Mill scale

Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, consisting of the iron oxides iron(II) oxide (FeO), iron(III) oxide (Fe₂O₃), and iron(II,III) oxide (Fe₃O₄, 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 mills. Mill scale is composed of iron oxides mostly ferric and is bluish black in color. It is usually less than 0.1 mm (0.0039 in) thick and initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this coating.