Corrosion is essentially an electrochemical phenomenon. Explain the reactions occurring during corrosion of iron kept in an open atmosphere.

Explanation

According to electrochemical theory of rusting the impure iron surface behaves like small electrochemical cell. In this any point of iron acts as anode and other iron surface acts as cathode. Moisture having dissolved `CO_(2)` or `O_(2)` or acts as an electrolyte. The reactions are given below.
At anode : `Feto Fe^(2+) + 2e^(-)` , `E_(Fe^(2+//Fe))^(@) = -0.44V`
At cathode: `2H^(+) + (1)/(2) o_(2) + 2e^(-) to H_(2)O` , `E_(H^+//O_(2)//H_2O)^(@) = 1.23V`
Overall reaction : `Fe+ 2H^(+) + (1)/(2)O_(2) to Fe^(2+) + H_(2)O`, `E_(cell)^(@) = 1.67`
The `Fe^(2+)0`ions are further oxidized by atmospheric oxygen to `Fe^(3+)` ions, which form hydrated ferric oxide (rust).
`2Fe^(2+) + (1)/(2) O_(2) + 2H_(2)O rightarrow Fe_(2)O_(3)+ 4H^(+)Rightarrow Fe_(2) O_(3) + xH_(2) O to Fe_(2)O_(3) XH_(2)O`