What Is Slag in Welding for Beginners

Written By: Liam Bryant

Slag in welding refers to the byproduct formed during welding, consisting of impurities and oxides from the weld pool. It protects the molten metal from atmospheric contamination and is typically removed after the weld cools and solidifies.

Once the process is done, and the area cools, the byproduct solidifies. What remains is called welding slag. You’ll want to remove this afterward, but it also makes sure the weld was of good quality.

What Is Slag?

Welding slag is nonmetallic, which is why it needs to be removed. If not, it could weaken your weld if you need to pass through it multiple times.

When welding with flux, it melts and produces gasses pushing out atmospheric gasses, such as oxygen and other contaminants. This protects your weld from oxidation.

The parts of the flux that didn’t evaporate are left in the weld zone, resulting in slag. It contains the impurities that have floated to the top of the welding pool, leaving only metal to cool and solidify underneath.

welding slag

Different Types of Slag

The characteristics of slag will differ depending on the kind of flux and process you use.

Rutile or Acidic Slag Systems

Using rutile or acid flux wires results in slag that’s easier to remove. These flux wires contain titanium oxide. Other minerals, like silica or zircon, can also be combined with these wires.

Rutile flux wires are known to be easier to use than basic flux wires. They offer smooth weld joints, low spatter, and good arc quality.

Basic Slag Systems

Basic flux wires are used to achieve tougher welds, suitable for those with mechanical properties.

These flux wires can comprise calcium fluoride (fluorspar), calcium carbonate (limestone), magnesite, and dolomite. These materials result in slag that’s more difficult to remove.

This process also makes your weld more prone to having slag inclusions, especially if the slag doesn’t float to the top and solidifies within the cooling metal.


Welding slag may seem like a mess you need to eliminate, but it’s also an important part of the process.

  • Slag protects your weld from oxidation.
  • It prevents contaminants from entering your weld, resulting in fewer defects.
  • It helps keep the shape and position of the weld pool as it cools.


Without cleaning up the welding area from previous use, the leftover slag may also cause defects—even if it was meant to help prevent this.

Without using proper welding techniques, pockets within your weld can trap slag.

Slag Inclusions

Slag inclusions can affect the quality of your weld. You can prevent this by having a clean weld zone before working. 

Removing the solidified slag from your weld before going over it again will also keep it from getting trapped inside. Use proper technique so that you don’t accidentally trap slag in the middle of the welding process.

Slag inclusions can cause porosity in your weld. Having chunks of slag solidify in the metal can create air pockets that trap some of the gas produced by the welding process.

How to Remove Slag

Once welding slag cools and solidifies, it’s now waste material. You’ll have to remove this to preserve the quality of your weld. Removing slag will also allow you to inspect your weld properly.

It’s good to do this as a final step as well, to improve the appearance of your weld. If you plan to coat the surface with oil or paint, you’ll need to clean up the slag to help the coating stick.

There are several tools you can use to remove slag efficiently.

  • Hammer and chisel
  • Flat files
  • Grinder
  • Needle scaler

Remember to wear protective gear, like goggles and gloves, to avoid getting injured by flying pieces of slag.

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Using a Hammer and Chisel

You can use a hammer and chisel to easily chip away the solidified welding slag. If you have a chipping hammer (a hammer that has one pointed end and a flat edge on the other side) you can use that to knock off the slag.

Using Flat Files

Filing down welding slag may not be the easiest technique, but it can still work for removal. It’ll leave you with a smooth surface afterward.

Using a Grinder

Power tools like angle grinders can remove welding slag quicker than a normal flat file. You can also finish off with a grinder or belt sander to give your piece of metal a polished surface.

Using a Needle Scaler

Another power tool you can use is a needle scaler for stubborn pieces of slag. The multiple needles will pound at the metal surface and remove the welding slag. This tool is also suitable for bigger welding projects because it can do the job quickly.

How to Prevent Slag Inclusions

Here are some simple tips to help you avoid slag inclusions in your weld:

  • Always clean your welds – Before starting your welding project, ensure there’s no residual slag in your workspace. You should clean slag before going over the same area too.
  • Use proper technique – While welding, use a dragging technique while keeping the angle as straight as you can. Applying heat right to the base of the weld will allow the slag to come up to the surface, instead of trapping it inside.
  • Work with the right speed and current – Using the appropriate speed (not too slow or fast) will reduce the risk of getting slag inclusions. Using excessive current can also form convex beads which cause slag to get trapped within the weld.

Wrapping Up

So, what is slag in welding? It’s a nonmetal byproduct that results from welding processes. It helps prevent contaminants from entering your weld but must also be removed afterward.

Dealing with slag is inevitable in this field. It may feel like a chore to keep cleaning it up, but it’ll save you more time in the long run.

Getting slag inclusions may force you to work on more taxing and costly fixes, so try to prevent them as much as possible.