Corrosion is a matter of common observation that certain metals are slowly eaten up on long exposure to atmosphere. For example: silver gets tarnished, copper develops green coating on its surface, iron gets rusted and lead loses its luster. In fact, such metals react with the gases or moisture present in the environment to form undesirable compounds. Corrosion is the process of slow conversion of metals into their undesirable compounds by reaction with moisture and other gases present in the atmosphere.

The more active metals are more prone to corrosion. Presence of impurities helps in setting up a corrosion cell and makes the corrosion to occur rapidly. Pure iron does not rust. Air and moisture are quite helpful in corrosion. The presence of gases like carbon dioxide and sulphur dioxide in air makes it still rapid. So if iron is kept in vacuum, no rusting is caused. Strains in metal also help in corrosion. In iron articles, rusting is more pronounced on the areas having bends, dents, scratches, nicks and cuts. The presence of electrolytes also makes the corrosion process faster. So iron rusts more rapidly in saline water in comparison to pure water.

Chemically speaking, rust is hydrated iron. It is generally caused by moisture of air. Rust is a non sticking brown colored material which can be easily removed by scratching. There are several methods followed for protecting metals from corrosion. A barrier film is introduced between iron and atmospheric oxygen and moisture by painting the surface, by coating the surface with a thin film of oil or grease and by electroplating iron with some non-corrosive metal such as nickel, chromium, copper etc. in this type of protection, if scratches or crakes appear in the protective layer then surface f iron may get exposed thus allowing moisture and oxygen to come in contact with iron and rusting starts. This rusting extends beneath the protective layer and eventually peels off the protective layer.

Sometimes the surface of iron may also be covered with a layer of more active metal like zinc. This active metal loses electrons in preference to iron and hence, prevents the rusting of iron. However, the covering metal gets consumed in due course of time, but so long as it is present, even the nearly uncovered surfaces of iron do not get rusted. The rusting of iron may also be prevented by electrical protection. The exposed surface of iron is protected by connecting it to some more active metal such as magnesium. The more active metal acts as anode and loses electrons in preference to iron. The alkaline solutions of some phosphate and chromate salts act as anti rust solutions. When iron articles are dipped into the boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the articles from rusting. Thus in true sense, rust is a material that needs both attention and prevention.