How Rust and Corrosion Work
Rust is the common name for iron oxide. The most familiar form of rust is the reddish coating that forms flakes on iron and steel (Fe2O3), but rust also comes in other colors including yellow, brown, orange, and even green! The different colors reflect various chemical compositions of rust.
Rust specifically refers to oxides on iron or iron alloys, such as steel. Oxidation of other metals has other names. There is tarnish on silver and verdigris on copper, for example.
Key Takeaways: How Rust Works
Rust is the common name of the chemical called iron oxide. Technically, it’s iron oxide hydrate, because pure iron oxide isn’t rust.
Rust forms when iron or its alloys are exposed to moist air. The oxygen and water in air react with the metal to form the hydrated oxide.
The familiar red form of rust is (Fe2O3), but iron has other oxidation states, so it can form other colors of rust.
The Chemical Reaction That Forms Rust
Although rust is considered the result of an oxidation reaction, it’s worth noting not all iron oxides are rust. Rust forms when oxygen reacts with iron, but simply putting iron and oxygen together isn’t sufficient. Although about 21% of air consists of oxygen,1 rusting doesn’t occur in dry air. It occurs in moist air and in water. Rust requires three chemicals in order to form: iron, oxygen, and water.
iron + water + oxygen → hydrated iron(III) oxide
This is an example of an electrochemical reaction and corrosion. Two distinct electrochemical reactions occur:
There is anodic dissolution or oxidation of iron going into aqueous (water) solution:
2Fe → 2Fe2+ + 4e-
Cathodic reduction of oxygen that is dissolved into water also occurs:
O2 + 2H2O + 4e- → 4OH-
The iron ion and the hydroxide ion react to form iron hydroxide:
2Fe2+ + 4OH- → 2Fe(OH)2
The iron oxide reacts with oxygen to yield red rust, Fe2O3.H2O
Because of the electrochemical nature of the reaction, dissolved electrolytes in water aid the reaction. Rust occurs more quickly in saltwater than in pure water, for example.
Keep in mind oxygen gas (O2) is not the only source of oxygen in air or water. Carbon dioxide (CO2) also contains oxygen. Carbon dioxide and water react to form weak carbonic acid. Carbonic acid is a better electrolyte than pure water. As the acid attacks the iron, water breaks into hydrogen and oxygen. Free oxygen and dissolved iron form iron oxide, releasing electrons, which can flow to another part of the metal. Once rusting starts, it continues to corrode the metal.
Rust is brittle, fragile, progressive, and weakens iron and steel. To protect iron and its alloys from rust, the surface needs to be separated from air and water. Coatings can be applied to iron. Stainless steel contains chromium, which forms an oxide, much like how iron forms rust. The difference is the chromium oxide does not flake away, so it forms a protective layer on the steel.