What Is Porosity In Welding?

By Joe Stephens •  Updated: 08/30/20 •  7 min read

Welding is an important trade, and welders produce products that hold a lot of responsibility. If welding is not done right, the result can be catastrophic because parts can break, people can get injured, and reputations can be ruined. One common weakness in welded materials is porosity. This article answers the question: “what is porosity in welding,” as well as the causes, how to prevent it, if it can be fixed, and the acceptable amount of porosity in a finished product.

Welding Porosity

Porosity is a form of metal contamination. It results in a weak, bubble-filled weld that does not meet code because it can weaken structures and cause them to collapse.

Example of Welding Porosity

Porosity in welding happens when a molten weld pool absorbs and traps hydrogen, oxygen, and nitrogen gas. Upon solidification, these gases get trapped inside the weld metal, resulting in cavities.

Porosity can be random: oriented at random, uneven distances. It can also be isolated: spaced one-inch apart from all sides.

There are also different types of porosity in welding as well. These include:

  • Distributed Porosity – Pores that are distributed throughout the weld.
  • Surface-Breaking Pores – Pores that break the surface.
  • Wormhole – Elongated pores that show up resembling a herringbone pattern on the radiograph
  • Crater Pipes – A shrinkage cavity at the end of the Weld Run caused by shrinkage during solidification.

What causes porosity in welding?

Now that you know what porosity is, you might be wondering what causes it. This is where things get a little difficult since there aren’t just one or two things that can cause porosity, but a whole heap of things.

Below, you’ll find a list of the more common reasons for porosity:

  • Gas in the cylinder.
  • Air drafts caused by overhead and floor fans, open doors, and machinery.
  • Moisture from morning dew, water, and condensation.
  • Weld spatter obstructing the nozzle opening.
  • Distance between the weld nozzle and the weld puddle.
  • Inappropriate angle between the GMAW, FCAW, or SMAW guns and the joint.
  • Contamination from paint, oil, glue, sweat, and grease.
  • Damaged, obstructed, or contaminated equipment, like a kinked gas hose.
  • Contaminated welding gas resulting in an improper dew point.
  • Open weld joint at the root.
  • Damaged o-ring seals on guns or torch caps.
  • Defective or damaged gas solenoid at the wire feeder.
  • Oxidation as a result of welding over rust.
  • Zinc plating compounds.
  • Improper care and storing of equipment.
  • High gas flow.
  • Improper application and use of anti-spatter sprays, gels, or compounds.
  • Contaminated weld filler metals, and soiled gloves that come into contact with the hot welding arc.
  • Contaminated GMAW gun liners.
  • Certain types of anti-spatter spray, along with excess amounts of it.

Besides this long list, it’s also important to know that each type of porosity we mentioned earlier has its own unique set of causes. Focusing on the different types and their causes can be easier than just focusing on everything all at once.

Distributed Porosity and Surface Breaking Pores


example of distributed porosity

Distributed Porosity


Gas line leaks, a high gas flow rate, excessive turbulence, and draughts in the weld pool often lead to porosity. In fact, if the shielding gas has just 1% of air entrainment, distributed porosity will result. Anything greater than 1.5% of air entrainment will result in surface-breaking pores.


Example of Surface Breaking Pores

Surface-Breaking Pores


If you are welding materials that have surface coatings, for instance, zinc coatings and primer paints, there is a high chance of porosity since these coatings generate large amounts of fumes when welding. T-joints are especially vulnerable, more so than butt joints, to porosity, and the risk is increased if the welder is fillet welding both sides.

There are also weldable primers, called low zinc primers. If these primers meet the appropriate thickness per the manufacturer’s recommendation, it is not necessary to remove any of the primers before welding.

Hydrogen gas commonly originates from moisture that occurs as a result of improperly dried electrodes, the workpiece surface, fluxes, or grease and oil on the workpiece or filler wire.



Example of Wormhole Porosity



Wormholes are caused by excessive gas that forms in crevices, especially in the gap that is located beneath the vertical piece in a horizontal-vertical, and T-joints that are fillet welded on both sides.

Like with distributed porosity, the primed vertical member in T-joints must meet the manufacturer’s recommended thickness, which is usually 20µm.

Crater Pipes


Example of Crater Pipe

Crater Pipe


When there is some gas porosity, a crater pipe can form during the final solidification. This is caused by shrinkage and is often exaggerated by the conditions present when the volume change happens during liquid to solid transformation. Practically speaking, the rapid solidification that occurs in the weld pool happens when the welding arc is extinguished. If you stop the welding wire from attaching to the weld pool before you extinguish the welding arc, a crater is more likely to form.

How to prevent porosity in welding

With all these issues that can produce porosity, it may seem impossible to prevent. However, it’s actually the complete opposite. Porosity can be easy to avoid and gets even easier as you get more welding experience.

The easiest ways to avoid porosity are precautions that every welder should already be practicing. These include:

  • Keeping your materials clean.
  • Checking your gas flow.
  • Checking your equipment.
  • Monitoring your workspace conditions.
  • Understanding the proper welding procedure.

Another easy way to avoid porosity appearing in your welds is to study and memorize all the different causes we listed above. This alone can drop the rate of porosity being produced dramatically.

Besides these main prevention methods, there are a lot of other things you can do depending on the type of porosity. These methods include:

Distributed Porosity and Surface Breaking Pores

  • Gas Source: Identify the gas source and remove it. It may be coming from an air leak, weld pool turbulences, improper application or deoxidant composition of filler, excessive-high shielding gas flow, and draughts. You need to inspect and address these issues.
  • Hydrogen: Dry the flux and electrode per the manufacturer’s recommendations, and clean the surface of the workpiece.
  • Surface coatings: Clean the joint edges right before welding, and check the thickness of the weldable primer.


  • Gas Generation: Clean surfaces where the weld will occur and remove surface contaminants like rust, grease, oil, and residue. Remove surface coatings, and check the primer thickness.
  • Joint Geometry: Avoid joint geometries that result in cavities.

Crater Pipes

  • Remove Stop: Run-off tags allow the welding arc to extinguish outside the welded joint. You need to grind out the weld runs top crater, and then you continue with the next electrode.
  • Welder Technique: Use crater fill or slope-down functions, and add filler in anticipation of weld pool shrinkage.

Welding Porosity Prevention Video Guide




How to fix porosity in welds

Even with all the different ways to prevent porosity, there will always be a small chance of it appearing in your welds. Thankfully, porosity can be fixed under certain circumstances.

You first need to detect the presence of porosity by using either a penetrant or a magnetic particle inspection technique, such as radiography or ultrasonic inspection (for subsurface imperfections). Small pores, especially in thicker sections, are difficult to detect.

The porosity itself cannot exactly be fixed. Rather, you can fix the weld by removing the localized gouging, or grinding it out. This is only if the porosity is contained, however. If it is widespread, you need to remove the entire weld, prepare the joint again, and weld according to the welding procedure.

How much porosity is acceptable in a weld

There is no general sound and true answer to this question, as it depends on your welding construction code. That being said, the American Welding Society mandates the following: diameters of visible porosity should be 3/8 inches (or 9.4mm) or less, in any linear inch of the weld, and be 3.4 inches (or 19mm) in a 12-inch length of the weld.

When in doubt, you need to check your welding construction code and redo the weld.


Porosity is a common defect that happens in welding, and it can be caused by a number of different things. Fortunately, the prevention rate is about 90%, and most rejects can be redone to meet welding codes. The important thing is to be aware, knowledgeable, and honest about the work you are doing, and fix the mistakes that you make.

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Joe Stephens

Joey has over 20 years of experience working in the welding industry and now works with providing readers with intensive reviews. Joey has also self-published an e-book and has written countless articles regarding welding information and safety.

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