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What is Mill Scale and How to Remove It

Apr 21st 2022

If you’re a welder or metalworker, mill scale is something to definitely look out for. All types of steel can develop residual mill scale from production. Its presence can contribute to corrosion and can ruin a weld, so it needs to be removed before working with the steel.

In this article, we will discuss what mill scale is and several options for mill scale removal.

What is Mill Scale?

Mill scale (also called scale) is the flaky surface that forms on hot-rolled steel. Its composition includes FeO (iron(II) oxide), Fe2O3 (iron(III) oxide), and Fe3O4 (iron(II,III) oxide).

You will find mill scaling on steel sheets, plates, and profiles. It arises when you roll hot steel billets or iron in rolling mills. When you heat, condition, and roll steel, it will encounter oxygen which oxidizes on the surface, leaving this flaky substance…mill scale.

It has a bluish-black appearance, and it is about 0.004 inches thick. The consistency can be powdery or flaky. Additionally, mill scaling has an uneven thickness.

Initially, mill scaling has a protective function. It adheres to the surface and protects the steel from atmospheric corrosion. However, this protection is temporary. Due to its non-uniform texture and thickness, mill scale chips readily.

Mixed iron oxides serve as cathodes to steel since they are less reactive. Any cracks in the coating will accelerate steel corrosion on the exposed surface.

Rapid oxidation occurs because of the influx of oxygen ions from moisture, the air, and deteriorating iron oxides. Excess oxygen contributes to high porosity levels that concentrate stress on the remaining metal.

You cannot prevent mill scale unless you hot roll steel in an inert environment.

If you wish to process the steel, the mill scale will get in your way. Any paint you apply will come off as the scale peels off. Furthermore, it becomes a nuisance during welding.

Mill Scale and Welding

Mill scaling has a detrimental effect on welding processes. While some welding techniques work, the scale can negatively impact steel properties.

Puddle Fluidity and Arc Stability

You need to perform mill scale removal before welding. The scale behaves as a thermal and electrical insulator. This behavior restricts puddle fluidity and creates arc instability.

Steel is a natural thermal and electrical conductor. Since the mill scale serves as an insulator, it will decrease the size of the puddle and arc.

The puddle will become less fluid, so your bead will have undercuts and become convex.

The current and voltage flowing through scale-ridden metal and ground will decrease and become unstable. This facet generates arc instability.

It will prevent you from starting the arc and lead to superficial penetration, spatter, and sparks.

Impurities

The scale has impurities like oxygen that it can release into the steel. As the mill scale degenerates, it releases oxygen ions into the weld metal. The oxygen will increase the porosity, which weakens the joint. The stress becomes concentrated on the non-porous parts of the weld metal.

Other impurities in the mill scale can create non-metallic inclusions in the weld metal. While the effects will depend on the resulting alloy, these impurities will impact the material properties of the steel.

Altered properties may change the capabilities of the steel and make it unsuitable for the desired application.

Lack of Fusion

Also, it can inhibit fusion so that the metal component will fall apart without external stresses. You will get uneven weld metal deposition at the toes, restricting fusion. If you weld out of position, the reduced fluidity will prevent fusion.

Since mill scale has a higher melting temperature than steel, you may not input sufficient heat into the metals. As a result, the steel component will not fuse.

Surface Defects

When the mill scale chips, it will lead to surface defects like undercuts and porosity. These defects will cause oxygen from the air and surrounding moisture to concentrate on the surface.

Since steel is highly reactive to oxygen, it will corrode quickly due to the protective barrier’s destruction.

How to Remove Mill Scale

Scale is detrimental to your metal component. If you want to learn how to remove mill scale, you’ve come to the right place.

Fresh mill scaling is well-adhered to the surface, so you will have trouble removing it. Most mill scale removal techniques will move it around on the surface.

Angle Grinder

You can use an angle grinder to remove the mill scale. Make sure you avoid using sanding discs or wheels that feature ceramics, zirconia oxide, or aluminum oxide. These will get clogged quickly. Instead, opt for one made from silicon carbide which won’t clog as easily.

You can use a silicon carbide disc made for metal, stone, or concrete. Keep in mind that you should only use the ones designed for stone or concrete on the mill scale and not the metal.

To further prevent clogging, remove oil from the workpiece before using stripper discs.

If you do not have heavy scaling, you could opt for nylon brushes and/or wheels or stripping discs. These abrasive products will not damage the surface underneath the scale.

However, nylon will not perform as well as a wire wheel. Wire wheels can remove thick layers of mill scale. Keep in mind that the wire will damage the surface under the scale, so you should only use it in the worst cases.

Shot Blasting

Metal manufacturers know that shot blasting can strengthen, polish, and clean metal. If you already perform shot blasting to treat the surface of your steel, you can use it to remove mill scale as well.

Shot blasting will remove any impurities from the surface, which will keep it clean enough to apply a protective coating.

While cost-effective and quick, shot blasting will require equipment like a tumble blast, monorail, conveyor, and swing table.

If you do not plan on treating or painting the surface, it might not be worth the trouble. Shot blasting is more time-consuming than using an angle grinder to remove the mill scale.

Flame Cleaning

With flame cleaning, you pass a hot oxyacetylene flame over the steel surface. It removes rust and mill scale, making the steel suitable for processing.

Flame cleaning can remove paint, burrs, bad welds, defects, and surface imperfections. This mill scale removal option can get costly, so you should only use it if you have more than just mill scaling to correct.

Also, avoid flame cleaning a thin workpiece. This process can warp the metal beyond repair.

Acids

Chemical removal of mill scale is quick and effective. You dissolve the iron oxides in oxalic, phosphoric, or hydrochloric acid. The acid will produce a chemical reaction on the steel’s surface, cleaning it thoroughly.

However, it is challenging to tell when the chemical reaction will stop. A prolonged reaction could damage the steel itself, potentially leading to more rust and weakened mechanical properties.

You need to stop the chemical reaction by adding another substance. Otherwise, you will damage your part. You may need to repeat the process if you do not let the reaction continue for sufficient time.

Moreover, chemical removal can pose concerns for the environment, people, and wildlife. It can burn your skin and release toxic fumes. If the acid contacts water, it can kill aquatic animals.

Another safer option, if you're willing to wait, is a vinegar bath. Soaking the steel in distilled white vinegar for up to 24 hours often makes mill scale removal a fairly easy process. This however, is best done when you have small amounts of steel that can fit into a container with the vinegar.

Welding Options without Removing Mill Scale

Gas-Shield Welding

Ideally, you would remove the mill scale before welding. However, removal processes add to the cost. If you are on a budget, you can try welding through the mill scale.

Gas-shielded welding is an excellent option if you work in a manufacturing environment. Solid wires often come with deoxidizers that destroy the scale while welding. You can process at high speeds and retain the quality.

However, solid wires can lead to arc instability and produce spatter.

A flux-cored wire can weld through thick scaling due to the slag system and high quantity of deoxidizers. Metal-cored wires can improve productivity and cut through moderate to thick scaling from the deoxidizers.

Keep in mind that flux- and metal-cored wires cost more than solid ones. Nevertheless, they improve productivity, increase travel speeds, reduce spatter, and can bridge gaps.

Submerged Arc Welding

Submerged arc welding (SAW) is a wire-fed process used on large parts. The wire goes through a torch that moves along the weld joint. This process relies on granular flux.

The arc gets submerged in the granular flux, so it is invisible during operation. This arc will melt the flux, wire, and base material to create a weld metal pool. After melting, the molten flux will deoxidize the surface and create a protective atmosphere for the metal.

SAW fluxes can be active or neutral. Active ones readily weld through mill scale while neutral ones are less effective. Nevertheless, neutral fluxes have better mechanical properties.

Self-Shielded Welding

If you weld outdoors, you can use self-shielded shielded metal arc welding (SMAW) or flux-cored welding (FCAW-S).

SMAW can weld through mill scale because its electrodes have a slag system and deoxidizers that destroy the scale. You can get special stick electrodes to deal with thick scaling, but most electrodes will work on thin to moderate scaling.

FCAW-S wires are less productive than gas-shield ones, but they are useful if you cannot haul gas cylinders to your site. These have a slag system and deoxidizers that can cut through scale.