There are various rust grades defined in by Swedish Corrosion Institute. They are as follows :
Rust Grade A : Steel surface covered completely with adherent mill scale and with little if any rust.
Rust Grade B : Steel surface which has begun to rust and from which the mill scale has begun to flake.
Rust Grade C : Steel surface on which the mill scale has rusted away or from which it can be scraped, but with little pitting visible to the naked eye.
Rust Grade D : Steel surface on which the mill scale has rusted away and on which considerable pitting is visible to the naked eye.