Why is a zinc coating on iron better than a tin coating?

In addition to the coating, zinc also acts as a "sacrificial anode". I.o.w., it oxidizes faster than iron, and certainly than tin.
As long as there is some zinc, the oxidation of the iron will be slowed down.
zinc does not react as fast to water as either iron or tin. therefore the nail and such can last longer because corrosion is slowed downed considerably.
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