Mill scale - WikipediaMill 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 (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.mill scale in steel,What is Mill Scale? - Definition from CorrosionpediaMill scale is a flaky surface that can be found on iron oxides and metals that have been hot rolled. Mill scale forms on exterior surfaces of sheets and plates as they are being manufactured through rolling steel billets and hot iron into rolling mills. Mill scale typically consists of iron oxides that are bluish black. The thickness is.