Question

why does corrosion of aluminium is not proved to be wasteful

Answer 1

The outer layer of aluminium is oxidized whenever it comes in contact with air resulting in an oxide layer all over.

The oxide layer is resistive in nature and remains attached to it.

As a result, it protects the inner layer from getting further oxidised.

Answer 2

corrosion forms a layer of oxides over iron

this is why iron bars lose the lustre and also get rusted

actually the first layer of oxides over the iron bars protect it from further oxidation