In this video we are going to learn about corrosion. Imagine four objects, an aluminium toy, an iron nail, a copper pipe and a gold ring are left out in the open for many years. Which of the four materials do you think will be in the best condition when they are rediscovered? Pause, think and continue and ready. If you think the gold ring is likely to be in the best condition, you're right. The copper pipe and the iron nail will appear in a worse condition, but the aluminium toy is also preserved very well. Most likely the iron nail will appear in a worse condition. This is because of corrosion. Corrosion is described as formation of compounds on the surface of a metal when it is exposed to air, and or water or an electrolyte like salt water. Typically once a metal corrode it forms compounds known as oxides or hydrated oxides. The nail, made of iron, rusted. This by the way, is the only time you should use the term rusting. All other metals are considered as having been corroded when they react with water and or oxygen in the air. But why would the nail be more corroded than the aluminium toy, the copper pipe or the gold ring? Have you any ideas? Pause the video and continue and ready. The reason is the metals all have different reactivities. Iron, corrode's faster because it is more reactive than the copper and the gold. But you may be wondering why didn't the aluminium toy corrode the fastest? The truth is the aluminium is more reactive than the iron, so it reacts fastest with the oxygen from the air, forming aluminium oxide. Aluminium oxide is very unreactive because once the oxide layer is formed on the surface, it binds very tightly to the surface of the metal, so no cracks can form and cause further corrosion. Here's a challenge. How might you use this idea to protect the iron now? The answer is that by covering a metal like iron with another more reactive metal, like zinc, is that the zinc forms the zinc oxide layer quickly. The iron underneath is protected. This process is called galvanising and can last up to 100 years. If you think of the metal on a bike, corrosion can be prevented by a paint in the frame, a static part, or greasing the metal parts like the chain which moves, or by covering it with a thin layer of less reactive metal, like tin, copper or gold. But that would be very expensive.