What Is Slag In Welding? [Pros & Cons]

by WeldingWatch

what is slag in welding

Slag in welding is a hard layer on the weld created from non-metallic elements of a weld, such as carbon and atmospheric gases. When the weld is complete, it needs to be removed.

How is Slag Formed?

Slag is produced when melted flux and metal combine in the weld pool. During the cooling process, not all of the flux evaporates. Some remains in the weld pool and rises to the surface. This does help protect the weld as it cools, but it then forms a hard layer on the surface, which needs to be chipped away.

Due to the different materials used in welding, it does not always have the same composition, and often also contains atmospheric gases and any impurities which were absorbed in to the flux as it melted. As it contains only non-metallic elements it must be removed from the weld.

Advantages of Slag

Even though it does need to be removed, slag does serve a purpose during a weld.

  • During the weld, it protects the weld from any atmospheric contamination and from oxidation.
  • It also helps keeps the weld pool in the right place, and in the right shape, during the cooling process.


  • If slag doesn’t always rise to the surface this can cause a problem with the weld, such as leave small holes and debris. This slag weld inclusion can be damaging to the weld.

Weld Slag Inclusion

If the slag has not risen to the surface, this can lead to problems with the weld.  Any non-metallic elements left in the weld are prone to corrosion or porosity, and over time, this can weaken the overall joint.

The main causes for slag inclusion are

  • The quality of the flux, or the flux not being used in accordance with the correct guidelines
  • The welding technique use, and the experience of the welder
  • The metal not being cleaned properly before being welded.

The only way to correct an inclusion is to remove that part of the weld, and redo it. Experienced welders will know how to minimize the occurrence of  weld slag inclusion.

  • Make sure the correct flux cored wire is used and that it is used at the correct temperature.
  • Make sure everything is thoroughly cleaned before you start to weld. All dust and grease should be removed before you begin as these will be melted in to the weld.
  • Welding at the correct angle will also help the weld pool to form correctly.
  • If you are competing a multi pass weld, remove the slag between each pass. This may sound time consuming, but it will save a lot of time having to redo parts of the weld if you simply weld over slag.

Removing Slag

Slag needs to be correctly removed from any weld and for this you will need

  • Chipping hammer
  • Wire brush
  • Needle scaler

You will need to chip the slag away from the weld, making sure you do not damage the weld or leave any small particles of slag behind. If the weld has been done correctly, the slag should not be bonded to the weld and so should be easy to remove.

You can also get filler metals which have a self-peeling slag.

There are several reasons why you need to remove the slag.

  • It’s impossible to fully inspect your weld if there is a layer of slag over it.
  • Completing a multiple pass weld means you will have to remove it between each pass
  • Your weld will not look the best if it’s covered in slag

Frequently Asked Questions:

Can you weld over slag?

Welding over slag can cause issues with the weld and if you attempt to weld over it, it can also produce more spatter. Slag is a covering over the weld, so it you leave it on and weld over it, it could also affect the overall strength of your joint.

What happens if you don’t remove slag?

Slag is made up of non-metallic elements and if they are left over time they can react to the weather, create rust and allow moisture to enter the weld.

What do I do if I get a slag weld inclusion?

These are best avoided, if possible, by simply following the correct welding guidelines and using the electrode as the manufacturer’s advice. If you do get a slag inclusion, it will need to be removed and that part of the weld will need to be redone for the best results.


Slag can be an important part of a weld, but if left on, it can cause problems. No matter how good a welder you are, you will come across slag, and it will need to be removed.

If you have completed a good weld, it should chip off quite easily. If the slag is messy, and has got in to the weld itself, the best thing to do is to start over.

There’s no way to avoid slag with certain welding processes, so learning how to work with it will serve you well.

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