Iron does not rust even if zinc coating on its surface is broken but the same is not true when coating is of tin.

Explanation

The `E^(@)` values of these metals are listed as follows :
`Zn^(2+)(aq)+2e^(-) to Zn(s) , " " E^(@)=-0.76" V"`
`Fe^(2+)(aq)+2e^(-) to 2Fe(s) , " "E^(@)=-0.44" V"`
`Sn^(2+)(aq)+2e^(-) to Sn(s) , " "E^(@)=-0.14" V"`
The data shows that zinc is more reactive than iron. This means that if a crack appears on the surface of iron coated with zinc even then zinc will take part in the redox reaction and not iron.In other words, zinc will be corroded in preference to iron. But same is not the case with tin. It is less reactive than iron. If a crack appears on the surface of iron coated with tin, then iron will take part in the redox reaction and not tin. Therefore, iron will be corroded under these circumstances.