The chemical corrosion of iron by oxidation with moist air to give hydrated ferric oxide is called resting and the produce is rust. The nesting process is accelerated by acidic oxide like CO2, SO2, NO2, etc.
2Fe + O2 + xH2O → Fe2O3. xH2O
There are many theories to explain the nesting of iron is any which most reliable theory is electrochemical theory.
Acc. to this theory, rusting of iron occurs due to formation of galvanic cell in the iron. At anode, iron atom oxides to Fe++ ion by losing 2 electrons.
At anode, Fe → Fe++ + 2e
The electon moves to cathode where oxygen reduces to OH by gaining electron.
At cathode=1\2O2 + H2O + 2e → 2OH
Fe++ and OH– ion combines forming Fe(Oh)2.
Fe+2 +2OH → Fe (OH)2
Ferrous wmpound are unstable so Fe(OH)2 is oxided by air in presence of water to Fe (OH)3. Which then decompose forming Ferric oxide which is rust.
2Fe(OH)2 + H2O +1\2O2 → 2Fe (OH)3
2Fe (OH)3 → Fe2O3+3H2O.
This theory is supported by fact that analysis of rust shows presence of Fe+3 stages.
Prevention of Rusting:-
This phenomenon can be prevented by:
- By applying layer of enamel, grease oil etc.
- By galvanisity iron by zinc.
- By making passive by dipping is conc. HNO3 or by passing steam over red hot iron.
- By cathodic protection, this process is generally used in ships Iron is kept in contact with more electropositive metals like Zn, Al, Mg, etc. which acts as anode where iron is protected as cathode. The metal used as anode is called sacrificial anode.