JEE Main · 2024 · 27 Jan 2024 · Shift 2 · Redox Reactions and Electrochemistry

Tin is less reactive than iron, so a tin coating protects iron only as long as it stays intact and physically blocks contact with moisture and air. Once the coating is scratched or peels off, the exposed iron and the remaining tin form a galvanic couple in which the more reactive iron becomes the anode; iron then corrodes faster at the exposed spot than it would if left uncoated. So tin coating does not continue to prevent rusting once it peels off — this statement is incorrect.

The other statements hold: above pH 9–10 a passivating film forms and rusting is suppressed; dissolved acidic oxides such as $S{O}_2$ and $N{O}_2$ lower the pH of the moisture film and accelerate rusting, acting catalytically; and rusting proceeds through the setting up of tiny electrochemical (anode–cathode) cells on the metal surface, with $Fe\to F{e}^{2+}+2e^{-}$ at the anode and $O_2+4H^{+}+4e^{-}\to 2H_{2}O$ at the cathode.