According to electrochemical theory metal undergoing corrosion acts as
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Corrosion Of Iron
Corrosion is the process in which a metal is destructed as a result of its reaction with environment. Corrosion of iron is known as rusting. Rusting is the hydrated ferric oxide. Other examples for corrosion are tarnishing of silver and development of green coating on copper and bronze. In corrosion metals undergo anodic oxidation to metal oxides.
Electrochemical theory of rusting
The rusting of iron is an electrochemical process involving the following steps.
The moister containing CO2 acts as electrolyte.
H2O + CO2 ------> H2CO3
H2CO3 ------> 2H+ + CO32-
The iron is oxidized by the removal of electrons and acts as the anode.
Fe ------> Fe2+ + 2electron
The H+ ions from the electrolyte accept electrons from the adjacent areas on metal surface and function as cathode.
4H+ + 4electron ------> 2H2
The atmospheric oxygen moves hydrogen as water.
2H2 + O2 ------> 2H2O
Adding the above two equations,
4H+ +O2 + 4electron ------> 2H2O
Fe2+ formed at the anode is further oxidized to Fe3+ by atmospheric oxygen in presence of moisture.