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.