What type of metal typically has black mill scale on its surface?

Hot rolled steel typically has black mill scale on its surface as a result of the manufacturing process. During hot rolling, steel is heated above its recrystallization temperature and then worked into shape. This high-temperature process causes oxidation to occur, leading to the formation of a layer of iron oxide, known as mill scale, which is blackish in color.

This characteristic is an important consideration in various applications. For instance, while the mill scale can provide some level of corrosion protection, it may also need to be removed prior to certain finishes or coatings to ensure better adhesion and the longevity of protective finishes.

In contrast, cold rolled steel does not develop a mill scale, as it undergoes processing at room temperature. Stainless steel is known for its resistance to corrosion and typically has a smooth, shiny surface without the presence of mill scale. Copper also does not have mill scale; instead, it tarnishes and develops patinas due to oxidation.