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Waste and Recycling

What is mill scale?


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July 09, 2009 11:22AM

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 millimetre 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 coating 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 since it 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 is 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.