Rusting

Rusting is a form of corrosion that occurs when iron or steel reacts with oxygen in the presence of water to form hydrated iron oxides and hydroxides, commonly called rust. The resulting material is typically reddish-brown and flaky.

Rusting is an electrochemical process that requires an electrolyte. In moist air, iron is oxidized at anodic sites, releasing Fe2+ ions and electrons. At cathodic sites, electrons reduce oxygen or water to form hydroxide. The overall reaction produces hydrated iron(III) oxide. The rust layer is usually porous and adheres poorly, which allows further exposure of fresh metal to continue the corrosion process.

Several factors influence rusting. The availability of liquid water and oxygen is essential, and the presence of salts or acids can accelerate corrosion by increasing conductivity and altering the local environment. Temperature, metal microstructure, and the presence of dissimilar metals in contact can also affect the rate, as can surface coatings and protective films.

Prevention and mitigation rely on limiting contact with water and oxygen or interrupting the electrochemical cycle. Methods include protective coatings such as paint or epoxy, zinc coatings (galvanization) that act as sacrificial anodes, and the use of corrosion-resistant alloys like stainless steel. Cathodic protection, corrosion inhibitors, and careful design to avoid crevices and standing water also reduce rusting risk. Regular maintenance helps detect early signs of rust and extend the life of iron-containing structures and components.