What is a Mill Scale?

Mary McMahon

Mill scale is a very thin layer of brownish to black material that forms when people pass steel through a hot mill to roll and shape it. Companies producing sheet metal usually utilize milling because it is cost effective, and the mill scale is an unwanted byproduct of the process. There are several options for removing it to expose the clean underlayer of metal so people can subject it to treatments like painting to prepare it for use.

Mill scale is an unwanted byproduct of passing steel through a hot mill.
Mill scale is an unwanted byproduct of passing steel through a hot mill.

The composition of the mill scale includes a mixture of oxidized metals. It actually acts as a protectant, keeping the material underneath safe from corrosion. As soon as it starts cracking, however, air and fluid can seep under the mill scale and damage the underlying material. Some factories remove mill scale immediately in chemical baths so their metal will be ready for use, while others may allow metal to weather in an exposed area to encourage the scale to fall off.

Sheet metal may come with a layer of mill scale.
Sheet metal may come with a layer of mill scale.

People cannot apply paints and other treatments to the mill scale, as it will start to flake, taking the treatment material with it. This leaves the steel underneath exposed. While removing mill scale can be time consuming and requires energy and personnel, it is necessary before people can proceed with using steel in manufacturing processes. Another option is to use cold milled steel, where no oxidation takes place, leaving the surface of the metal clean and ready to use.

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When people purchase sheet metal for projects, they may find it has a layer of mill scale because the company did not treat it or it did not have chance to weather. It is important to remove this layer before moving on with a project, either by leaving the metal out so the scale can flake off or bathing it in a material like vinegar to loosen the scale. Once the scale starts to come loose, people can buff 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 product. Depending on how the company handles metals and what kinds of temperatures it uses, the end quality of steel products can vary considerably. Some steels are more springy and robust, while others are very strong but brittle, and cannot withstand shearing or bending forces. Depending on what a company is making metal for, the makeup of the scale can shift as the company uses differing materials in its steel production.

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