 Imagine four objects, an aluminium toy, an iron now, 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 when ready. If you think the gold ring is likely to be in the best condition, you're right. The copper pipe and the iron now will appear in a worse condition, but the aluminium toy is also preserved very well. Most likely the iron now will appear in the worst 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 corrodes, it forms compounds known as oxides or hydrated oxides. The now, 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 it in the air. But why would the now be more corroded than the aluminium toy, the copper pipe or the gold ring? Have you any ideas? Pause the video and continue when ready. The reason is the metals all have different reactivities. Iron corrodes 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 galvanizing and can last up to 100 years. If you think of the metal on a bike, corrosion can be prevented by painting the frame a static part or greasing the metal parts like the chain which moves, or by covering it with a thin layer of a less reactive metal, like tin, copper or gold. But that would be very expensive.