Rust

Rust is a reddish-brown oxide that forms when iron or iron-based materials such as steel are exposed to oxygen and moisture for extended periods. It is a type of corrosion, meaning the chemical degradation of metal when it comes into contact with the surrounding environment.

Rust differs from the protective oxides that form on other metals. Aluminum oxide, for example, creates a hard, dense layer that prevents further corrosion. Iron rust, by contrast, is porous and flaky; it peels away from the surface, exposing fresh metal beneath it to further rust. For this reason, rusting continues until all of the iron has been converted to oxide.

How rust forms

The formation of rust requires three things: iron, oxygen, and water. When the latter two combine on the iron surface, an electrochemical reaction occurs. Part of the iron becomes an anode from which electrons flow; another part becomes a cathode where oxygen and water accept those electrons. The result is the production of iron ions that combine with oxygen and water, forming hydrated iron oxide, which is what we call rust.

Salt accelerates this process, because saltwater conducts electric current well, which intensifies the electrochemical reaction. This is why ships, coastal structures, and vehicles in salt-spray regions rust quickly. Similarly, atmospheric impurities such as industrial gases increase the rate of corrosion.

Prevention

The most common method is to seal the iron surface from air and moisture through painting, varnishing, or oiling. Another method is galvanizing, which involves coating steel with zinc; zinc corrodes preferentially to iron, protecting the iron instead. Stainless steel, which contains chromium, offers another solution; chromium forms an exceptionally dense and impermeable oxide layer.

Economic and historical significance

Rust is one of the largest expenses in the maintenance of metal structures: harbors, water and oil pipelines, ships, and iron buildings. Governments and companies continually monitor rust-affected areas, as rusted iron loses its structural strength. On the other hand, iron oxide has practical applications: iron oxides form the basis of red and brown pigments used in painting, and have been sources of these colors in art for millennia.