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Rust is a type of corrosion. It happens to metals when they are exposed to air or water for a long time. Rust slowly decomposes metals into other chemicals, because of a reaction with the oxygen molecules. Air and water contain oxygen so they are usually the reason for rust. Almost all metals rust, but they can be protected with paint. Alloys (mixes of metals) such as stainless steel, rust much slower than simple metals like pure iron. When a piece of metal rusts, it becomes a different color (for example, iron becomes red or brown), and the metal eventually decays.
Rust appears on metal if it is left outside in the damp air. For example rust occurs mostly in cracks. If it rains, water will enter the cracks and make it easier to rust because it would be hard to remove the water from the cracks. So since the water stays in the cracks, the metal starts to corrode. Eventually it becomes rust. Iron cannot be used or reused once it rusts.
Some metals, such as aluminium, titanium, and stainless steel form a very thin coating of corrosion on the metal. The metal cannot continue corroding because the coating isolates the rest of the metal from the source of oxygen. This is why aluminium keeps its shine. It also makes aluminium seem very unreactive, even though it can react with water.