Question

mention a type of corrosion which is helpful to the metal

Answer 1

Like corrosion at copper , silver ,gold
Does not harm metal it become like covering layer
It can be remove easily without harming metal

Answer 2

Corrosion is actually the eating of metal by natural as well as by artificial means.

A type of corrosion which is helpful to the metal happens when the corrosion is over surface which actually prevents the contact of metal with environment.