Slag inclusions are a common weld defect that can happen when using welding methods that involve flux, like stick and gasless MIG. They’re not always the easiest problem to see, especially when they’re buried in the weld, but they do need to be dealt with.

We’ve covered what slag inclusions are, how to spot them, what causes them, how to avoid getting them, and what to do if you end up with slag inclusions in your weld.

First things first, though, let’s go over what slag is and the role it plays in a weld.

What Is Welding Slag?

Welding slag is a non-metallic byproduct that forms when using gas-shielded or self-shielded flux-cored or flux-coated welding processes. It is the hardened layer that is left on top of a weld after the flux has melted out and solidified.

Four welding processes use welding flux:

• Shielded Metal Arc Welding (SMAW), also known as Manual Metal Arc (MMA) welding or stick welding
• Flux-Cored Arc Welding (FCAW), also known as gasless MIG
• Submerged Arc Welding (SAW)
• Electroslag Welding (ESW)

Welding flux is made up of a combination of silicate and carbonate materials. These materials are chosen because their density is lower than the weld metal’s when the flux melts, so it can float to the surface.

When welding, the heat from the arc melts the filler material (including the flux) and base metal to create the weld pool. The melting flux produces its own gases that push the atmospheric gases out of the weld, preventing oxidation and nitrogen reactions.

Thermodynamic processes in the weld pool force any elements that aren’t included in the formation of the pool, including the flux, out to the surface. These non-metallic elements, which include oxides of aluminium, silicon and calcium, solidify into slag. It also forces internal gases like nitrogen, hydrogen and carbon out of the weld.

The hardened layer of slag forms a protective cover over the finished weld bead. The characteristics of the slag that is produced will depend on the materials used in the flux.

The flux and slag in a welding electrode or wire serve several purposes, including:

• To protect the weld bead from atmospheric gases and contaminants
• To hold the weld bead in the joint while it cools, which is important when welding out-of-position
• To thermally insulate the weld and control its cooling rate

While the primary purpose of slag is to protect a weld, in some instances, it may not float to the top of the weld, and you can be left with slag inclusions.

What Are Slag Inclusions?

Slag inclusions occur when bits of slag become trapped inside the weld pool, contaminating it as it solidifies. Slag inclusions can be seen on the surface of the weld or can be buried inside the weld bead and are only detectable by x-raying the weld.

Why Is It Bad?

Slag inclusions are a weld defect that leaves you with a weld full of weak spots, whether they’re internal or on the surface.

Slag is a non-metallic material, so it doesn’t have the same strength properties that the weld metal does. The trapped slag reduces the overall structural integrity of the weld, as the weakened spots can’t handle as much load, which can lead to failure, especially in critical applications.

They also reduce the mechanical integrity of the weld because they interrupt the natural interactions between individual grains.

The slag-filled spaces in the weld can allow moisture and other gases into them, which can begin to rust. Depending on its chemical makeup, the slag itself can also speed up the corrosion process if it’s prone to rusting.

Slag inclusions can make the weld look irregular and rough, which, as well as being a detriment to the weld, is also not aesthetically pleasing. 

How to Find & Identify Slag Inclusions

There are several ways to tell if your weld has slag inclusions in it. If you have slag inclusions on the surface of your weld, they’ll be visible to the eye and easy to spot.

Surface inclusions usually look like wormholes or porosity in a weld, small black spots that are embedded into the weld and leave dark divots, often along the toes of the weld. They are usually scattered randomly in clusters or sometimes as secluded spots. 

If the inclusions are buried inside the weld bead, you’ll need to use non-destructive testing (NDT) methods to check your welds. NDTs include dye penetration testing, x-rays or ultrasound testing in order to pinpoint them.  

What Causes Slag Inclusions?

While slag inclusions are limited to welding processes that involve flux, there are still several ways you can get them in your weld.

The Type of Flux Used

Flux turns into slag, so the composition of the flux plays a role in determining whether you end up with slag inclusions. A flux’s composition determines the weldability, surface tension, how fast it freezes, and oxygen levels in a weld and can impact the arc’s performance as well.

Materials like titanium oxide (TiO2), iron oxide, silica and lime have unique effects on flux and the resulting slag.

High levels of titanium oxide mean the flux is faster freezing, which is good for out-of-position welding. However, too much titanium oxide can also make the slag sticky.

The addition of iron oxide in flux can lower the surface tension of a weld and create a more concave weld bead, which helps with wetting into the base metal. Iron oxides also help with providing higher deposition rates.

Silicates produce a glass-like slag that often detaches itself from a weld. Limestone (calcium carbonate) produces a harder slag that is difficult to remove from a weld.

There are several different flux types depending on what it’s made from.

Rutile (or acidic) fluxes are mostly made up of titanium oxide (the mineral rutile) but can also contain silica, alumina, and zircon. Rutile fluxes tend to have a high surface tension and create a flatter weld bead.

The slag this flux produces rises to the surface of a weld easily and is generally a more welder-friendly flux to work with. Rutile flux also helps to make arc ignitions smoother and easier.

Basic fluxes are mostly made up of calcium carbonate (limestone) but can also contain calcium fluoride (fluorspar), magnesite, or dolomite. The high levels of limestone help reduce the oxygen levels in the weld, but have a slower freezing slag.

The slag that basic fluxes produce is harder to work with, as it’s difficult to remove. It also tends to remain in the weld pool rather than rise to the surface, so it’s much more prone to leaving slag inclusions in your weld.

Poor Welding Technique

When you’re working with flux, your welding technique can be the deciding factor on whether or not you get slag inclusions. If you’re pushing your weld instead of dragging it, you’re going to get slag inclusions, as the slag is pushed out in front and welded over rather than left to melt out as you travel along.

If your travel angle is too shallow (completely perpendicular to the weld or vertical) or too steep (almost flat in the joint or horizontal), then you won’t have enough control over the arc and the weld pool, which can cause slag inclusions.

If your arc length is too long or isn’t being held steady, you also won’t have as much control over the puddle. As well as your arc length, you want to keep the weld itself steady. Slag can get trapped if your welds are uneven or wobbly, or if you’re getting a convex weld bead or undercut.

Not Cleaning Your Welds Between Passes

If you’re doing a multi-pass weld, you need to clean each weld bead of slag before moving on to the next. If you leave any slag on the previous weld, especially along the toes of the weld, it’ll become trapped as you weld along the top of it, creating slag inclusions and preventing the weld from getting proper fusion.

Poor Filler Storage or Quality

Leaving your stick electrodes or flux-cored wire out in the open is going to cause problems. If they’re not stored properly, they’re more likely to absorb moisture, and flux-cored wires can rust, which will ruin the flux and give you slag inclusions.

Using filler material that contains low-quality flux will also result in slag inclusions.

How to Prevent Slag Inclusions

Trying to fix slag inclusions after your weld is done isn’t ideal, so the best thing you can do is prevent them instead.

Use the Correct Welding Technique

MIG or stick, if you’re working with filler material that involves flux, you need to drag your welds. If you try to use a pushing angle while stick welding or gasless MIG welding, you’re inviting slag inclusions.

When working on a flat or horizontal joint, you want to have your electrode pointed in the joint (this angle will depend on your joint type) and tilted slightly sideways by 10°-15° at a drag angle. Your torch should be hovering over where you will be welding, not hovering where you’ve already welded.

There is one exception to the always drag rule: when you’re welding vertical up. You’ll need to use a slight push angle up the joint to get proper penetration and keep the weld in the joint. Vertical up welds are typically more prone to slag inclusions because of this.

You want to keep your 10°-15° travel angle, but this time at a pushing angle, with the end of the electrode tilted slightly down.

Keep your arc length short. A good rule of thumb when you’re stick welding is that your arc length shouldn’t be longer than your electrode diameter. If you’re using a 2.6mm electrode, your arc length shouldn’t be longer than 3mm. However, you don’t want to be so close that your electrode is touching the metal, as it will stick.

Remove the Slag After Every Weld

Whether you’re doing single-pass or multi-pass welds, you need to remove the slag from your weld. It’s more important when doing multi-pass welds, as the slag needs to be completely cleaned off before you can begin the next weld. If there’s any left on your previous weld when you lay the next one, you’ll end up with a slag inclusion.

You can use a chipping hammer and wire brush to get rid of slag. The type of flux in your filler material will determine how hard the slag is to remove. For really difficult slag, and to be extra sure it’s all gone, you can use a wire wheel on a grinder to clean your welds.

Quick Tip

When using a chipping hammer and facing challenging slag, don’t beat at it. You’ll likely make it airborne and spray it across the room, creating a bigger mess. Instead, use the pointed end of your hammer to drag along the top and chip at the sides with the flat end, alternating with the wire brush as well to remove it.

Use Good Quality Electrodes That Are Stored Properly

First things first, make sure you’ve gotten yourself some good-quality electrodes or flux-cored wire that isn’t going to cause you issues from the get-go.

Store your opened filler materials somewhere dry to keep the moisture out, preferably in a sealed container to ensure they remain moisture-free.

If you don’t have somewhere dry and sealed to store these, or they become filled with a bit of moisture, you can re-bake your electrodes in an electrode oven to dry them out. An electrode oven will re-bake the flux coating on an electrode or the flux core in MIG wire, evaporating any moisture and leaving them ready to weld.

If you’re baking flux-cored wire, make sure it’s on a metal basket. If you’ve got a plastic wire spool, it’s going to melt the plastic as it bakes the flux.

How to Fix Slag Inclusions

To fix a slag inclusion, it needs to be removed. Unfortunately, you can’t just chipping hammer or wire brush a slag inclusion out, even if it’s on the surface. The weld has still been compromised, and the weak spot is still there.

On non-critical welds, for example, welds on furniture, grinding off the section of the weld with the inclusion will work, and you’ll only need to partially redo the weld.

On structural and critical welds, the whole weld will need to be removed and redone. You should also reference the welding procedure for the recommended remedy for your specific job.

Like with most weld defects, if you’ve gotten slag inclusions in your weld, you’ll need to remove them and start again. The best way to avoid getting them is to take preventative measures.