Mill scale is a layer of iron oxides (FeO, Fe₃O₄, Fe₂O₃) that forms on the surface of steel during hot rolling when the hot metal is exposed to air. It is bluish-black, brittle, and can spall off. For most painting or galvanizing applications, mill scale must be removed because it is poorly adherent and can compromise coating life. Common removal methods include:
Abrasive Blasting: The most effective industrial method (shot or grit blasting).
Pickling: Immersion in an acid bath (hydrochloric or sulfuric).
Power Tool Cleaning: For smaller-scale or on-site work.