why zinc is used in the galvanisation of iron and not the copper?
More reactive metal is galvanized over less reactive metal.
Zinc is more reactive than iron, it is used to protect it from corrosion because when oxygen in the air reacts with the surface of zinc, a very dense and impermeable coating of zinc oxide is formed and it is this physical barrier that protects the zinc surface from further attack.
Copper is a less reactive metal and it is difficult to form alloys with iron and go on a separate layer. Therefore it is not used for galvanization.