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Definition: Corrosion is a reaction which occurs only on the surface of a metal. The surface of the metal reacts with the surrounding air or water. The surface changes colour to a dull colour. Here is an example – a shiny silver tray and a dull silver clock. The silver clock has been left standing and became dull over time.
I can make the surface of the silver clock shiny again by using Brasso and a cloth. The surface was black to start with because of corrosion. The surface reacted to the air and changed to a dull colour. The cloth is dirty now and the clock is shiny again. Perfect!
There is another good example of corrosion – the Statue of Liberty in America. It is made of copper metal and was originally brown but over the years the colour of the statue changed to green because of corrosion.