Corrosion is the deterioration of a material through chemical or electrochemical interaction with its surrounding environment. It is especially important for metals because many metals are thermodynamically capable of returning towards more stable oxidised forms. In aqueous environments, corrosion often involves local electrochemical cells on a metal surface. At anodic sites, metal atoms can lose electrons and enter solution as ions, while the electrons move through the metal to cathodic sites where reduction reactions occur. Oxygen reduction and hydrogen evolution are common cathodic processes. Corrosion is therefore broader than simple oxidation because its rate and pattern depend on electrode reactions, electrolyte properties, surface structure, transport, material composition, stress, temperature, and environmental chemistry.
Corrosion has several distinct forms. Uniform corrosion produces relatively even material loss over a surface. Galvanic corrosion occurs when electrically connected dissimilar metals are exposed to an electrolyte and develop different electrochemical tendencies. Pitting corrosion creates highly localised cavities that can penetrate deeply while leaving much of the surrounding surface apparently intact. Crevice corrosion develops in shielded regions where stagnant electrolyte produces different local chemical conditions. Intergranular corrosion preferentially attacks susceptible grain boundaries. Stress corrosion cracking combines a susceptible material, a particular environment, and mechanical stress and can produce cracking with limited general surface damage. Distinguishing these mechanisms is important because each requires different inspection, material selection, design, and prevention strategies.
Rusting is a familiar example of corrosion involving iron and iron based materials. Iron can undergo anodic oxidation, producing iron ions and electrons. The electrons can participate in cathodic reactions involving dissolved oxygen and water, while subsequent chemical reactions produce hydrated iron oxide and related corrosion products. Rust is generally porous and poorly adherent, so it does not reliably isolate the underlying metal from the environment.
Aluminium behaves differently because it can form a thin, adherent oxide film that strongly reduces the rate of further attack under many conditions. The protective nature of surface films is therefore crucial to corrosion behaviour and explains why different metals exposed to the same environment can have dramatically different durability.
Environmental conditions control corrosion rates and mechanisms. Water provides an ionic medium, dissolved oxygen can participate in cathodic reactions, and dissolved salts can increase conductivity. Chloride ions are particularly important in many forms of localised corrosion because they can interfere with protective passive films and promote breakdown at susceptible surfaces. Acidity, temperature, humidity, fluid flow, pollutants, deposits, microorganisms, and stagnant regions can all alter corrosion behaviour. Marine environments are especially demanding because of salt content and continuous exposure to moisture. Industrial environments may introduce acidic gases, alkaline chemicals, dissolved contaminants, or high temperatures. Corrosion assessment therefore requires understanding the actual service environment rather than relying solely on generic material labels.
Material selection is a major corrosion control strategy. Stainless steels, aluminium alloys, nickel based alloys, titanium, copper alloys, and specialised steels can provide improved performance under particular environmental conditions. Alloying can alter passive film composition, strength, electrochemical behaviour, and resistance to specific corrosion mechanisms. Coatings provide another major defence by separating the material from the environment. Paints, polymers, metallic coatings, conversion coatings, sealants, and multilayer systems can all be used. Surface preparation is critical because contamination, poor adhesion, defects, or trapped moisture can undermine a coating. Design also matters because sharp crevices, poor drainage, stagnant liquid, and inaccessible surfaces can create environments where corrosion develops rapidly.
Cathodic protection uses electrochemical principles to reduce corrosion of structures such as pipelines, storage tanks, ships, offshore installations, and buried infrastructure. A protected structure is maintained at a sufficiently cathodic potential that anodic dissolution is suppressed. Sacrificial anodes made from more readily oxidised materials can provide electrons while being consumed, whereas impressed current systems use an external power supply to regulate the structure potential. Anodic protection can be used for selected materials and environments where a stable passive state exists. These approaches demonstrate that corrosion is not simply a materials problem but an electrochemical process that can sometimes be controlled by deliberately manipulating electrical potential and reaction pathways.
Corrosion creates major economic, engineering, environmental, and safety consequences. It can reduce structural thickness, weaken components, cause leakage, contaminate products, increase maintenance costs, and produce sudden failures. Bridges, aircraft, ships, pipelines, vehicles, buildings, chemical plants, power infrastructure, and medical equipment can all require corrosion management. Inspection methods include visual examination, thickness measurement, ultrasonic testing, radiography, electrochemical monitoring, coating inspection, and environmental monitoring. Effective corrosion management can combine suitable material selection, protective coatings, inhibitors, drainage, cathodic protection, controlled environmental conditions, inspection, and scheduled maintenance. Designing a structure so that water cannot accumulate and susceptible surfaces remain accessible for inspection can prevent problems that might otherwise require expensive material replacement.
Corrosion is not synonymous with rust. Rust mainly describes corrosion products associated with iron and iron based materials, while corrosion can affect many metals through different mechanisms. Corrosion is also not necessarily visible, because localised attack can occur beneath deposits, inside crevices, or under coatings. Some materials form protective passive films and can remain highly resistant under one environment while becoming vulnerable under another.
Understanding corrosion requires concepts from electrochemistry, materials science, thermodynamics, kinetics, fluid transport, mechanics, surface chemistry, and environmental chemistry. The practical goal is not simply to find a material that never corrodes, because no material is universally immune, but to understand the relevant degradation mechanism and manage material, design, environment, and protection together.
