Corrosion and rust

At the beginning of summer, Aadesh bought a new bike. Now it’s nearly autumn, and the weather will start getting more and more humid. Aadesh’s wondering if it’s okay to leave his bike outside. He’s worried it might corrode. When a material corrodes, it reacts with air, water, or other substances in the environment, such as salts or acids.

As a result of this reaction, the material slowly changes its qualities and deteriorates. Corrosion can affect different types of materials, for example polymers, such as plastics or rubber. But more often corrosion affects metals. Most metals are highly reactive, so when they come in contact with oxygen, water, or some other substances, they easily form new compounds. When iron, for example, is exposed to oxygen and water, it undergoes a type of corrosion called rusting.

Let’s take a closer look at how it happens! In the presence of oxygen, the iron atoms can lose electrons and become positive iron ions. We call this an oxidation reaction. Meanwhile, the atoms in the oxygen molecule gain electrons, to form negative ions. This is a reduction reaction.

The reaction is much faster when there is water present The product is hydrated iron oxide, also known as rust. Rust forms a reddish-brown flaky coating on the surface of the metal. Rust has very different properties to iron — it is weaker and much more brittle. Rust is also porous and peels easily. This makes it easier for water and oxygen to get deeper into the metal, causing even more rust to form.

Not all types of corrosion are as destructive as rust. Actually, for some metals corrosion can be beneficial. For instance, aluminium reacts with oxygen very readily, forming a layer of aluminium oxide on the surface. Unlike rust, this solid layer of aluminium oxide prevents oxygen reaching the aluminium below. It acts as a protective coating, so no more corrosion can occur!

Thanks to this protective layer, aluminium can withstand contact with various substances. This makes it useful in the food industry, where aluminium is used to package liquids or acidic products, such as carbonated drinks or tomato sauce. There are also certain materials that are highly resistant to corrosion. Gold, platinum, palladium and other noble metals do not lose electrons easily, so they are much less reactive. They do not oxidise or corrode easily, not even on the surface.

Ceramic materials, such as pottery, are usually very resistant to corrosion too. Unfortunately, Aadesh, your bike is not made of any of these corrosion-resistant materials. It’s made mostly of steel, which is an alloy of iron. If you leave it outside in the humid air for too long, there is a good chance it will rust. If you want to keep it in good condition for longer, you’d better take it inside, somewhere dry!