Mechanism Of Rusting- Factors affecting and prevention

Hello students let’s learn the mechanism of rusting, the factors affecting rusting and it’s prevention.

Corrosion

Corrosion is a process of deterioration of a metal as a result of it reaction with air or water surrounding it .

When metals are exposed to atmospheric conditions, they react with air or water in the environment to form undesirable compounds (usually oxides). Almost all metals except the least active metals such as gold, platinum and palladium are attacked by environment i.e., undergo corrosion.

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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 dry air and in vacuum.

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Electrochemical theory of rusting

The phenomenon of corrosion can be explained with the help of electrochemical theory which involves oxidation and reduction reactions.

A film of moisture with dissolved CO₂ constitutes electrolytic solution covering the metal surface at various places. The schematic representation of mechanism of rusting of iron can be shown as-

In the small electrolytic cells, pure iron acts as anode while cathodes are impure portions. The overall rusting involves the following steps-

1.At a particular spot of an object made up of iron, oxidation takes place and the spot behaves as an anode- 

Fe(s) ————> Fe²⁺ (aq) + 2e^–

Thus, the metal atoms in the lattice pass into the solution as ferrous (Fe²⁺) ions, leaving electrons on the metal itself. These electrons go to another spot on the metal and reduce atmospheric oxygen to water in the presence of H⁺ ions. The H⁺ ions are obtained either from water or from acidic substances (e.g., CO₂) in water.

2. At cathode the reduction takes place-

  4H⁺+O₂+4e^–———> 2H₂O

Overall reaction is  

2Fe(s) + 4H⁺ + O₂  ————>2Fe²⁺ (aq) + 2H₂O

The ferrous ions are oxidized further by dissolved oxygen or atmospheric oxygen from air to form ferric oxide with further production of H⁺ ions. The ferric oxide then undergoes hydration and the hydrated form of iron (III) oxide is known as rust. 

4Fe²⁺ (aq) + O₂(g) + 4H₂O————>    2Fe₂O₃ + 8H⁺ 

Fe₂O₃ +x H₂O ————>    Fe₂O₃. xH₂O

Prevention from rusting

1. Barrier protection–

This is one of the simplest methods to prevent rusting of iron. In this method a barrier is placed between iron and atmospheric air. The barrier protection can be achieved by any of the following methods-

1.The surface is coated with paint or some chemicals (e.g., bisphenol).

2.The surface is protected by applying a thin film of oil or grease.

3.The metal is electroplated with metals like tin, nickel, zinc 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.

  Zinc is commonly used for covering iron surfaces. The process of covering iron with zinc is called galvanization.

The galvanized iron materials maintain their luster due to the coating of invisible layer of basic zinc carbonate, ZnCO₃. Zn (OH)₂ on the zinc film. This is so because the reduction potential of zinc is less then the reduction potential of iron.

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.

4.Using anti-rust solutions.

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. For example, iron articles are dipped in boiling alkaline sodium phosphate solutions, when a protective insoluble sticking film of iron phosphate is formed