Corrosion

Summary: The deterioration of a metal due to reaction with substances in the environment (e.g., oxygen, water). Tags: igcse chemistry definition metals Created: 2026-07-14 Last Updated: 2026-07-14

Corrosion is the gradual destruction or deterioration of a metal resulting from chemical or electrochemical reactions with substances in its surrounding environment, such as oxygen, water, acids, or salts. At its core, corrosion is a redox (oxidation-reduction) process in which the metal atoms lose electrons and are oxidised to form metal ions or compounds — for example, when iron rusts it is oxidised to hydrated iron(III) oxide (Fe₂O₃·xH₂O), a flaky, porous, reddish-brown solid that weakens the metal structure. It is important to distinguish between general corrosion and the specific term rusting: rusting applies only to iron and steel and requires both oxygen AND water to occur, whereas other metals corrode by different mechanisms (e.g., aluminium forms a protective oxide layer, silver tarnishes by reacting with sulfur compounds). The economic impact of corrosion is enormous, with billions spent annually worldwide on prevention, maintenance, and replacement of corroded infrastructure, pipelines, vehicles, and ships. Prevention methods taught at IGCSE include: barrier methods (painting, oiling/greasing, plastic coating, electroplating), which physically exclude oxygen and water; sacrificial protection, where blocks of a more reactive metal (e.g., zinc or magnesium) are attached to iron and corrode preferentially; and alloying, such as making stainless steel by adding chromium and nickel to iron. In the IGCSE exam, students must be able to explain sacrificial protection using the reactivity series, describe experimental conditions needed for rusting (controlled experiments with nails in test tubes with water only, air only, both water and air, and with salt), and link rust prevention methods to the exclusion of oxygen and/or water.

Key term — IGCSE Chemistry (0620/0971) glossary.