Tin-plated iron corrodes faster than unplated iron once the coating is scratched, whereas zinc-plated iron does not. The reason is

1. Atin forms a porous oxide and zinc a non-porous one
2. B$E^{\circ }(\mathrm{Sn}{}^{2+}/\mathrm{Sn})$ is less negative than $E^{\circ }(\mathrm{Fe}{}^{2+}/\mathrm{Fe})$, so exposed iron becomes the anode; zinc is more negative and corrodes in place of iron
3. Czinc is a better conductor than tin
4. Dtin catalyses the reduction of oxygen