Most commonly used metals are strong, ductile and malleable. When in use, these get exposed to the environment. Many of them lose their shine, and get covered with colored layer on them. Some even lose their strength and become weak and brittle. This happens due to chemical interaction between the metals and their environment. The spontaneous destruction of metals due to their interaction with environment is called corrosion. Corrosion takes place on the exposed surface. When the upper layer of the metal gets corroded, then the inner surface of the metal gets exposed, and the corrosion then continues up to certain depth. Corrosion is said to have taken place when for example, silver gets tarnished, i.e., it loses its shine, iron gets coated with a brittle brown colored layer, copper and brass get a green-colored deposit, aluminum surface becomes dull, and loses its shine, and so on. General corrosion affects the entire surface of the metal; while the local corrosion is confined to certain localized spot-on the surface. Corrosion causes enormous damage to the buildings, bridges, ships and many other things made of iron.
Corrosion of metals can be prevented in many ways. Some commonly used methods are Described below.
(i) By surface coating: Corrosion of metals can be prevented by coating their surfaces with any of the following:
- By applying oil, grease, paint or varnish on the surface.
- By coating/depositing a thin layer of any other metal which does not corrode. For example, iron surface can be protected from corrosion by depositing a thin layer of zinc, nickel or chromium on it. Copper/brass can be protected by coating it with a thin layer of tin. Tinning of brass utensils is a very common practice in our country.
(ii) By connecting metal to a more electropositive metal: A metal can be protected from corrosion by connecting it to a more electropositive metal. As long as the more electropositive metal is there, the given metal does not get corroded. For example, iron can be protected from corrosion by connecting it to a block/plate of zinc or magnesium. This method of corrosion protection is called cathodic protection.
(iii) By forming insoluble phosphate or chromate coating: Metal surfaces are treated with phosphoric acid to form an insoluble phosphate coating on the surface. This phosphate coating protects the metal from corrosion. Formation of a thin chromate layer also prevents the corrosion of metals
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