Corrosion and Its Prevention

Corrosion can be defined in many ways

• It is the derivation of metals by natural redox process.
• It is a spontaneous redox process which causes a huge damage to cars, buildings, bridges, planes etc.
• It is a natural process in which metals are oxidized to oxides or sulphide. It is the reverse of isolation of metal from their oxides. 

Conditions of  Corrosion: 

Corrosion of iron is due to series of complex redox reactions (common example is rusting of iron).

1. Iron does not rust in dry air. Hi
2. It does not rust in air free water (oxygen must be there).
3. Iron rust more quickly in acidic conditions or low pH.
4. Iron rust more quickly when it is in contact with ionic solutions.
5. Loss of iron due to corrosion results in rust formation and deposition of rust takes place at different point, position and places of an object.
6. Iron rust move quickly when it is in contact with less active metals e.g. Cu.
7. lron rust slowly when it is in contact with active metals e.g. Mg
8. Salt water accelerates corrosion as salt water increases the conduction of electrolyte solution.

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Mechanism of Corrosion:

Dents, strains, cracks and ridges etc are the sites of corrosion. They are the anodic region where oxidation will occur.

1) The very first step involves the formation of Fe⁺² ions that are the results of spontaneous oxidation of iron. The Fe⁺² ions formed entered the water droplets.

Reaction at anode:

2Fe à  2Fe⁺²  + 4e^-

2) Free electrons move to a region where concentration of oxygen is high. That region is cathodic region and here reduction takes place.

Reaction at cathode:

4H⁺ + O₂ + 4e^- à 2H₂O

Redox reaction:

2Fe + 4H⁺ + O₂ + 4e^- à2Fe⁺²  + 2H₂O              e.q.9.1

3) Fe⁺² further oxidized and results in the formation of rust.

2Fe⁺² + ½ O₂ + (2+n) H₂O  à  Fe₂O₃ + nH₂O + 4H⁺      e.q.9.2

Adding e.q.9.1 and 9.2 we got:

2Fe + 4H+ + O2 + 4e- à2Fe+2  + 2H2O 2Fe+2 + ½ O2 + (2+n) H2Oà Fe2O3 + nH2O + 4H+

2Fe+ 3/2 O2 + nH2O  à  Fe2O3.nH2O

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• Introduction To Electrochemistry
• Ostwald Dilution Law
• Electrochemical Cell and Its Construction