Iron rusts because it can be contacted by water-or the moisture in the air- and the metal takes the oxygen from the water forming iron oxide-rust. Paint or plating (like chrome plating) prevents the H2O molecules from reaching the surface of the metal so it can't get the oxygen.
Because the paint over the iron of the car helps to keep the iron from coming in contact with air, and that's why cars in junkyards are usually rusted, because the paint is chipped or gone.
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