Simply put, rust is the common term for iron or iron alloy corrosion. Though most of the metals present on Earth eventually go through a corrosion phase, we only refer to the iron metal corrosion when we say the word rust. Iron corrosion represents a set of chemical reactions between iron, water and oxygen, which end with the transformation of the whole mass of metal into a mix of iron oxides and water that leads to the disintegration of the respective iron structure.

Usually, metals, such as aluminum, create compact layers of corrosion covering the whole surface of the metal, thus protecting it for long periods of time from further reactions with the environment. Unfortunately, this is not available for iron, as iron rust is porous - meaning that it actually accelerates the corrosion process, rather than slowing it down.

Against general belief, iron present in a pure water, or in a pure-oxygen environment, does not rust. It is only when the two agents are combined that a corrosion process is initiated, which forms iron(III) oxide. Iron(II) oxide, on the other hand, is a highly-compact substance that can easily adhere to the surface of bulk metal in order to protect it. The corrosion process will only stop when all the iron(0) metal is oxidized by the action of oxygen, water, carbon dioxide or sulfur dioxide, or until these substances are being removed from the system.

The reaction is initiated by oxygen through an electrochemical process, in which electrons are being transferred from iron to oxygen gas, a reaction catalyzed by electrolytes, such as road salt that accelerates the corrosion of automobiles. Then, ionized oxygen gas reacts with water to create hydroxide ions that have low pH, thus are acid. The second step is represented by the actual reaction between iron(II) and (III) with hydroxide, which results in hydrogen and iron(II) and (III) hydroxides, that will dissociate through dehydration into iron oxide and iron oxide(II).

As I said earlier, rust is porous and cannot prevent further metal corrosion. Thus, to protect iron and iron alloys, they usually get coated with a series of metals that form compact corrosive layers, such as chromium and zinc. The process is known as galvanization. Even the galvanization of iron may fail from time to time, as in the case when the coating is pierced, then rust may occur. Alternative protection methods against the development of rust are cathodic protection, bluing and paint coating.