Why Do Nails Rust?
Nails are generally made of steel, which does not rust. But there is often a good amount of iron within the nail. Specifically, rust is a product of iron, but it is made from oxidation of the metal by water and air. Similar oxidation occurs with other metals, so it is a much more appropriate term when referring to metals in general, rather than just iron. The oxidation of silver causes a darkened oxidation layer to form, commonly known as tarnish ‘š this can be easily wiped off with a cloth or brush to reveal the untouched silver underneath, but it will keep returning, even though it returns slowly, until a protective coating is used. Copper as well also is oxidized into a green patina; the most famous example of this is, of course, the Statue of Liberty. The patina actually acts as a protective layer over the remaining copper, and once the patina has formed, nothing underneath is damaged, very much unlike iron rust. When iron begins to rust, it severely damages the internal structure of the iron object, weakening it and especially damaging the integrity of the whole structure of whatever the iron is used for. This includes nails, and as they rust, their strength diminishes.
The reason that iron rusts and other metals oxidize is because rust and other oxidation forms are a lower energy and thus more stable than the metal form. The combination of the metal iron atom and an oxygen atom releases energy as they combine, and rust would actually feel hot if rusting were a much faster process. To return the rust to an un-oxidized state, energy would needed to be added to force the two atoms to separate.
Some nails also contain copper or bronze, rather than iron, so they would not oxidize so easily. It really depends on which kind of nail you are using.