Mill scale is the flaky surface of hot worked steel and is one of the wastes generated in steel plants, representing around 2 % of the produced steel. Mill Scale is found on all hot-rolled steel products unless processed in a protective inert atmosphere. It is formed by the oxidation of the steel surface during reheating, conditioning, hot rolling, and hot forming operations. Mill scale consists primarily of magnetite, Fe3O4, of characteristic blue-gray "steely" color. An extremely thin outer film of hematite, Fe2O3, is invisible to the naked eye. The inner portion of the magnetite contains fine metal grains and residual ferrous oxide as wüstite, FeO, typically black colored.
