125.

What is corrosion? Describe the electrochemical phenomenon of rusting of iron.

Corrosion is the process of slowly eating away of the metal due to attack of the atmospheric gases on the surface of the metal resulting into the formation of compounds such as oxides, sulphides, carbonates, etc.

The corrosion of iron is called rusting.
According to theory of rusting, impure iron surface behaves as a small electrochemical cell in the presence of water containing dissolved oxygen or CO₂.
The pure iron acts as anode and impure surface as cathode.
At Anode : Iron atom undergo oxidation spontaneously forming Fe²⁺ ion.
Fe → Fe²⁺ (aq) + 2e^- E°_cell = – 0.44 V
Fe²⁺ ions move into solution and electrons into cathodic area where they are picked up by H⁺ ions of the solution.
At cathode:
             $2{\mathrm{H}}^{+}+\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{e}}^{-} \to {\mathrm{H}}_2\mathrm{O} \mathrm{E}{°}_{\mathrm{red}} = 1.23 \mathrm{V}$
H⁺ ions are produced by secondary reaction either from H₂O or from H₂CO₃ (CO₂ + H₂O)
H₂O ---->H⁺ + OH^–

H₂CO₃------> H+ + HCO₃^–

The overall reaction of the corrosion cell may be represented as:
$$\begin{gathered} \mathrm{Fe}\left(\mathrm{s}\right)+2{\mathrm{H}}^{+}\left(\mathrm{aq}\right)+\frac{1}{2}{\mathrm{O}}_2\to {\mathrm{Fe}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O} \\ {{\mathrm{E}}^0}_{\mathrm{cell}} = 1.67 \mathrm{V} \end{gathered}$$

The Fe²⁺ ions move through water and come at the surface where these are further oxidized into Fe³⁺ ions by atmospheric oxygen to form hydrate ferric oxide known as rust, Fe₂O₃.xH₂O.
$$\begin{gathered} 2{\mathrm{Fe}}^{2+}+\frac{1}{2}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O} \to {\mathrm{Fe}}_2{\mathrm{O}}_3+4{\mathrm{H}}^{+} \\ {\mathrm{Fe}}_2{\mathrm{O}}_3+{\mathrm{xH}}_2\mathrm{O} \to {\mathrm{Fe}}_2{\mathrm{O}}_3.{\mathrm{xH}}_2\mathrm{O} \\ \mathrm{Rust} \end{gathered}$$