What Is Mill Scale in Welding: And How to Weld Through It

ByAndrew Valsamis Updated on Guides

Mill scale covers hot-rolled steel, causing difficulties for welders. After reading this post, you will understand what mill scale is, if it is necessary to remove it, and how to weld over it.

While scale covers all hot-rolled steels, this article will cover mild steel and the most well-known arc welding processes to keep things simple.

Table of Contents

A photo of partially removed mill scale on hot-rolled mild steel
Partially removed mill scale on hot-rolled mild steel

What is mill scale on steel?

Mill scale is a uniform layer of various iron oxides and other impurities that covers the surfaces of hot-rolled steel. When machinery rolls the hot steel to form it, the air’s oxygen reacts with the hot iron and creates these oxides.

Mill scale has a black or black-blueish color and spreads evenly over the entire surface. Its thickness is usually around 0.004″ (0.1 mm), but depending on the metal’s thickness, it can reach up to 0.04″ (1 mm). 

When scale is fresh, it protects the steel from rust, but it gets dry and eventually starts to break. Cracks in dry scale let moisture contact the steel, and rust will begin to appear.

Many steel manufacturers or suppliers remove scale from hot-rolled steel and ship it covered in protective oil or primers. It looks like, but it’s not the same as cold-rolled steel, which has precise dimensions.

It’s more helpful to buy hot-rolled steel with the scale removed. Although it’s slightly more expensive, you will save time, cost, and effort if you consider removing it for painting.

Is welding over mill scale harmful?

Welding over mill scale generates additional fumes. But they are not as harmful as fumes from galvanized or, even worse, painted metal. Furthermore, oil may exist on scale used for lubrication during hot-rolling or rust protection. Burned oil generates hazardous fumes.

Moreover, the suitable welding processes that can burn through scale generate many fumes.

You will also choose consumables that contain high levels of deoxidizers that will generate additional fumes.

Similarities and differences in mill scale and rust for welding

Mill scale is a mixture of iron oxides and other impurities formed in high temperatures. Rust is also a mixture of iron oxides but is created in the presence of moisture in everyday temperatures. 

For the same thickness, rust is far easier to remove than fresh scale. If scale is old, dry, and rusty, it is almost as easy to remove.

Scale has a fixed thickness, it never gets thicker, and doesn’t destroy the steel. Unfortunately, rust digs deeper into the steel as time passes by, and it will corrode and ruin mild steel easily.

While welding over scale is possible, welding over thick rust is much harder.

Weldpundit has a dedicated article on welding rusty metals.

Do you need to remove mill scale before welding?

If you weld fabrications that take heavy impacts or endure harsh conditions, you must remove the mill scale completely. However, if you weld small fabrications or repair work that will endure low static loads, you do not need to remove it.

As a general rule, to get the best overall results, you should always remove the scale. Removal is faster and cheaper than correcting a weld with defects. 

Weldments must perform as expected in the long run, not just when produced in the workshop. Perfect welds need clean metals.

However, there are situations when you do not want, or don’t have the means to remove it.

In situations like these, and since the welds are not critical, you can weld over scale with the proper process and electrodes and save time and money.

How can you remove mill scale?

The simplest and fastest way to remove mill scale is by using an angle grinder with a grinding wheel or flap disc covered with silicon carbide abrasive material.  

When mill scale is fresh, it’s firmly attached to steel and hard to remove. If you use manual or mechanical ways to remove it, you will notice that it tends to move around and remain on the metal. 

If you want to use an angle grinder, avoid wheels or flap discs covered with aluminum oxide, zirconia oxide, or ceramic material. The scale will clog flap discs after a short time. Instead, save these for clean metal.

Only a wheel or flap disc from silicon carbide material can give you consistent results to remove fresh scale without moving it around or clogging the discs.

Discs for concrete or stone or more efficient to remove scale, but don’t use them on the metal itself.

Use one to remove the scale 1/2″ (1.25 cm) to 1″ (2.5 cm) from each side of the joint, depending on the base metal’s thickness. 

Many manufacturers produce wheels and discs to remove scale without mentioning their material. Most of the time, these products are pretty expensive and might still get clogged.

Other methods to remove fresh mill scale are:

  • Sandblasting has the best results overall if you want to clean the entire surface. But it’s too much trouble if you only want to weld the joint.
  • You can use several acids, but like sandblasting, it’s time-consuming if you only want to weld.
  • Flame cleaning with oxyfuel equipment can remove fresh scale locally, but it costs and might warp the workpiece if it is thin.

Inefficient methods to remove fresh mill scale are:

  • Wheels and discs covered with aluminum oxide, zirconia, and ceramic material.
  • Stripping discs for paint.
  • Wire wheels for angle grinders.
  • Needle scalers.
  • Hand wire brushes.

However, you can use these methods when the scale has dried up or has signs of rust.

How mill scale affects welding?

Mill scale acts as an electrical and heat insulator resulting in arc instability and restricted puddle fluidity. Scale also releases oxygen and other impurities in the puddle. The worse defect is lack of fusion to such a degree that even small fabrications can fall apart by themselves.

In the presence of mill scale, the amperage and voltage running through the electrode and the working (or grounding) clamp will be reduced and unstable, affecting the arc’s performance.

You will have a harder time starting the arc, and low penetration at the bead’s beginning. In addition, the arc will be unstable until increasing sparks and spatter.

Scale has a higher melting point than steel and, in combination with the lower heat and current conductivity, it will make the arc and the puddle smaller.

The puddle will move slower and will not flow to the bead’s sides as expected, resulting in a narrow and convex bead with undercut.

At the joint’s sides (or toes), the weld metal deposition will be uneven and without reliable fusion.

You will also have trouble when welding out of position. For instance, vertical up welding needs excellent puddle fluidity to provide reliable fusion, and scale decreases fluidity.

If you use the wrong settings and technique that will input low heat, scale will prevent their fusion to the point that you can easily separate them with a breaking test.

Furthermore, scale will break down, releasing oxygen in the weld metal. The excess oxygen will create porosity. Porosity will make the joint weaker by concentrating stress on the remaining weld metal. 

Scale contains many impurities that will remain in the weld metal as nonmetallic inclusions. These inclusions can reduce the weld metal’s mechanical properties, for instance, ductility (elasticity).

Finally, surface porosity and undercuts, concentrate moisture and cause corrosion given enough time.

Can you weld through mill scale?

You can weld through mill scale if you choose the suitable welding process and electrodes and adjust the settings and technique accordingly.

To weld through mill scale, you should use a higher amperage to provide more heat. Heat will help to burn it and increase the puddle’s fluidity.

A slightly slower traveling speed will also help counter the puddle’s slower speed, but you should be careful not to overdo it. The electrode must always point to the leading edge of the puddle, not over it.

Because removing scale is not always possible or desirable, manufacturers produce electrodes to give acceptable results. Electrodes that combine with flux material are the best for this kind of job.

Manganese and silicon are the main deoxidizers all rods, wires, and filler metals have. These elements combine with oxygen creating oxides that will float to the bead’s surface.

Furthermore, silicon can significantly enhance the puddle’s fluidity so it can reach and fill the joint’s toes.

Can you stick weld through mill scale?

Stick welding is the best process to weld through mill scale. The stick rods are covered with a thick layer of flux material. The flux contains many deoxidizers and other impurity scavengers that can remove scale from the puddle.

Stick welding (SMAW or MMA) always creates a forceful arc. If you set an amperage high enough to meld the stick rod, the generated heat is enough to burn through scale and fuse the joint.

The joint’s fusion will be enough to hold small fabrications, but never for critical work, you always clean the joint.

Furthermore, stick rods are the cheapest consumables compared to the other processes.

What are the best stick welding rods for mill scale?

Now let’s see which rod is the best and how the other rods compare.

The best stick rods for mill scale are the cellulosic rods, for example, the E6010 and the E6011. These rods can weld through the thickest mill scale and penetrate deep into the base metal.

Only the stick welding process has cellulosic electrodes. These rods generate the strongest arc than any other rod. That’s about 70% stronger with the same amperage.

Cellulosic rods also generate high amounts of shielding gasses. The gas coverage is large enough to manipulate the rod with a whipping forth and back movement. 

When you pass the rod over the scale, the powerful arc can burn it and preheat the base metal. Then, when the puddle arrives, the scale is not there to restrict good fluidity, and the metal is preheated for improved fluidity and fusion.

Rutile rods such as the E6013 create a smooth arc and medium gas coverage. However, while rutile rods can weld over thin scale, they cannot offer the same efficiency as cellulosic rods.

Finally, basic rods such as the E7018 create a smooth arc and low gas coverage. Basic rods need relatively clean base metal, but can still give good results if the scale is not thick. 

Basic rods are also called low-hydrogen rods. That’s because they are used for welding hardenable steels that need to avoid high hydrogen levels inside them to prevent cold cracking.

Scale contains impurities with hydrogen, making low-hydrogen welding impossible without removing it.

Can you weld through mill scale with flux-core wires?

Flux-cored wires are very efficient to weld through mill scale. They contain flux material with large amounts of deoxidizers. These materials counter the negative effects that scale has on the puddle.

However, flux-cored wires are expensive and need proper storage to avoid going bad from moisture absorption.

There are two types of flux-cored wires:

  • Self-shielding wires that create their own shielding gases. These are more efficient against scale.
  • Gas-shielding wires that need external gas coverage like MIG. These are for high-production work.

Another thing to keep in mind with flux-cored wires is that most of them use the DC- polarity to burn properly, while MIG welding uses DC+. So if you forget to switch the polarity, you will end up with heavy spatter and a bad-looking bead.

Self-shielded flux-cored wires

The best self-shielded flux-cored wire for mill scale is the E71T-14. This is because it has the highest amount of deoxidizers. But, E71T-14 is a single pass wire only and hard to find. 

E71T-14 has such high amounts of deoxidizers that a second pass will concentrate them so much that may crack the weld right after stopping the arc. We call this hot cracking.

The general-purpose E71T-11 self-shielded wire is suitable enough. It can weld through thick scale, but not like a cellulosic stick rod. In addition, it is easy to find, a bit cheaper, and can do multipass welds. 

Gas-shielded flux-cored wires

The best gas-shielded flux-cored wire for mill scale is the E71T-2. It is designed to weld over mill scale and rust with excellent results. But it is a single-pass wire, expensive, and available in thick wires and heavy spools.

E71T-2 welds with DC+, and there are two versions. E71T-2C uses 100% CO2, and E71T-2M uses mixed 75% argon and 25% CO2.

The E71T-1 is the general-purpose gas-shielded wire and can weld through scale if it’s not very thick.

Many manufacturers produce ExxT-G wires. These wires always have different characteristics. Many of them are specifically designed for welding over mill scale. You must read and trust the manufacturer’s description for each one.

Can you MIG weld through mill scale?

You can MIG weld through a light layer of mill scale but not a thick one. Under poor settings and torch handling, MIG welds can suffer from a severe lack of fusion. The presence of mill scale can worsen fusion problems to dangerous levels. 

MIG and surface impurities as scale do not combine well. This is because MIG wire does not come with flux material that traps impurities. As a result, MIG needs a relatively clean metal to work well. 

If you set low voltage or wire speed, the heat that goes into the base metal is not enough for good fusion. The same happens if you do not point the wire at the puddle’s leading edge but over the puddle.

If there is scale, the lack of fusion worsens greatly. As a result, the bead will attach to the workpiece without fusion.

If you want to MIG weld over scale, you can weld on similar scrap metal and do breaking tests. This way, you will be certain that your welds will be strong enough.

One tip to prevent lack of fusion is to always point the arc in front of the puddle. The following image shows the V pattern technique that offers better results than the circular one.

An image of a V pattern for a horizontal fillet weld
V pattern for a horizontal fillet weld

What is the best MIG wire for mill scale?

The ER70S-2 is the best MIG wire for mill scale because it has three special deoxidizers, titanium, zirconium, and aluminum, that have a strong cleaning effect. After that, the popular ER70S-6 wire gives good results against thin scale.

All MIG wires have manganese and silicon that act as deoxidizers. While these elements can work against scale, they are more suitable to capture free oxygen. 

Titanium, zirconium, and aluminum can capture the scale’s oxygen creating new oxides that will float to the surface while leaving the remaining iron in the weld metal.

Even if a small percentage of the new oxides remains, they are not as harmful as scale. 

However, the ER70S-2 wire is hard to find and more expensive than the standard ER70S-6 wire. 

The ER70S-6 wire does not contain titanium, zirconium, and aluminum, but it has the highest manganese and silicon amounts. With proper attention to avoid lack of fusion problems, ER70S-6 can give acceptable results over thin scale to make simple fabrications. 

If you have a MIG welder, you can buy a small spool of self-shielded flux-cored wire as an alternative to weld over thick mill scale.

Can you TIG weld through mill scale?

You cannot use TIG welding to weld through mill scale and receive acceptable results, even if the mill scale is thin. TIG always needs a clean base metal.

Mill scale will release large amounts of oxygen that TIG cannot handle. Oxygen will lead to severe porosity both inside and on the surface of the bead.

TIG welding on perfectly clean metal generates minimum sparks or spatter, but they will be abundant in the presence of scale. Sparks, spatter, and oxygen will contaminate the tungsten electrode, resulting in frequent stops to resharpen it. 

If you tried to clean the joint, but there are scale traces, use an ER70S-2 rod to get somewhat better results. 

A more efficient thing you can try is to weld with an austenitic stainless steel rod such as the 309L. This rod has high amounts of nickel that can dilute various contaminants. But 309L rods are much more expensive.

Conclusion

Mill scale is a leftover byproduct on hot-rolled steels, undesirable for welding. It is necessary to remove it for critical welding, but you can weld over it for simple projects.

The fastest way to remove scale along the joint is with an angle grinder and a silicon carbide wheel or flap disc.

The best procedure to weld through scale is stick welding with cellulosic rods. After that, flux-cored welding with self-shielding wire.

MIG can weld over thin layers of scale with the correct settings, but TIG requires removing it completely.

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