Mill scale is an unwanted by-product of passing steel through a hot rolling mill.
Mill scale is a very thin layer of brownish-black material that forms when people run steel through a hot mill to roll it into shape. Companies that produce sheet metal often use milling because it is cost effective and mill scale is an unwanted by-product of the process. There are several options for removing it and exposing the clean metal undercoat so people can put it through treatments like painting to make it ready for use.
Sheet metal may come with a layer of mill scale.
The mill scale composition includes a mixture of oxidized metals. In fact, it acts as a protector, keeping the material below protected from corrosion. However, once it begins to crack, air and fluid can seep under the mill scale and damage the underlying material. Some mills remove mill scale immediately in chemical baths so the metal is ready for use, while others may allow the metal to expand in an exposed area to encourage scale shedding.
People cannot apply paints and other treatments to mill scale, as it will begin to flake off, taking the treatment material with it. This leaves the steel underneath exposed. While removing scale from mills can be time-consuming, energy- and manpower-intensive, it is necessary before people can continue to use steel in manufacturing processes. Another option is to use cold-milled steel, where rust does not occur, leaving the metal surface clean and ready for use.
When people buy sheet metal for projects, they may find that it has a factory scale coating because the company didn’t treat it or didn’t have a chance to weather it. It’s important to remove this coating before proceeding with the project, either by leaving the metal out so the scale can peel off or by dipping it in a material such as vinegar to loosen the scale. When the scale begins to loosen, people can grind and polish to remove the last flakes of material and clean the metal.
Steel mills use a variety of processes to control the quality and composition of their products. Depending on how the company handles the metals and what type of temperature they use, the final quality of the steel products can vary considerably. Some steels are more elastic and robust, while others are very strong but brittle and do not resist shear or bending forces. Depending on what a company is doing for the metal, the composition of the scale can change as the company uses different materials in its steel production.