ruostuu

Ruostuu is a Finnish verb meaning to rust or corrode, especially in reference to iron and steel. The related noun is ruoste, and the process is rusting, the corrosion of iron due to reaction with oxygen and water. Rust is primarily hydrated iron(III) oxide formed by electrochemical reactions at active sites on the metal surface.

The basic chemistry can be described by oxidation of iron in the presence of water and oxygen, producing iron oxides and hydroxides. A commonly cited simplified overall reaction is 4 Fe + 3 O2 + 6 H2O → 4 Fe(OH)3, which dehydrates to Fe2O3·nH2O. In practice rust is a complex, layered mixture of hydrated oxides rather than a single compound.

Several factors promote rusting, including moisture, electrolytes such as salts, acids, and temperature fluctuations. Metals that are not pure iron or that are protected by coatings may rust more slowly. Stainless steel, for example, forms a protective chromium oxide layer that resists rusting, while coatings such as paint or oil reduce exposure to water and oxygen. Protective strategies also include galvanization, cathodic protection, and the use of corrosion-resistant alloys.

In addition to its literal meaning, ruostua can be used metaphorically in Finnish to describe equipment or practices that have become less functional or obsolete due to neglect or aging.

The term ruoste refers to the oxide products themselves, often visible as a reddish-brown coating on affected surfaces.