Mill scale is iron oxide that forms on a surface during hot rolling of steel. During rolling, the high temperature involved causes oxide to form over the entire surface, which only increases in quantity as the rolling process progresses. The spots are often hard, but at the same time very brittle. The combination of the high temperature and the large amount of pressure exerted during the rolling process ultimately leads to a blue-grey surface, which we call the rolling skin.
We now know what the mill scale is and how it is formed, but does it actually do any harm or does it need to be removed? In theory, the mill scale could be a uniform, well adhering layer on the steel and the perfect protective layer. Unfortunately, the opposite is the case.
In practice, mill scale has no connection with the underlying surface. The oxides are of course pressed into the lower layer with great force, but are not actually attached to it. Over time, for example during processing of the material, the mill scale will therefore slowly but surely loosen, causing it to lose its protective function. A tricky point, since any coatings applied directly to the mill scale will also be removed from the object in this way. This is why it is important to remove the mill scale before processing and coating the product, as sooner or later it will fail.
In the past, in the construction and maritime industries, the choice was often made to let the mill scale that had formed on the rolled material rust first. By storing the objects outside, the hard rolling skin was affected by the weather and turned into a layer of rust. This method of removing the mill scale was very time-consuming and also took up a lot of space. And that was not the only disadvantage. The rusting of the mill scale also caused severe damage to the object and created many irregularities. Therefore, through the years, several other methods have been adopted to remove the mill scale from metal pipes or sheets.
One of the most common ways to remove the mill scale from steel is chemical removal. It is a thorough method that also works very quickly and efficiently. With chemical cleaning, the mill scale layer is dissolved in acid, with acids like n hydrochloric acid, phosphoric acid or oxalic acid. A major disadvantage of dry cleaning is that it is difficult to estimate when the chemical reaction of the acid stops. If the reaction continues for too long, the object will be damaged by the aggressive effect of the acid and will immediately start to rust again. This chemical reaction can only be stopped by adding other substances. So, chemical cleaning can be a fast and efficient method for removing mill scale, but it also has its disadvantages. Further disadvantages are:
Hydrochloric acid is extremely aggressive and can cause nasty wounds if it comes into contact with people. In addition, when working with hydrochloric acid, a lot of vapor can be produced, which is just as dangerous. This vapor spreads through the air and can harm the environment in many ways. When this vapor ends up in the water, it is also lethal to fish and other aquatic animals.
Fortunately, the chemical method is not the only option for removing the mill scale from steel. The mill scale can also be removed by blasting or grinding. Although this does not have the harmful effect that chemical cleaning does, these methods for removing the mill scale also have their disadvantages. For example, not all objects are suitable for blasting or grinding. In addition, blasting or sanding can cause spattering, which then adheres to the piece that has just been treated. Therefore, it is not easy to remove the mill scale and leave a flawless surface. All in all, the removal of mill scale therefore requires a lot of work and preparation, which is why it is always recommended to have this done by a professional.
Another method is to apply abrasive blasting. This method has the same end effect as grit blasting, but uses mechanical energy by means of a rotating brush.
The abrasive blasting technology (think for instance about the brand Bristle Blaster®) is equipped with a rotating brush with specially designed, patented curved tips. As these curved brush tips are released with great force from behind the accelerator bar of the power tool, the tips immediately retract from the surface to be cleaned. This not only neatly removes the mill scale, but also creates a new adhesive layer through the imprint of the tips. The technology of the power tools makes the abrasive blasting technology not only a tool for the removal of mill scale, but also, for example, the alternative to sandblasting.