Question

why does corrosion of aluminium is not proved to be wasteful?

Answer 1

It is because of aluminium oxide layer. Aluminium outer surface layer is oxidized whenever it came in contact with the air and so result in aluminium oxide layer which is resistive in nature and remain attached to it. So due to this it act as protective layer to inner aluminium to get oxidized with air while in iron as the surface layer of iron is oxidized, it form rust which is not remain attached to iron and fall off exposing the inner iron for further oxidation.

Answer 2

al corrosion is not wasteful becoz after corrosion it forms a layer of al oxide,i.e.,Al2O3, which prevents it from further corrosion