Why Great Welding Needs Good Welding Slag

By Bill Byers / December 29, 2018
Welding Slag Title Image

You've seen it over your welds, heard its name, but what is it?

Why does it form?

Why sometimes is it so difficult to chip off?

I'm going to answer those questions and more, and I will also tell you why great welding needs good welding slag.

​What is Slag in Welding?

​Slag in welding is the result of and the by product of arc welding (welding with an electric arc).

Slag is a coating. The layer on top of your weld left after you have welded.

Welding Slag is formed by metal oxides and other chemical compounds.  

Slag needs to be removed so that you can;

1. Look at the weld

2. Prepare your weld project for protective painting or galvanization

3. Run a second weld bead for projects where you need to lay down multiple beads to build up a wide or deep weld joint.

Weld Slag Easily Removed

Weld Slag Easily Removed

​Is All Slag in Welding the Same?

​No. It isn't ...

What type of slag you have depends on whether you are MIG welding and using shielding gas and solid wire.

Or whether you are welding with flux core wire?

Let me explain the difference.

​Welding Slag From MIG Welding

​Some welders think there should be no slag on a MIG welded joint and they have a point.

The stuff we call slag - that residue that forms after MIG welding - over the weld isn't strictly speaking slag at all.

I explain what slag really is later.

This coating is Mill Scale. The same kind of coating you see when you buy brand new steel.  It is a thin layer of metal oxides and will either simply fall away.

Or be easily removed with the use of a wire brush or chipping hammer.

For example, when you are MIG welding Aluminum then this coating will consist of Aluminum Oxide.

What is called slag in MIG welding is in fact highly oxidized metal - Mill Scale - that could also contain small impurities from your weld and silicon as glass.

And once chipped off, you can quite happily dispose of this slag coating as you would scrap metal.

​YouTube Video of MIG Welding Slag on Stainless Steel

This one minute 15 second video from Hobart Filler metals is a bit high on the sales pitch but it does show how easy MIG welding slag removal should be with the right settings.

I've set the video to start 40 seconds in. Watch till 51 seconds if you don't want to get too bored.

New Stainless Steel Wires Offer Easy Slag Removal

Video Credit: Hobart Filler Metals

​What are the Causes of Slag in MIG Welding?

​When mild steel reaches the high temperatures necessary for MIG welding, a layer is formed. This coating of oxides, more commonly known as Mill Scale is a result of a reaction of the surface of the welded metal with air.

And forms over the molten metal as it cools.

In addition some welding wires for MIG welding have chemical scavengers and deoxidizers such as silicon and manganese as part of the welding wire. ​

The aim of these scavengers is to trap small contaminants in the hot weld pool. The scavengers and silicon when molten are designed to be lighter than the hot pool of metal and float to the top.

When the weld pool solidifies after your weld pass, the contaminants in the now hardened silica, are on the top included in the metal oxides. The silicon looks like glass.

Weld With Silicon On The Top

​Is Slag in MIG Welding is a Good Thing?

A light slag or Mill Scale covering shows you have reached a good temperature for your weld. A temperature high enough to produce Mill Scale, often signals a good weld.

Although having slag doesn't necessarily mean that you have achieved good joint penetration.

The little shards of glass on the surface when you're using solid welding wire that includes scavengers, means that those small impurities have made it to the top of the weld.

And shouldn't be stuck within your weld where you really don't want them.

If you have cleaned your metal well before you MIG weld then the slag covering should be light and easily peeled away and not be something to be concerned about.

​Slag From Welding with Flux Core Wire

Welding with flux core wire is very different from MIG welding where you weld with a solid metal wire. I'll explain it briefly here.

​But should you want to know more details about the flux core welding process, have a read of my document here on the site - What is Flux Core Welding for Beginners and Everyone Else - I go into more detail there.

Whether it is a flux core welder, a stick welder or submerged arc welding, the wire used is a wire where the core is made up of solid metal and it is surrounded by a covering. ​

Flux Core Wire

Flux Core Wire

This covering is called the flux. The flux contains chemical compounds that when melted by the arc forms the shielding gas to protect the weld pool from the surrounding air.

Flux core welding slag is formed from a reaction of the shielding agent compounds in the flux with foreign particles.

Where Welding Slag is Formed

​Where Welding Slag is Formed

​What causes Slag in Flux Core Welding?

The flux core wire itself.

Not only does the flux produce the gas that shields the weld from the air, another crucial feature of flux core wire is that is designed to leave a coating.

This coating is a true slag coating compared to MIG welding where the coating is really Mill Scale.

​What is the Purpose of Slag in Flux Core Welding

​The slag in flux core wire welding is a secondary method of protecting the weld pool. The primary method is the flux on the flux core wire burning off as a gas.

When arc welding part of the flux burns off forming your shielding gas. The gas holds back the oxygen and nitrogen in the air that would otherwise react with the metal and ruin your weld.

Then the remaining flux melts into the weld pool scavenging for contaminants in the weld pool and from the base metal.

The contaminants are trapped and raised to the top of the weld where the flux then forms a slag cover.

A cover that protects the weld pool as it cools from damage from the surrounding air.

When manufacturers design flux core wire​ the compounds in the flux are picked so that the weight of the flux is lighter when compared to the weight of the metal. This makes the flux float.

It floats to the top of the weld with the aim of leaving pure metal behind. The action of the flux moving through the molten metal also helps your welding wire and the base metal to mix well together.

The slag also slows down the cooling of the molten metal in the weld pool, which again contributes to a good solid weld forming underneath.

​32 Second YouTube Video

​Here is a Youtube video I found that is an example how slag would normally come off from a weld. It is not quite the easy slag removal the title claims, perhaps this YouTuber has had tough slag removal previously.

Ideally, the slag just sits there and is easily peeled. Anyway it is a good example of what to expect of your welding slag.

Easy Slag Removal

Video Credit Nathan LePage

​Problems Caused By Welding Slag

There are instances where welding slag can cause you problems and it is worth being aware of them.

1. In multipass welding. Where you need to take multiple weld passes to help build up a weld of a gappy joint or when welding thicker metal.

The slag can get trapped in the weld as you build up each layer of the weld.

The slag caught in the joint, because it contains oxidized metal and foreign particles can corrode over time causing your weld to fail early.

2. Again in multipass welding not overlapping the welds sufficiently. This can cause a gap where slag can form because the weld pool is not hot enough or is too wide to lift the slag to the surface where it can be removed.

Then the slag is left behind leaving contamination in the weld.

3. Incorrect settings on your welder while you are welding could mean that the slag does not completely float to the top leaving behind what are known as slag inclusions. Basically random foreign objects in the weld stuck between the your new weld and base metal of your weld project.

Sometimes these defects can be clearly seen with the naked eye. Or they are so engrained that they can only be seen on x-ray inspection of the weld.

Flux Core Weld Porosity Example

Flux Core Weld Porosity Example

​They are then classed as a weld defect.

4. Baked on slag that is difficult to remove could mean damaging your welding project in your keenness to remove it. Careful use of grinder and a flat flap disc will remove baked on welding slag

5. Welding slag that's not completely removed will cause any paint protection of the joint to fall off and protection by galvanization to fail.

Although there are problems that can be caused by welding slag it is a good thing and necessary for a great weld.

​Is Welding Slag in Flux Core Wire Welding a Good Thing?

​Yes, it is a feature of flux core welding with a flux core welder.  If you had no slag production then you know you definitely had a problem with your welding.

It is a good thing because;

1. Without a slag covering oxidization of your metal would happen and your weld would fail.

2. In flux core welding you want the slag coating to flow freely over the surface of your weld pool giving the best chance for a good weld to form underneath.

3. The slag contains the imperfections, the small impurities that you don't want in your weld. You want them on top where you can easily chip them off when you are done welding.

4. It helps the metals mix - the metal in your welding wire to mix in with the base metals of your welding project, filling your weld properly to form a good solid weld.

​Removing Welding Slag

​Now I have explained what is slag in welding let us have a look at removing this necessary by product.

Ideally, the slag residue whether it is from MIG welding or from flux core welding should be brittle and easily removed.

Before attempting to remove welding slag it is good practice to use and wear safety goggles.

As chipping away at slag can send small particles up off the weld, into the air and into your eye.

Not something you want.

Weld Slag Tapped By Chipping Hammer

Weld Slag Tapped By Chipping Hammer

1. Slag that comes away easily can be brushed with a wire brush.

2. Or a chipping hammer to remove the pieces of slag that are a shade more difficult to remove.

3. Or a grinder with a flap disk or wire brush attachment.

4. Or remove slag using a pair of MIG welding pliers.

​Nine Second YouTube Video

​I found this short 9 second YouTube video illustrating how a weld can be done so that the welding slag literally falls off. Enjoy.

When the Slag falls Off

​Video Credit: Jeremiah Kreuder

​Slag That is Difficult to Remove

​If your welding slag is really baked on and is taking quite a log of effort to remove. It has effectively been 'baked on' by the welding arc.

You're likely using an amperage setting a shade too high.

Lower your amp a little and adjust your welding angle to aim for a 10 to 15 degree angle to the weld.

You may also find increasing the speed you travel along your weld will help prevent the slag from baking on.

It is worth knowing that welding slag is alkaline.

That means using a chemical bath to remove slag won't be affective. The bath will be acidic and the alkaline in the slag covering will naturally resists the acid bath.

It is better to adjust your weld setting to form slag that is more easily removed.

Although easy slag removal does not guarantee a good weld.

i.e. A weld that will hold.

It is likely that when you have slag that is easy to remove, underneath you will have a good weld.

​Other Tips for A Good Slag Covering

​1. The mantra of good welding is clean, clean, clean and prepare your weld.

Yes, any scavengers in your welding wire will trap those contaminants but have too many and your slag just wont contain them all.

Clean Your Metal before Welding

Clean Your Metal before Welding

​2. When welding multiple passes aim to remove all slag before the next weld pass. Don't risk trapping slag in your next weld pass.

3. Use a wire brush on mild steel and have a different brush for stainless steel and definitely a separate one for Aluminum. This will prevent small pieces of slag metal not of the type you are welding, from cross contaminating your weld.

​Last Words

​Thank you for reading my article "Why Great Welding Needs Good Welding Slag". I hope I have convinced you that slag over your weld is a good thing.

And that easy slag removal is the aim as it is a great indication of the right settings, travel speed and weld angle on a weld.