This Is What Causes Porosity in MIG Welding

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Metal inert gas welding is one of the most common methods of welding. It is also less commonly known as gas metal arc welding (GMAW). However, popular as it might be, as with any method of welding, MIG welding is susceptible to porosity.

There are many things that can cause porosity, with the most common causes of porosity in MIG welding are an excessive gun angle during the weld, having the wire too far from the nozzle, contaminated or wet gas cylinders, a dirty base material, and/or an incorrect gas level.

With proper training and experience, porosity in MIG welding can be prevented. Porosity is a common problem with MIG welding, but if you fully understand what porosity is and its causes, you will be able to spot the problem and know exactly how and why it happened. So, read on to find out all about it.

Porosity with MIG Welding

Porosity is the occurrence of an unwanted void or cavities in the weld material. When it comes to MIG welding, it is tiny gaps within the weld that weaken it and prevent proper weld adherence. It is caused by improper welding techniques. Some of the reasons that the cavities occur are:

  • Gases become trapped in the weld 
  • The weld cools and solidifies over the gas, traps it in, and creates the cavities

Porosity is considered a defect in the weld, and a good welder will always check their weld to ensure there are no defects. If they identify porosity, it can be fixed with proper procedures, but it takes lots of extra time.

To properly fix, the weld with porosity must be ground off and replaced. Since removing and reapplying a weld is more than 2X the time it takes to do it correctly in the first place, porosity should be avoided at all costs. Learning about and avoiding porosity saves time, money, and frustration, so it is an important part of welding to learn about.

Top Causes of Porosity in MIG Welding 

Porosity is caused by a wide variety of reasons in MIG welding. It takes practice and experience to prevent porosity, and even a seasoned welder will still routinely have porosity occur. That is why it is recommended that you be mindful of the condition of your equipment, your speed, your gun, and other factors that could impact the weld.

To fully understand the causes of porosity, the American Welding Society has a published reference guide. The AWS B1.11 “Guide for the Visual Examination of Welds” explains all the details from a technical standpoint. This guide is a great reference tool for experienced and amateur welders alike.

Improper Gas Flow

When it comes to gas flow, there are some things you need to consider. In fact, there are certain things you do and do not want when it comes to gas flow:

  • You do not want too little of a gas flow
  • You do not want too high of a gas flow
  • You want your gas flow just right

While it seems not to say much, there is a delicate balance that needs to be struck. If you have too little or too much gas flow, you will have issues. Therefore, it is very important to check for certain things:

  • Moisture in your gas
  • Kinks and pinches in your hoses
  • Leaks at the connections or in the hose itself

By paying proper attention to these things, you will find yourself avoiding potential problems before they have a chance to occur.

Air Drafts 

Welding is a hot career, and cooling off with a cold breeze through the window may sound like a relief, but it could be disastrous to your weld! Strong breezes and air drafts can blow your shielding gas away and infiltrate the weld puddle with atmospheric gas, ruining your hard work.

Prevent this by observing certain protocols when you are welding. A few things to do when welding are:

  • Keep the windows closed
  • Keep the gun the proper distance away from your weld material
  • Watch the arc length for any changes

Additionally, keeping air drafts to a minimum and ensuring there is enough shielding gas at all times will prevent porosity bubbles from popping up.

Welding Too Fast or Shaky 

If you do not weld at a proper speed and move too weak (or even too slowly), you can have issues with the reel tension and provide extra wire. The extra wire can then land in your weld puddle and contaminate it before it can freeze.

Make sure your wire feeding system is operating properly, and be steady as you weld, keeping a nice, even keel. When you rush, you can introduce porosity by improper shielding gas coverage and wire feed supply issues.

Dirty Equipment 

It is important to maintain your welding equipment. Make sure everything is clean and dry before starting your next welding adventure. Oil, rust, powder, and dirt on your welding gun can infiltrate your weld puddle and easily make porosity.

You should also take note to clean the material(s) you are welding. You should check for and remove things like:

  • Oil & water
  • Rust & oxidation
  • Dirt
  • Coatings (like zinc plating)
  • Paints & Laminates

It is a good idea to get a decent grinder and sander to remove coatings, paints, laminates, rust, and oxidation more easily.

Contaminated Gas 

If your gas supply is bad, you might as well start over. Contaminated gas is a sure-fire way to introduce a lot of porosity into your welds. Since your shielding gas is what prevents atmospheric gas from getting to your weld, applying a contaminated version of it directly to your weld puddle is an issue.

If you are not careful, you could end up in a situation akin to pouring your hot coffee on your computer.

Weld Gun Angle and Nozzle Distance

When MIG welding, you should always pay attention and make sure you are holding your gun mostly perpendicular to the weld joint, at most a 15-degree angle. If you hold the weld gun at too wide of an angle, the gas flow will spread out too wide and may suck in the atmosphere from the backside.

Not holding the nozzle close enough to the weld puzzle causes issues too. In fact, it can cause several problems, such as:

  • A decrease in the amount of shielding gas covering the weld puddle
  • An uneven weld
  • Allow atmospheric gas to come in contact with the weld puddle

If you hold the gun at too wide of an angle and the nozzle too far away, you are welding incorrectly. It is okay, though, with this problem only requiring practice.

Moisture Presence 

Water is necessary for all life as we know it. Conversely, water is not needed for a good weld. In reality, water presence will make the weld bad by causing porosity.

If the weld material is moist when it is welded, the heat will evaporate the liquid and turn it into hydrogen. The hydrogen gas will then cause porosity in the weld. Dry the weld material and try again.

An easy moisture presence fix is to heat the material to above boiling temperature (212°F) to evaporate it!

Sprays and Anti-spatter Compounds Misapplied 

To prevent porosity with MIG welding, do not overdo it with the gels, sprays, and anti-spatter compounds.

If you put too much of it on the weld gun, it can boil off into a gas in your weld puddle. It could also drip into the weld puddle and contaminate it. Do not use excess. Remember, use just the right amount.

Other Prevention Tips

Check your weld gun for grease, oil, and dirt. Weld shops are dirty and can easily contaminant your equipment. Also, check your equipment routinely, making sure that:

  • All O-rings are in good condition
  • Solenoids work properly
  • Hoses have no holes and nicks
  • Liner is clean
  • Electrodes are dry

You should also make sure that what you are welding is not opened on the backside. If you are welding in the front with the back exposed, the liquid weld will be exposed to the atmosphere and be unprotected by the shielding gas.

Forms of Porosity and How to Identify and Prevent Them

Porosity can take on four main forms. Each form of porosity presents itself in a different way, so you should be able to identify the type visually or through the use of x-rays.

Using all prevention methods will help you stop weld porosity. However, if you have identified a specific form of porosity on your welds, you should first focus on the solution to that form of porosity.

Distributed Porosity 

Distributed porosity mostly occurs because of fine, small cavities throughout the weld bead. Distributed porosity occurs because of improper gas shielding. Because of the poor gas shielding, small amounts of atmospheric gas (nitrogen and oxygen) gets absorbed into the molten weld pool.

Improper gas shielding can be caused by a few factors, and they all relate to your welding gas supply. Either there are leaks or kinks in your gas line that are causing too high or too low gas flow. It can also occur through the absorption of hydrogen gas from moisture on the gun or the weld piece.

Surface coatings like zinc plating can be another cause of porosity since it creates a ton of excess fumes when it is heated by the weld.

To prevent distributed porosity, you should make sure the weld material is stripped of any coating or paint and ensure all moisture is dried from the electrodes and weld material. You should also check your gas line for kinks, holes, cuts, and leaks to make sure you are supplying the proper amount of shielding gas.

Crater Pipe Porosity 

Crater pipe porosity is a long, narrow cavity on the weld bead that occurs after the weld pool solidifies and then collapses into a cavity.

The defects are called craters because they generally look like divots or dents in the weld. This might be more easily identified than some of the others, so keep an eye out for any craters in your welds.

Wormhole Porosity

Wormhole porosity is an elongated type of pore that occurs through the weld bead. It is a problem that you might only be able to see through a radiograph.

Wormholes are usually caused by a high amount of surface contamination, typically rust, paint, and/or coatings. It is most commonly found on welds near or on crevices and gaps in the material. 

To prevent wormhole porosity, you will want to excessively clean your weld material. Remove all paint, coatings, and rust before you start welding, and avoid welding the materials where there is bad geometry for welds. Use something to remove all surface contamination before welding, such as:

  • A grinder
  • A wire brush
  • A cloth

Surface Breaking Pores

Surace breaking pores are a type of porosity that comes in the form of pores that break through the top of the weld like small holes. Surface pores should be the easiest to identify.

Additionally, surface breaking pores are caused by the same reason as distributed pores. They are a form of distributed pores that broke through the surface of the weld during cooling.

Since they are caused by the same issues, they have the same solution. To prevent it, make sure everything is dry, make sure you have a proper supply of shielding gas, and remove any coatings and paint from your weld materials.


Additional Prevention Techniques

When you are welding, always make sure you are following the best weld practices recommended by the American Society of Welders. These are the basics but are easily overlooked, especially if you or overworked or in a rush.  

  • Wear Proper Safety Gear: Wearing the proper safety gear, like flame retardant clothing, your face shield, boots, gloves, and a covering for your hair, not only protects you but also prevents potential contaminants from getting in touch with the weld puddle.
  • Prepare Your Equipment: Check everything and check it twice. Make sure there is the proper tension to drive the wires, that the gas flow is set correctly, the polarity is correct, and that all the cables are connected securely. Also, remove any spatter, oil, or dirt and check for worn-out contact tips.
  • Select the Proper Gas: For welding carbon steel, stick with 75/25, which is 75% argon and 25% carbon dioxide gas. This is the perfect ratio for shielding gas to prevent porosity.
  • Maintain Around ⅜” Wire Stick-Out: This is the ideal length for welding. If the sizzling does not sound right, your wire stick-out is probably too long.
  • Prepare the Metal Properly: MIG wires collect contaminants and corrode very easily! Always grind away at it to clean it enough before striking a welding arc. 

Select the Proper Equipment and Settings

You should also select a proper wire. The two most common wires are AWS ER70S-3 and ER70S-6. ER70S-3 wire should be your go-to and is the best for all-purpose welding. ER70S-6 should only be used in cases where you are welding on rusty steel. It has extra deoxidizers that combat rust contamination.

Choose the proper amperage and voltage for the weld. The right voltage and amperage will depend upon the metals you are welding, the thickness, joint type, wire speed, and more. You can reference the chart usually located at the door of your wire feed system for a reference table.

Use the Proper Weld Techniques and Positions

To prevent porosity from forming in your welds, you should also follow the proper welding techniques for the joint types you are working on. Use the “push” or the “pull” method and concentrate on having the correct travel and work angles. 

Lastly, you should know how to weld from all positions to avoid porosity, as failing to apply your techniques properly due to position can cause porosity.

  • Vertical Position: This is one of the harder positions because you are fighting gravity. Making a vertical down motion is easiest on thin metals, whereas the vertical up technique is better suited for thicker metals because the penetration is deeper.
  • Horizontal Position: This position is also affected by gravity. For horizontal welding, hold the gun at a 0 to 15-degree work angle for best results.
  • Overhead Position: Lower your voltage way down to keep the weld puddle small and prevent it from falling and travel fast!
  • Flat Position: The flat position is the easy weld position because gravity works in your favor. For a simple butt weld, hold the gun at 90 degrees to the weld piece and have a travel angle of 5 to 15 degrees, then use a steady back and forth motion to join the pieces.

Each type of porosity forms for different reasons. If you keep finding the same issues over and over in your welds, you need to make changes now to prevent it! Porosity prevention will save you time and money.

What are the Outcomes of Porosity? 

The potential outcomes of unnoticed porosity are not good. Because of the nature of porosity (a cavity within the weld), the strength of the weld will be severely compromised. Depending on the severity and type of porosity, the material can break at the weld, corrode at the weld, or the weld may crack.

Porosity that is not immediately found can show up months or even weeks later when corrosion and rust start to show up on and around the weld. Obviously, this is not ideal, so take every action you can to prevent porosity and produce high-quality welds!


In summary, the top causes of porosity issues in MIG welding are wet or contaminated gases, an incorrect weld gun angle, placing the wire not close enough to the nozzle, a dirty or coated base material, or an incorrect shielding gas level. Also, proper welding techniques and safety measures should always be followed to help prevent porosity.

There are four types of porosities: distributed, surface breaking, wormhole, and crater. They can all be identified and troubleshot through various means. Now that you know more about porosity, you can do your duty to prevent it!


MIG Welding: The Basics for Mild Steel |

Guide for the Visual Examination of Welds | American Welding Society (

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