Ruoste

Ruoste, the Finnish word for rust, is the common corrosion product formed when iron or steel reacts with oxygen in the presence of water. The resulting material is a mixture of hydrated iron(III) oxides and hydroxides, typically iron oxide hydroxides such as goethite or lepidocrocite, and often hematite as well. Rust is usually porous and flaky, and its formation involves electrochemical processes in which iron dissolves at sites that act as anodes and oxygen is reduced at cathodic sites.

The rusting process begins with the anodic oxidation of iron to iron ions, while oxygen from the air is reduced in the surrounding electrolyte to hydroxide ions. These ions combine to form iron hydroxides, which gradually dehydrate and convert to iron oxides. The resulting rust typically occupies more volume than the original metal, inducing stresses, cracks, and loss of structural integrity. Rust formation is favored by moisture, salts, acids, temperature, and the presence of electrolytes that enhance electrical conductivity; dissimilar metals in contact can create galvanic couples that accelerate corrosion.

Prevention and control strategies focus on limiting moisture and oxygen access, isolating iron from corrosive electrolytes, or disrupting the electrochemical reactions. Common methods include protective coatings and paints, sealants, and waxes; metallic coatings such as galvanization (zinc coating) or electroplating; use of corrosion inhibitors in fluids; design improvements to avoid crevices; and cathodic protection. Stainless steels, which form a passive chromium-rich oxide layer, offer greater resistance in many environments.

Rust remains a major concern for infrastructure, transportation, and industry because it reduces strength and increases maintenance costs. Regular inspection, cleaning, and timely protective measures are essential to manage rust formation.