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Mill scale is the flaky surface of hot rolled steel, iron oxides consisting of iron (2, 3) oxide, hematite
and magnetite. Mill scale is formed on the outer surfaces of plates, sheets or profiles when they are
being produced by rolling red hot iron or steel billets in rolling or steel mills. Mill scale is composed of
iron oxides mostly ferric and is bluish black in color.
It is usually less than a millimeter thick and initially adheres to the steel surface and protects it from
atmospheric corrosion provided no break occurs in this coating. Because it is electro-chemically cathodic to steel, any break in the mill scale coating will cause accelerated corrosion of
steel exposed at the break. Mill scale is thus a boon for a while until its coat breaks due to handling
of the steel product or due to any other mechanical cause.
It is a nuisance when the steel is to be processed. Any paint applied over it is wasted will come off
with the scale as moisture laden air get under it. Thus mill scale has to be removed from steel
surfaces by flame heating, acid pickling or grit/sand blasting. All tedious operations wasteful of
energy. This why shipbuilders used to leave steel delivered freshly rolled from mills out in the open
to allow it to weather till most of the scale fell off due to atmospheric action. Now a days most steels
mills can supply their produce with mill scale removed and steel coated with shop primers over which
welding can be done safely.
Iron mill scale