Mill scale is a flaky, bluish-black coating that forms on the surface of hot-rolled steel as it cools after being processed at high temperatures. It’s basically a layer of iron oxides (magnetite, hematite, and wüstite) created when the hot steel reacts with oxygen in the air. Key Points: • Where it comes from: During hot rolling (when steel is shaped at very high temps), the outer layer reacts with oxygen and forms this oxide crust. • Appearance: Dark, rough, and brittle flakes that can sometime