What is rust?
Rusting is the common term for corrosion of iron and its alloys, such as steel. Although oxidation of other metals occurs, these oxides are not commonly called rust. Rust is a compound that is formed by oxidation of iron. The chemical composition of rust is mainly hydrated iron(III) oxide.
Rusting requires iron, oxygen, and a electrolyte. Salt water will cause rust to develop faster than fresh water because it is a better electrolyte. Like most chemical reactions, heat also speeds rust; a good rule of thumb is that the rate of a given chemical reaction, such as rusting, will double with every 10 centigrade degree increase in temperature.
Why is rust a problem? Since rust has higher volume than the originating mass of iron, as it expands it often forces apart adjacent components. An employee in a yard in Ft. Lauderdale quoted an expansion ratio of 6 to 1: 1 mm of steel will expand to form 6 mm of rust. Rust will flake off and expose fresh metal to further corrosion.