesser copper content.

Another corrosion mode of brass is dezincification, whereby the metal can become brittle and weakened due to the zinc dissolving out of the material. This is most commonly caused by prolonged exposure to an acidic environment.

An indication that dezincification is in progress is a subtle change from brass’ normal yellow to a more pink color.

Bronze

Like brass and copper, bronze can corrode, a thin oxide layer forms on the outer surface, which generally prevents any further corrosion from occurring. Bronze corrosion causes its distinctive green patina, as often seen on artifacts from history such as coins and reliefs.

Notable Examples of Copper Corrosion

The Roof of the Minneapolis City Hall, Minnesota

An incredibly striking contrast to the grays and browns of downtown Minneapolis, the bright green roof of the city hall is a landmark in the city. Built in 1909 it is currently adorned with a roof made of copper that reportedly weighs over 180,000lbs.

The copper oxide coating that has formed from years of being open to the elements gives the building a distinctive appearance.

The Statue of Liberty, New York

Surely the most famous example of copper oxidation, the outer layer of the 305ft tall statue atop Liberty Island is constructed entirely of copper. In the years since its construction in 1876, the sea air has caused its distinctive green coloring.

Originally a gift from France, the statue weighs approximately 450,000 lbs and is visited by millions of tourists every year.

Buy Copper for Your Project at Metals Supermarkets

We stock a wide variety of copper products, including flat bars, plates, tubes and sheets. For ease and convenience have your copper stock cut to size, removing the need to buy full lengths!