Definition - What does Mill Scale mean?
Mill scale is a flaky surface that can be found on iron oxides and metals that have been hot rolled. Mill scale forms on exterior surfaces of sheets and plates as they are being manufactured through rolling steel billets and hot iron into rolling mills.
Mill scale typically consists of iron oxides that are bluish black. The thickness is about 1mm and bonds to steel surfaces, protecting them against atmospheric corrosion given that there are no breaks in the coating.
Corrosionpedia explains Mill Scale
Mill scale is a severe nuisance in steel processing. Any coating applied on top of the scale goes to waste. This happens because the steel can be laden with moisture when air gets into it. Electrochemically, mill scale is cathodic compared to steel, so any kind of breakage in the scale could lead to accelerated corrosion in the steel at the point of breakage.
Thus, the scale is considered an advantage for a short time until the coating cracks due to mechanical factors such as the handling of steel as well as other processes such as abrasive blasting, pickling and flame cleaning.
Therefore, those who are involved in the industry of ship building prefer to have the steel delivered fresh out of the mills to allow ample time for weathering or until almost all of the scale is removed by atmospheric action.
Almost all modern steel mills are capable of supplying products with the mill scale eradicated and the steel painted with primers that are suitable for welding.