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Question:

Tin/Zinc coat to prevent Rusting of Iron ?

why does tin can when scratched rust much quicker than an iron can coated with zinc (galvanised) when it becomes scratched ?

Answer:

I am not sure that it does. Once exposed (in my opinion) iron will rust at the same rate given all else being equal like moisture in the air. I suspect that the scratch on the tin coated instance has less thickness of tin through which to penetrate compared to the thickness of a coating of zinc which despite being scratched has not yet penetrated all the way in to the iron.
The coating simply prevents direct contact between the iron and any corrosive agent inside. Tin is a more noble metal than iron, and when combined as a set of electrodes, the iron will act like a sacrificial electrode to all oxydizing agents attacking the tin surface, provided the iron has somewhere to spill its cations formed in the process. If there is a rupture in the coating, rust can grow out of the iron there. Rust is way more voluminous than metallic iron and will not form a sheathing layer but crumble away, providing a medium for the iron ions to go into. That's why a damaged tin will rust faster than uncoated iron.
Iron is more reactive than tin so it will protect the iron from rusting better since iron can react better with oxygen in the air.

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