Question

how corossion is useful as well as harmful

Answer 1

Corrosion of metals is harmful as it destroys the metals but in the case of aluminium, it is useful as when aluminium gets corroded, the corroded layer acts as a protective layer which protects aluminium.

Answer 2

corrosion is harmful as it destroys the upper surface of metal ..making it non-usable..i am doubtful that it is good ..but oxidation of metal surface benefits few metals ..preventjng from futher oxidation(eg-aluminium , process is called anodising)