When water falls on iron, it reacts with the oxygen in the water and forms iron oxide. This is rust. The drop of water turns reddish in colour and the rust is suspended in the water. When the water drop evaporates, the rust remains there and forms a reddish coat on the iron itself.
Once started, rust spreads even in dry air. This is due to the fact helps whatever moisture is available in the air to condense in it. It also attracts the air and holds it. Here, the phrase, Prevention is better than cure is apt because it is easier to prevent rust from starting than to prevent it from spreading once it has started. Iron articles can be prevented from rusting by coating them with a special paint.
Below Iron Rust info from Wikipedia (Read More):-
Rust is a general term for a series of iron oxides, usually red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture. Several forms of rust are distinguishable visually and by spectroscopy, and form under different circumstances.[1] Rust consists of hydrated iron(III) oxides Fe2O3·nH2O and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3). Rusting is the common term for corrosion of iron and its alloys, such as steel. Other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust. Given sufficient time, oxygen, and water, any iron mass eventually converts entirely to rust and disintegrates. The corrosion of aluminium is extremely slow because the resulting aluminium oxide forms a conformal coating, which protects the remaining aluminium. This process is known as passivation.
http://en.wikipedia.org/wiki/Rust
