What is a mill scale?

Mill scale is an unwanted by-product of passing steel through a hot mill.

Mill scale is a very thin layer of brownish-black material that forms when people run steel through a hot mill to roll and shape it. 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 to expose the bottom layer of clean metal so people can subject it to treatments like painting to get it ready for use.

Sheet metal may come with a layer of mill scale.

The mill scale composition includes a mixture of oxidized metals. It actually acts as a protector, keeping the material underneath protected from corrosion. Once it starts to crack, 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 the metal is ready to use, while others may allow the metal to expand in an exposed area to encourage scale fall.

People cannot apply paints and other treatments to the mill scale, as it will start to flake off, taking the treatment material with it. This leaves the steel underneath exposed. While removing scale from mills can be time-consuming and require energy and personnel, it is necessary before people can continue to use steel in manufacturing processes. Another option is to use cold milled steel, where oxidation does not occur, leaving the metal surface clean and ready for use.

When people buy sheet metal for projects, they may find it has a layer of factory scale because the company didn’t treat it or didn’t have a chance to weather it. It’s important to remove this layer before proceeding with the project, leaving the metal out so the scale can peel off, or bathing it in a material like vinegar to loosen the scale. When the scale starts to loosen, people can polish and polish to remove the last flakes of material and clean the metal.

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Steel mills use a variety of processes to control the quality and composition of their products. Depending on how the company handles metals and what type of temperature it uses, the final quality of steel products can vary considerably. Some steels are more elastic and robust, while others are very strong but brittle and do not withstand shearing or bending forces. Depending on what a company is doing for the metal, the scale composition can change as the company uses different materials in its steel production.

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