Mill scale is the flaky residue that forms on the surface of hot rolled steel, comprising iron oxides such as iron(II,III) oxide, hematite, and magnetite.

During the production process of rolling red-hot iron or steel billets into plates, sheets, or profiles in rolling mills, mill scale develops on the outer surfaces. This scale is primarily made up of iron oxides, predominantly ferric, and has a bluish-black hue. Typically less than 1 mm thick, it initially adheres to the steel surface, acting as a protective coating against atmospheric corrosion as long as no breaks occur in this layer.

The mill scale produced in rolling mills is gathered and sent to a sinter plant for recycling.

In 2012, UNIMINES traded more than 250,000 metric tons of mill scale materials sourced from the  Brazil.