Mill scale

Mill scale is a thin layer of iron oxide that forms on the surface of steel or iron during the manufacturing and processing of metal, particularly during hot rolling processes. It is typically composed of various iron oxides, primarily FeO (wüstite), Fe₂O₃ (hematite), and Fe₃O₄ (magnetite). Mill scale is usually removed before welding or further processing because it can interfere with the quality of the weld and the adherence of coatings.