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We have learnt that metals that are chemically active get corroded in the presence of a moist atmosphere. Corrosion is an oxidation reaction with atmospheric oxygen in the presence of water on the surface of a metal. Iron corrodes more quickly than most other transition metals to form an iron oxide. Corrosion or rusting of iron is accelerated in the presence of CO2 and also in the presence of salt solution.
i.e., rust is hydrated iron (III) oxide
Rusting is oxidation because it involves iron gaining oxygen (Fe  Fe2O3) or iron atoms losing electrons (Fe - 3e-  Fe3+). The equation is not meant to be balanced and the amount of water 'x' is variable, from dry to soggy.
The major problem of corrosion occurs with iron (or steel) as it is used as a structural material in industries like construction, infrastructure, bridges, rail transport power transmission, shipbuilding, automobiles, heavy industries etc.
Aluminium, another useful structural metal, also undergoes an oxidation reaction, but does not oxidize and corrode as quickly as its reactivity suggests. Once a thin oxide layer of Al2O3 has formed on the surface, it forms a barrier to oxygen and water to prevent further corrosion of the aluminium. Aluminium can be made harder, stronger and stiffer by mixing it with small amounts of other metals (e.g., magnesium) to make alloys.
The Group 1 alkali metals like sodium used in chemistry laboratories and in some chemical industries rapidly corrodes in air and need to be stored under oil.
Copper and lead are both used in roofing situations because neither is very reactive. The compounds formed on the surface do not flake away as easily as rust does from iron. Lead corrodes to a white lead oxide or carbonate and copper corrodes to form a basic green carbonate (combination of the hydroxide Cu(OH)2 and carbonate CuCO3). In the past both metals have been used for piping but as lead is considered too toxic copper is usually used.
Non reactive metals like gold, platinum, mercury do not corrode.
Prevention of Corrosion
Iron and steel (alloy of iron) are most easily protected by paint which provides a barrier between the metal and air/water. Moving parts on machines can be protected by a water repellent oil or grease layer. Covering the surface with enamel and lacquers is another method.
Sacrificial Protection
'Rusting' can be prevented by connecting iron to a more reactive metal (e.g., zinc or magnesium). This is referred to as sacrificial protection or sacrificial corrosion, because the more reactive protecting metal is preferentially oxidized away, leaving the protected metal intact.
Alloying
Iron or steel along with other metals can also be protected by 'alloying' or mixing with other metals (e.g., chromium) to make non-rusting alloys. Stainless steel is an example of a non-rusting alloy of iron and carbon. Brass, an alloy containing copper is another metal alloy which is less expensive and non reactive.
Galvanizing
Coating iron or steel with a thin zinc layer is called 'galvanizing'. This layer is produced by electrolytic deposition. Dipping the iron/steel object in molten zinc and using it as the negative cathode zinc is coated on it. Zinc preferentially corrodes or oxidizes to form a zinc oxide layer that does not flake off like iron oxide rust. Also, if the surface is scratched, the exposed zinc again corrodes before the iron and continues to protect it.
Electroplating
Coating the surface with metals like tin, chromium, nickel etc. by electroplating is also utilized to prevent corrosion. Steel cans are protected by relatively un-reacted tin and works well as long as the thin tin layer is complete.
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