Corrosion:
When metals are exposed to atmospheric conditions, they react with air or water in the environment to form undesirable compounds (usually oxides). This process is called corrosion. Almost all metals except the least active metals such as gold, platinum and palladium are attacked by environment i.e., undergo corrosion. For example, silver tarnishes, copper develop a green coating, lead or stainless steel lose their lustre due to corrosion. Corrosion causes enormous damage to buildings, bridges, ships and other articles made of iron.
‘Corrosion may, therefore, the process of slow conversion of metals into their undesirable compounds (usually oxides) by reaction with moisture and other gases present in the atmosphere.’
In case of iron, corrosion is called rusting. Chemically, rust is hydrated form of ferric oxide, Fe2O3. xH2O. Rusting of iron is generally caused by moisture, carbon dioxide and oxygen present in air. It has been observed that rusting takes place only when iron is in contact with moist air. Iron does not rust in city air and in vacuum.
Prevention of Corrosion:
There are several methods for protecting metals from corrosion (iron from rusting). Some of these methods are—
1) Barrier Protection:
In this method a barrier film is introduced between iron and atmospheric oxygen and moisture. The barrier protection can be achieved by any of the following methods:
(i) By painting the surface
(ii) By coating the surface with a thin film of oil or grease.
(iii) By electroplating iron with some non-corrosive metal such as nickel, chromium, copper, etc.
2) Sacrificial Protection:
In this method iron is protected from rusting by covering it with a layer of a metal more active than iron. This prevents loss of electrons from iron. The active metal loses electrons in preference to iron and goes into ionic state. Therefore, the covering metal is consumed with time, but as long as it is present on the surface of iron, the latter is not rusted. This type of process in which rusting of iron is protected is called sacrificial protection.
Zinc is commonly used for covering Iron surfaces. The process of covering iron with zinc is called galvanization. The galvanized iron materials maintain their lustre due to the coating of invisible layer of basic zinc carbonate, ZnCO3, Zn(OH)2on the zinc film.
3) Electrical protection:
This method is used for protecting iron articles which are in contact with water such as underground water pipes. The article of iron is connected with more active metals like magnesium or zinc. The active metal has lower reduction potential than iron and will lose electrons in preference to iron. For example, magnesium has lower reduction potential than iron.
Protection of rusting of underground iron pipes by electrical protection method. Therefore, magnesium will be oxidised in preference to iron and therefore, it will protect iron from being rusted.
4) Using anti-rust solutions:
To retard the corrosion of iron, certain anti-rust solutions are used. For example, solutions of alkaline phosphates and alkaline chromates are generally used as anti-rust solutions. Due to the alkaline nature of these solutions, the H ions are removed from the solutions, and rusting is prevented.