Mill scale is often misunderstood and frequently ignored in the coatings industry. I have seen numerous coatings failures that we caused by coating over mill scale. While most professionals in the coatings industry are very familiar with the importance of removing mill scale, some in our trade are not. I am hopeful that a good understanding of all aspects of mill scale will encourage its removal prior to applying protective coatings.
First, what is mill scale? Mill scale is the thin layer of iron oxides that form on hot rolled steel. There are three distinct chemical compositions in mill scale, including FeO, Fe3O4, and Fe2O3. Mill scale can vary in thickness from 2 to 100 mils, depending on the quality of the steel and the rolling process. Mill scale is approximately 70% iron. At the steel mill, mill scale is collected and used for recycling. At blasting and coating shops, it is just a nuisance that must be removed. Unless the mill scale has been removed at the steel mill, the steel will be blue gray in color (see photo below). This blue gray layer is the mill scale, which is flaky and brittle. Upon atmospheric exposure, the mill scale will soon break, allowing moisture to reach the steel. Because mill scale has a different chemical composition and is cathodic in relation to the steel that it covers, corrosion will occur at the anode (the steel) when moisture (the electrolyte) reaches the steel. This is an important concept because the presence of mill scale will actually accelerate the corrosion process of the steel in the presence of moisture.
Many steel fabrication shops routinely apply primers over mill scale without removing it. Coating over mill scale is often done on steel beams and columns that will be used in dry, low corrosivity interior applications, often enclosed in walls or ceilings. For these applications, the expense of removing the mill scale may not be justified by an increase in performance that is not deemed to have significant value. For other applications, removal of the mill scale is absolutely necessary for long term performance of the protective coatings system. These applications include areas which will be subjected to mechanical impact or flexing , visible areas of interior locations, corrosive areas, and all exterior applications. The most common method of removing mill scale is by abrasive blasting. A mixture of steel shot and grit is often used effectively with automated blast equipment in manufacturing plants. The steel shot cracks and loosens the mill scale, while the steel grit removes it and provides an acceptable surface profile to the underlying steel. For manual blast operations, coal slag is an effective non-recyclable blast media. Removing mill scale by hand tool cleaning is nearly impossible, and the use of power tools such as a needle gun is a very slow process. With any removal method, it is wise to let the steel weather outdoors to accelerate the cracking and loosening of the mill scale if time permits. A few months in the rain will make the removal much faster and less costly.
I often see complete disbondment of mill scale, even indoors, when the importance of mill scale removal prior to coating operations is ignored. Examination of the coatings removed from these areas will reveal the mill scale completely covering the back of the chips. A great protective coatings system can quickly become a big waste of money when the mill scale disbands. Fortunately, these failures are completely avoidable. Removal of mill scale is a wise investment if you want to realize the benefits of a protective coatings system. If you don’t plan to remove the mill scale, be prepared for an early coatings failure.