The integrity of structural steel depends in part on how effectively it can be protected from rust. The standard way to achieve this is to cover the steel in a protective coating. However, it is crucial that the coating is applied on steel that is clean, otherwise corrosion may occur below the surface of the coating – creating bubbles over weakened parts of the metal.
As steel comes freshly rolled from the mill, it is already covered in mill scale, a thin layer of iron oxide. As soon as the mill scale layer is broken by handling, impact or friction, it allows corrosion of the base metal. To make matters worse, any paint applied over mill scale will be ineffective, as the corrosion will take place below the paint surface, damaging the metal itself. So it is imperative that mill scale is removed from structural steel before it is coated – for instance, by abrasive blasting.
The surface to be covered should be free of not only mill scale, but any dirt, grease or oil, and it should be perfectly dry. The aim is to remove anything that may hamper the adhesion of the coating to the metal and to create some roughness on the surface to improve adhesion. A protective coating works only as long as it sticks to the metal surface and its own surface remains unbroken.
Four grades of cleanliness are generally specified:
- White metal blast-cleaned – No visible mill scale, rust or dirt.
- Near white metal blast-cleaned – A small, set percentage of visible impurities allowed.
- Commercial blast-cleaned – Nearly all mill scale, rust and dirt removed, but some variation in appearance allowed. This is usually good enough for most uses of structural steel.
- Brush-off blast-cleaned – No loose mill scale, rust or dirt. This is still better than what usually achieved with power tools or hand sanding.
Other than simply looking at the metal surface, the easiest way to test how clean it is is to wipe it with a clean, white cloth, pressing down firmly. Then check the material for residue, whether rust, mill scale or dirt.
Once the surface is cleaned, it is recommended that you get it covered in a protective coating within four hours or less, otherwise corrosion may start again.