Zinc can be coated on iron to produce galvanised iron but the reverse is not possible. It is because

Explanation of the Concept

1. Electrochemical Series: The difference in behavior between zinc and iron when it comes to galvanization can be understood through the electrochemical series. In this series, metals are arranged according to their standard electrode potentials. Zinc is more reactive than iron, meaning it has a higher tendency to lose electrons and thus corrode.

2. Galvanized Iron Process: When zinc is coated on iron, it serves as a protective layer. Zinc corrodes preferentially to iron, which means that if the coating is damaged, the zinc will corrode away before the iron, thereby preventing rusting and extending the life of the iron.

3. Why Reverse Coating is Not Possible: Coating iron with zinc would not provide the same protective effect if iron were placed as a layer over zinc. When exposed to the environment, the less reactive iron would allow the more reactive zinc underneath to corrode first. Essentially, iron would not protect zinc; instead, it could lead to accelerated corrosion of the zinc.

Summary

In conclusion, the primary reason why zinc can coat iron to produce galvanized iron, but iron cannot coat zinc, lies in the relative reactivity of the two metals as described by the electrochemical series. Zinc, being more reactive, serves as a sacrificial anode, while the reverse scenario would lead to detrimental effects for the zinc layer. This illustrates the importance of the properties of metals in protective coatings.