Hot-rolled steel - characteristics

During the hot-rolling process a thin layer of oxides (known as millscale) is formed on the surface which when new adheres tightly to the base metal. On weathering this layer undergoes further oxidation and changes from a tightly adhering blue/black material to a yellow/red layer which adheres poorly enabling it to be detached easily. Even if the surface has been coated this change still takes place and both the millscale and the paint coating become detached. It is therefore essential to remove millscale to ensure long-life surface-coating protection.