For most people Rust is the red product on old iron nails, gates, and every other metal object.
A common question is “What is Rust?”, and the correct answer should be “Rust is the result of the corrosion process of iron and is composed by several different iron oxides”. What we see on a rusted metal object is indeed the most external layer composed by red iron oxides called “hematite”, chemical formula Fe2O3, while the inner layers are composed by black oxide called “magnetite”, chemical formula Fe3O4.
The damaging effect of rust is due to the nature of the iron oxides that are soft, not adherent to the metal surfaces, and have a volume that is approx three times the volume of metal corroded.
When a thick layer of rust develops it fall off exposing the iron surface to the corrosive environment.
Other metals, like for example copper, corrodes but they develops oxides, the typical Stop Rust green patina on copper roof, that are tightly adherent to the surface and are not easily removed and that protect the metal beneath from further corrosion.
A copper object can be left exposed to the atmosphere for long time, and it will experience little corrosion limited to the external surface while an iron object will be severely damaged and potentially, after a long exposure, vanish transforming in an indefinite mass of rust.
As an example the external surface of the Statue of Liberty is made with copper and after more than one hundred years of atmospheric exposure it is virtually intact.
Almost every metal if exposed to the “right” environment corrodes and develops metal oxides, so basically every metals “rust”, even if “rust” is used to define the corrosion product of iron.
What is important to understand is the correlation existing between corrosion and rust and why metals experience corrosion.
Metals are generally stable in their natural form that is the native ores found in nature, from which the metals are extracted in processing. The primary cause of metallic corrosion is the natural tendency of a metal to return to its natural state.
Iron ore are in fact composed by iron oxides, the same component of rust on a corroded iron object. Every metals have a different tendency to react with the environment of fluid and this tendency is listed in what is usually called “Galvanic Series” or “Galvanic Chart”.
Iron when exposed to the atmosphere corrodes because of the presence of water, rainwater or condensing water, and the reaction with oxygen dissolved into water.
The most common method used to stop or reduce iron to rust when exposed to the atmosphere is to use a paint or coating that prevents the contact between iron and water and the resulting corrosion process. Once rust is formed it is possible to restore the iron object, if it has not been severely damaged by corrosion, removing the rust layer with chemical or mechanical methods.