Over 80 percent of all known elements are metals. Experts classify them according to their physical and chemical properties, such as density, conductivity, melting point, and reaction to certain chemicals. Arguably, one of the simplest ways to classify metals is through their decomposition.
Like any other material, metals decompose when exposed to another element or compound. We call this process corrosion. While all metals corrode, some have higher corrosion resistance than others. In most cases, different metals corrode differently when exposed to the same chemical. For instance, iron turns into rust and zinc turns into a white powder when they come into contact with air or moisture.
In terms of corrosion, there are two broad classifications of metals – ferrous and non-ferrous. Ferrous metals contain iron, while non-ferrous metals do not. Iron is particularly notorious for being susceptible to corrosion. Oxygen is its main catalyst. When exposed to any oxygen-containing substance, such as air or water, oxygen atoms from that substance combine with iron atoms and form iron oxide or rust.
Does copper rust?
Copper is a non-ferrous metal. It doesn’t contain iron, so it won’t turn into rust when exposed to oxygen. Instead, when oxygen molecules land on copper’s surface, they combine with copper atoms and form copper oxide.
Unlike iron oxide, copper oxide does not disintegrate over time. It stays on copper’s surface and gradually thickens until it becomes copper carbonate. This new layer of material, commonly known in the metal world as patina, serves as a shield against the elements, preserving the unspoiled copper inside for a very long time.
The Pros and Cons of Copper Corrosion
Generally, corrosion is considered detrimental to metals because it takes away their useful properties. For instance, rust causes iron to lose its tensile strength, rendering it useless for construction applications as a result.
Corrosion of copper metal, on the other hand, is different. Instead of destroying the metal, it gives it an elegant and unique appearance. Furthermore, it doesn’t diminish copper’s key properties, such as workability and conductivity. If anything, the outer covering produced from corrosion gives copper added protection, allowing it to last for millennia.
Copper Alloys and the Effects of Corrosion
Copper is among the most flexible metals around. It can easily bend and stretch due to its high malleability and ductility. Unlike other metals, copper is an excellent base material for alloys to boot. Of the many different copper alloys available today, the most popular are brass (copper and zinc) and bronze (copper and tin).
To produce different versions of these alloys, metallurgists alter their content proportion. They add a miniscule amount of other metals (sometimes non-metals) into the mixture to create more variations.
Because copper alloys contain other metals, they corrode differently from how pure copper corrodes. For example, most types of brass turn golden brown during the final stage of corrosion, while copper turns green. In fact, one can tell how long copper or its alloys have been corroding by their color. This gives artisans a wealth of choices when decorating with copper-based sculptures and fixtures.
Then again, some applications require copper or its alloys to be in their pristine state. Copper-based machine components such as copper rods and plates, as an example, work optimally when their surfaces are free of patina. The same goes for copper wires; they are most electrically conductive in their purest form.
Copper corrosion is a slow process, so it’s easy to preserve the sheen of a copper, brass, or bronze item. In fact, it takes decades for copper to develop a greenish top layer. In addition, all it takes to polish these metals is an over-the-counter metal polish or homemade mix and a clean cloth. Some copper alloys, however, tarnish fast, so you have to polish them more often.
Forcing Copper Corrosion
As previously mentioned, copper corrodes very slowly. It only begins to change color after months or years of exposure to air and moisture. You may wonder how architects and interior designers are able to find copper fixtures and furniture pieces with the exact shade and color they need for their projects in a short span of time.
There’s no way they’ve been waiting for those items to patinate prior to their project’s commencement. What they actually do is force copper to corrode by applying certain chemicals on its surface. Oxygen is not the only element that can cause patina to form on copper’s surface. A lot of other harsher compounds are actually more effective, turning copper from red to brown in a matter of minutes.
Here are some of the popular methods for creating patina on copper items:
Incubate the item with hot crushed boiled eggs.
After boiling a few pieces of eggs, place and crush them in a sealable plastic bag. Bury the metal piece into the crushed egg and let it sit for 30 minutes to an hour. To achieve a dark shade of patina, incubate the metal piece for several hours.
Spray or apply a vinegar-salt solution on your copper item.
Both vinegar and salt are potent enough to accelerate the formation of patina on copper. With the right mixture, it can even give your copper item a bluish patina. To get a more specific shade or hue, you can add sawdust and chips into the mixture.
Suspend the copper piece in saltwater and non-detergent ammonia vapor.
Put the mixture in a container with a cap. Find a way to put the copper item in the container without touching the solution. You only need to expose it to the vapor. This means you have to close the container as well.
There are other chemicals that you can use to force corrode copper or its alloys. Ferric nitrate, sodium thiosulfate, and sulfureted potash are the most common. Each of these solutions produce a different patina color on copper. However, other factors such as temperature and humidity may also be at play.
Probably the least noticed but one of the most influential factors is the quality of the metal piece. Even the tiniest impurities within the metal could have a serious effect on patina formation. This is the reason why it’s crucial to only obtain your materials from the right copper sheet supplier. Here in North America, your best option is Rotax Metals. They have nearly a century of experience with copper and its alloys, so you can be sure that they provide the highest quality copper plates, sheets, rods, and anything in between.