Mill scale is a common, dark-colored byproduct that forms on the surface of steel during its manufacturing process. It appears as a flaky, bluish-black layer on hot-rolled steel products, such as plates, sheets, and structural shapes. This layer is a mixture of iron oxides created under high-temperature conditions and is an inherent part of steel production. Removing mill scale is important before applying protective coatings or fabricating finished products.
How Mill Scale Forms
Mill scale is the result of high-temperature oxidation occurring when steel is heated and mechanically shaped. This process, known as hot-rolling, exposes the red-hot steel to oxygen and water vapor in the atmosphere. The resulting chemical reaction creates a layered structure of iron oxides on the metal’s surface.
The scale is composed of three distinct layers of iron oxides, which vary in their chemical composition and thickness. The layer closest to the steel substrate is wustite (FeO), which contains the lowest oxygen content and is often the thickest layer, sometimes accounting for up to 95% of the total scale. Above the wustite is the intermediate layer, magnetite (\(\text{Fe}_{3}\text{O}_{4}\)), which is harder and contains a medium amount of oxygen.
The outermost layer is hematite (\(\text{Fe}_{2}\text{O}_{3}\)), which has the highest concentration of oxygen. This hematite layer is usually the thinnest, typically making up only about 1% of the scale. The formation process is continuous as the steel is manipulated, causing the scale to be brittle and only loosely attached to the underlying metal.
Why Mill Scale Must Be Removed
The primary reason for removing mill scale is that its presence interferes with the adhesion of protective coatings and promotes accelerated corrosion. Mill scale is hard, brittle, and non-adherent, meaning it is not securely bonded to the steel surface. If paint or a protective coating is applied directly over the scale, the coating will eventually flake off as the underlying scale loosens and detaches. This failure leaves the steel exposed to the environment, defeating the purpose of the applied protection.
A more serious problem is the creation of a localized corrosion cell when moisture penetrates the scale layer. Mill scale is electrically conductive and acts as a cathode, while the underlying bare steel acts as an anode in the presence of an electrolyte, such as water. This difference in electrochemical potential drives galvanic corrosion, similar to what happens inside a battery.
Because the mill scale is generally intact with only tiny breaks or cracks, the exposed steel acts as a small anode next to a very large cathode (the scale). This unfavorable surface area ratio causes the anodic steel to corrode rapidly in the exposed spots, leading to deep pitting and accelerated rust formation. The mill scale concentrates and accelerates the corrosion process on the small areas of exposed metal rather than protecting the steel uniformly.
Common Removal Techniques
Removing mill scale is necessary for preparing steel for fabrication or applying long-term protective finishes. The selection of a removal technique depends on the size of the steel, the budget, and the required surface finish.
Mechanical cleaning is one of the most widely used methods, particularly abrasive blasting, often called sandblasting. This technique involves propelling abrasive materials like crushed glass, garnet, or specialized ceramic media at high velocity against the steel surface. Abrasive blasting strips away the brittle oxide layer and creates a surface profile that improves the mechanical adhesion of subsequent coatings.
For smaller areas, mechanical methods include using power tools like angle grinders fitted with abrasive discs or wire wheels. These methods offer precise control but can be time-consuming for large components.
Chemical cleaning, frequently referred to as acid pickling, is another effective industrial method. This involves submerging the steel in a bath of dilute acid, most commonly hydrochloric or sulfuric acid, to dissolve the iron oxides. Phosphoric acid is sometimes used because it can leave a thin, protective phosphate coating that may enhance paint adhesion. A constraint of chemical cleaning is the need for careful neutralization after the process to prevent the residual acid from causing immediate corrosion on the bare steel. Thermal cleaning, such as flame cleaning, uses a high-temperature torch to cause the scale to detach due to rapid heating and expansion.