Mill scale - Wikipedia
Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, consisting of the mixed iron oxides iron(II) oxide (FeO), iron(III) oxide (Fe 2 O 3), and iron(II,III) oxide (Fe 3 O 4, 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.