According to Swedish Standard 055900-1967:
A: Steel surface covered completely with adhering mill scale and with little or nor rust.
B: Steel surface which has started to rust and from which the mill scale has begun to flake.
C: Steel surface where the mill scale has rusted away or from which it can be scraped with little pitting visible to the naked eye.
D: Steel surface where the mill scale has rusted away and where pitting is visible to the naked eye.
Grade A is normally the condition of the steel surface a short time after rolling. Grades B, C and D are normally the state of the surface after it has been exposed outdoors, without protection against rusting in a fairly corrosive atmosphere for two or three months, then a year or so, and about three years, respectively.