Rusting of Iron is catalyzed by:

The correct answer is Option (1) → \(H^+\)

Rusting is an electrochemical process. A tiny region of the iron surface acts as an anode and another as a cathode, with the water film on the metal as the electrolyte:

    \( \displaystyle \text{Anode: } \ce{2Fe(s) -> 2Fe^2+ + 4e-} \)

    \( \displaystyle \text{Cathode: } \ce{O2(g) + 4H+(aq) + 4e- -> 2H2O(l)} \)

    \( \displaystyle \text{Net: } \ce{2Fe(s) + O2(g) + 4H+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \)

The \( \ce{Fe^{2+}} \) ions are then oxidised further by air to hydrated iron(III) oxide, \( \ce{Fe2O3.xH2O} \), which is rust. The cathode reaction uses up hydrogen ions, so the more \( \ce{H^{+}} \) present, the faster the iron rusts; this is why iron rusts faster in moist, polluted or salty air. So rusting is promoted (catalysed) by H⁺ ions.

Why the other options are incorrect

• Option (2) \(O_2\) — Incorrect. Oxygen is a reactant that is used up at the cathode; a catalyst is not consumed.

• Option (3) \(Fe\) — Incorrect. Iron is the metal that is oxidised (corroded); it cannot be the catalyst of its own corrosion.

• Option (4) \({CO_3}^{2-}\) — Incorrect. Dissolved carbon dioxide helps by forming carbonic acid, which supplies \( \ce{H^{+}} \); the carbonate ion itself is a base and does not supply hydrogen ions.

NCERT (2026-27 edition):
Corrosion – …