Corrosion of iron is which type of phenomenon?

The correct answer is Option (2) → Electrochemical phenomenon

• In the corrosion (rusting) of iron, the metal surface acts like a tiny galvanic cell. At one spot iron is oxidised, so that spot is the anode. The electrons move through the metal to another spot, the cathode, where oxygen is reduced in the presence of \( \ce{H^+} \) (from carbonic acid formed when carbon dioxide from air dissolves in the water):

      Anode: \( \displaystyle \ce{2Fe(s) -> 2Fe^2+ + 4e^-} \qquad E^{\circ}_{\ce{Fe^2+/Fe}}=-0.44\,\mathrm{V} \)
      Cathode: \( \displaystyle \ce{O2(g) + 4H^+(aq) + 4e^- -> 2H2O(l)} \qquad E^{\circ}=1.23\,\mathrm{V} \)
      Cell reaction: \( \displaystyle \ce{2Fe(s) + O2(g) + 4H^+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \qquad E^{\circ}_{\text{cell}}=1.67\,\mathrm{V} \)

• The ferrous ions are then oxidised by air to ferric ions, which come out as rust, hydrated ferric oxide (\( \ce{Fe2O3.xH2O} \)).

Oxidation and reduction at separate spots, linked by electron flow through the metal, make corrosion an electrochemical phenomenon.

Why the other options are incorrect

Option (1) Electrolytic phenomenon — Incorrect. In electrolysis an external source of electricity drives a non-spontaneous reaction. Corrosion needs no outside current; it is spontaneous (positive E°_cell), like the reaction in a galvanic cell.

Option (3) Photolytic phenomenon — Incorrect. Rusting does not depend on light; it needs only water and air, with oxidation at one spot and reduction at another.

NCERT (2026-27 edition):
2.8 Corrosion – “In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot … of an object made of iron, oxidation takes place and that spot behaves as anode … Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H⁺ (which is believed to be available from H₂CO₃ formed due to dissolution of carbon dioxide from air into water. …). This spot behaves as cathode … The overall reaction being:

    \( \displaystyle \ce{2Fe(s) + O2(g) + 4H^+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \qquad E^{\circ}_{(\text{cell})}=1.67\,\mathrm{V} \)

The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe₂O₃. x H₂O) and with further production of hydrogen ions.”