In this video, we're going to look at corrosion, and in particular at why things rust and how we can prevent it. Corrosion is a general term that refers to the process by which metals are slowly broken down by reacting with substances in their environment. One example of this is rusting, which is where iron reacts with oxygen and water from the environment to form hydrated iron oxide, which is more commonly known as rust.
If we think about what's happening here at the atomic level, the iron atoms in our original block of metal are each losing three electrons to become iron ions, and at the same time, the two oxygen atoms in each oxygen molecule are gaining two electrons, so a total of four electrons, to become oxygen two minus ions. So the iron is being oxidized because it lost electrons, while the oxygen is being reduced because it's gaining electrons. And this means that the overall reaction is a redox reaction, because both oxidation and reduction are taking place in the same reaction.
The next thing we need to cover are the conditions that are required for rusting to take place. We said earlier that rusting involves iron reacting with oxygen and water from the environment, so both of these have to be present for rusting to take place. To see this in practice, let's imagine three test tubes, each one with an iron nail in it.
The first test tube contains water but no oxygen. This is because the water has been boiled to remove any of the oxygen that was previously dissolved in it, and this layer of oil at the top stops any new oxygen, that's in the air, from dissolving into the water. The second test tube contains air, and so oxygen, but no water, and this is achieved by having the calcium chloride at the bottom, which will remove any water vapour from the air.
And then the final test tube contains both air, so oxygen, and water. So if we now left these test tubes for a while, only this nail in the last tube would rust, because it's the only one that contains both oxygen and water. One important thing to understand with corrosion is that it's only the surface of a metal that corrodes.
This is because only the surface is exposed to substances in the environment, like water and oxygen. When iron rusts though, the rust that forms at the surface gradually flakes off, revealing new metal underneath. This means that the next layer can also rust, and over time that bit will fall away as well. So if left long enough, the entire chunk of iron can break down until there's nothing left. In contrast, when metals like aluminium corrode, it's only the surface atoms that are affected.
This is because when the top layer of aluminium atoms is oxidised to form aluminium oxide, they effectively form a protective layer around the piece of aluminium, which protects the rest of the metal from the environment. The last thing we need to look at is how we can prevent iron from rusting. Generally speaking, we can split the methods into two main categories, barrier methods and sacrificial methods.
The barrier methods work by preventing the oxygen and water from ever touching the iron, so that rusting can't take place. And there are three main types to know. One is to paint it, which works well for really big pieces of iron, like the side of a ship.
The second is to oil or grease it, which is necessary if the object is moving parts like a bike chain. And the third type of barrier protection is electroplating, which is kind of complicated, but basically involves using electrolysis to cover the iron in a thin layer of another metal, which will protect the iron underneath. The other option is called the sacrificial method. This involves adding a more reactive metal, like aluminium or zinc, to the iron. This means that if the object gets exposed to oxygen, the more reactive metal will be oxidised instead of the iron, and so the iron won't rust.
An example that uses both of these methods is galvanising. This involves coating the iron object, in a layer of zinc, which is effectively the barrier method, because it's forming a protective coating around the entire nail. However, if the zinc coating gets scratched, and the underlying iron gets exposed, the zinc will still react with any oxygen that goes near the nail, because the zinc is so much more reactive.
So it's actually an example of the sacrificial method as well. Anyway, that's everything for this video. So if you enjoyed it, then please do give us a like and subscribe.
And we'll see you again soon.