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It is the gradual destruction of metals by chemical or electrochemical reaction with the environment. It is a natural process which converts a metal into its oxide, hydroxide or sulphide so that it loses its metallic characteristics.
Rust is chemically known as hydrated ferric oxide (it is formulated as Fe2O3.xH2O). Rusting results in the formation of scaling reddish brown hydrated ferric oxide on the surface of iron and iron containing materials.
i. Dry Corrosion or Chemical Corrosion: The corrosive action in the absence of moisture is called dry corrosion. It is the process of a chemical attack on a metal by a corrosive liquids or gases such as O2, N2, SO2, H2S etc. It occurs at high temperature. Of all the gases mentioned above O2 is the most reactive gas to impart the chemical attack.
ii. Wet Corrosion or Electrochemical Corrosion: The corrosive action in the presence of moisture is called wet corrosion. It occurs as a result of electrochemical reaction of metal with water or aqueous solution of salt or acids or bases.
i. Alloying: The metals can be alloyed to prevent the process of corrosion. E.g: Stainless Steel
ii. Surface Coating: It involves application of a protective coating over the metal. It is of the following types:
a) Galvanization: It is the process of coating zinc on iron sheets by using electric current.
b) Electroplating: It is a method of coating one metal over another metal by passing electric current.
c) Anodizing: It is an electrochemical process that converts the metal surface into a decorative, durable and corrosion resistant. Aluminium is widely used for anodizing process.
d) Cathodic Protection: It is the method of controlling corrosion of a metal surface protected is coated with the metal which is easily corrodible. The easily corrodible metal is called Sacrificial metal to act as anode ensuring cathodic protection.
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