Electrons released at this anodic spot move through the metal and go to another spot on the metal and reduce oxygen at that spot in presence of H+ (which is believed to be available from H2 CO3 formed due to dissolution of carbon dioxide from air into water in moist air condition of atmosphere. Hydrogen ion in water may also be available due to dissolution of other acidic oxides from the atmosphere). This spot behaves as cathode with the reaction is.


Cathode:

O2(g) + 4H+ (aq) + 4e- 2H2 O(l)

The overall reaction is: 2Fe(s) +O2(g) +4H+(aq) 2Fe2+ (aq) +2H2 O(l)

The Ferrous ions (Fe2+) are further oxidised by atmospheric oxygen to Ferric ions (Fe3+) which come out as rust in the form of hydrated ferric oxide (Fe2 O3 .XH2 O) and with further production of hydrogen ions.

Prevention of corrosion

Prevention of corrosion is of prime importance. It not only saves money but also helps in preventing accidents such as a bridge collapse or failure of a key component due to corrosion.

One of the simplest methods of preventing corrosion is to prevent the surface of the metallic object to come in contact with atmosphere. This can be done by covering the surface with paint or by some chemicals (eg: bisphenol).

Another simple method is to cover the surface by other metals (Sn, Zn etc) that are inert or react themselves with atmosphere to save the object. This is generally done by electroplating.

An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn etc) which corrodes itself but saves the object.

Do you know?

Alloying is a method of improving the properties of a metal. We can get desired properties by this method. For example, iron is the most widely used metal. But it is never used in its pure state. This is because pure iron is very soft and stretches easily when hot. But, if it is mixed with a small amount of carbon, it becomes hard and strong. When iron is mixed with nickel and chromium we get stainless steel which will not rust. Pure gold, known as 24 carat gold, is very soft. It is, therefore, not suitable for making jewellery. It is alloyed with either silver or copper to make it hard. Generally in India 22carat gold is used for making ornaments. It means that 22 parts of pure gold is alloyed with 2 parts of either silver or copper.



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