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

This can be explained from the position of these three elements in the electrochemical series. In comparison to Iron, Tin has lower reactivity than Iron where as Zinc is more reactive than Iron. When the coating of Zinc on the surface of Iron is broken even then Zinc will corrode in preference to Iron metal, whereas when coating of tin on Iron is cracked it exposes both the metals to external oxidizing conditions under this condition, Iron will oxidize in preference to Tin metal.