What is mill scale?

Mill scale is a form of iron oxide that develops on the surface of steel during the manufacturing process, particularly in hot rolling. When steel is heated, oxidizing conditions can occur, and as the metal cools, the surface can lose iron atoms, leading to the formation of a thin layer of oxide. This layer is often dark and flaky and can be problematic during surface preparation for coatings, as it can hinder adhesion if not properly removed. Understanding mill scale is crucial for coatings inspectors because it affects the performance of protective coatings applied to steel surfaces.

The other options describe different concepts related to coatings and corrosion but do not accurately define mill scale. A protective coating is applied to prevent corrosion, corrosion refers to the gradual destruction of materials, and paint is a liquid applied to surfaces for aesthetic or protective purposes but is not itself mill scale.