Copper is a widely used metal that possesses a unique combination of properties, such as being malleable, ductile, and conductive. Like any other metal, it undergoes a decomposition or oxidation process which involves physical and chemical changes.
Now, let’s delve deeper into copper corrosion.
Does Copper Rust or Corrode?
Corrosion is a natural process that occurs as metals react with the atmosphere, chemicals, or other specific conditions. This transformation leads to a different appearance along with changes to the metal’s mechanical properties and a weaker structural integrity. Copper forms a reddish-brown cuprous oxide layer during its electrochemical reaction with the environment.
Rust forms when metal alloys containing iron undergo the oxidation process. However, copper is a non-ferrous metal, meaning it doesn’t contain iron. As iron content is a prerequisite for the formation of rust, copper certainly does not rust, it corrodes or oxidises as oxygen molecules land on its surface and combine with copper atoms to form copper oxide.
Unlike iron oxide, copper oxide does not disintegrate over time. It forms a protective film on the surface of the copper which gradually thickens until it becomes copper carbonate. This new layer of material, called patina, serves as a shield that preserves the unspoiled copper inside. What’s more, damaged patina regenerates itself.
Copper corrosion is a slow process, especially in unpolluted environments. Therefore, it takes months or even years for the surfaces to tarnish and gradually turn dark brown or black and finally into a distinctive blue-green colour.
The formation of patina can be forced, as for some applications a specific look is often desired while no one has the time to wait for the copper to achieve this look naturally. This is achieved by treating the copper surfaces with various chemicals or corrosive agents, such as ferric nitrate, sodium thiosulfate, and sulfuretted potash. By using different methods and exposing the copper to different temperatures and moisture levels, various shades and colours can be obtained.
Some applications of copper are most efficient when the patina is removed altogether and copper is in its cleanest form. An example of this would be copper wires, which exhibit their most electrically conductive state without the patina. Wax coating, polishing, and solutions will seal copper from corroding agents, preventing it from oxidising and tarnishing.
Conditions That Contribute to the Corrosion of Copper
There are specific conditions that promote or accelerate copper corrosion. These include:
- Exposure to environmental conditions that contain saltwater, heat, or acidic compounds deteriorate the copper surface.
- Induced direct or alternating currents flowing in the soils accelerate the corrosion rate for underground copper pipes.
- Galvanic corrosion takes place when dissimilar metals are in contact with copper. An example of this is a copper pipe in contact with a steel pipe, where differences in electrical conductivity promote corrosion. The easiest way to prevent galvanic action from taking place is to insulate copper from other metals.
- Abnormally aggressive soils can facilitate corrosion in copper when it has high concentrations of chloride, sulfate, ammonia compounds, and moisture.
- Contact with high amounts of organic and inorganic acid deteriorates the metal surface of the copper, removing the protective film.
- Corrosion fatigue may occur from constant stress applied to the ductile copper metals. High velocity and turbulent water flow inside copper tubes may create localised erosion and corrosion. Periodic contraction and expansion of copper tubes induces stress which promotes fatigue.
- Elevated levels of oxygen atoms present in the environment corrode the metal’s surface through accelerated oxidation.
Copper Corrosion Example – Statue of Liberty NYC
A great example of copper corrosion can be seen on the Statue of Liberty in New York. Erected in 1886, the statue was originally shiny brown, but it took about 10 years of exposure to the natural environment by the water in New York for its colour to turn to a bluish-green patina. Another 15 years later, the patina was full-blown.
There were even politicians who suggested that the statue be painted shiny brown again, but luckily the wider public didn’t like this plan one bit. Today, the blue-green look is loved by everyone. Some have argued that the statue should be polished every 50 years, so every generation could relive the gradual tarnishing and change in colours. However, this idea isn’t practical. As the statue is only about 2.4 millimetres thick, it would end up being too thin after a few cycles, and soon there would be no statue left.
Corrosion had an effect on the statue’s original design, which had a combination of an iron skeletal structure with copper skin. Galvanic corrosion occurred between the two elements, with rainwater acting as an electrolyte. The iron structure was applied with a zinc coating while the corroded parts were replaced with stainless steel.
Major restoration required 8,000 square feet of copper sheet to replace parts, such as the roofing and torch, while the torch flame was replaced with solid copper and gold leaf. The replacement torch was pre-patinised to match the rest of the statue before being mounted in 1986. The artificial patina wore off within a few years, exposing the dulled copper. It took over twenty years for the dull brown copper to develop its own patina.
The Effects of Corrosion on Copper Alloys
Copper is commonly combined with other metals, as it is an extremely malleable and ductile metal element. Common alloys of copper include bronze (88% copper, 12% tin) and brass (66% copper, 34% zinc with some traces of iron and lead).
Copper alloys are different compared to pure copper and corrode differently from how pure copper corrodes. A copper alloy may turn a different colour than green as it undergoes corrosion. For example, brass takes on a golden brown colour while bronze may turn lime green to dark brown.
The corrosion behaviour between copper alloys varies depending on their physical and chemical properties, environment, stress, and other factors.
Copper alloys demonstrate exceptional corrosion resistance under particular conditions. Here are a few examples:
- Aluminium brass is highly resistant to impingement corrosion from high-velocity saltwater.
- Aluminium bronze is resistant to chemical attacks from sulphite solutions.
- Copper-silicon alloys offer substantial resistance against stress-corrosion cracking compared to brass.
- Nickel silvers offer excellent protection against corrosion from freshwater and saltwater exposure.