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What Is Porosity in Welding
You finish a weld bead, clean it with a wire brush, and the bead still looks wrong. You may see tiny pinholes, shallow pits, or dark spots along the weld. Sometimes the surface looks fine until you grind it back or cut it open, and then small voids show up inside the metal.
That can be confusing when you are still learning. A rough bead is one thing. Holes in the weld are another. In the shop, people may simply call them “holes in the bead.” In welding, one common cause is porosity, especially when small holes keep showing up in or around the same weld area.
Photo by @DanWelds (TikTok)
What Is Porosity in Welding?
Porosity in welding is a weld defect caused by gas trapped in the molten weld pool as it solidifies. It may appear as pinholes, pits, or hidden voids inside the weld. Common causes include dirty metal, moisture, poor shielding gas coverage, wind, long stickout, and damp filler material.
Are Small Holes in a Weld a Problem?
Small holes can be a warning sign, but they do not always mean the weld has porosity. A mark that looks like a hole may come from trapped gas, an end crater, slag being chipped away, burn-through, or aggressive cleanup.
A visual check is usually enough for everyday welding. It helps you understand what is happening and spot common surface issues.
When welds are critical and involve strength or safety, visual inspection alone is not sufficient. In inspection practice, visual testing (VT) is often combined with radiographic (RT) or ultrasonic testing (UT) to confirm internal weld quality.
Quick Weld Hole Diagnosis Guide
| What you see | Most likely issue | What to check first |
|---|---|---|
| Pinholes | Gas porosity | Shielding gas flow, leaks, wind |
| Pits after grinding | Subsurface porosity | Base metal contamination |
| Holes at weld end | Crater defect | Arc termination technique |
| Clustered holes | Cluster porosity | Oil, rust, dirty spot |
| Tunnel voids | Wormhole porosity | Gas coverage, travel speed |
Common Weld Porosity Appearance
Holes in a weld do not all mean the same thing. The size, shape, and location can tell you where to look first.
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Pinholes: Small round holes on the bead often suggest porosity, especially if several appear close together.
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Small pits: Shallow pits may come from porosity, cleanup, slag removal, or contamination.
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Holes after grinding: These often indicate subsurface porosity exposed during material removal.
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Crater marks: A hole at the end of a bead is often an end crater from arc termination, not porosity.
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Burn-through: Burn-through is usually caused by excessive heat input, or poor fit-up causing metal collapse.
Photo by @RichardCranium83176 (TikTok)
Types of Porosity in Welding
Porosity does not always look the same. Sometimes it appears on the surface. Other times, it stays hidden until the weld is opened up.
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Surface porosity: Holes are visible on the bead. Check cleaning, shielding gas, wind, nozzle condition, stickout, and gun angle.
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Internal porosity: Voids stay hidden until you grind, cut, or inspect the weld. Check joint prep, moisture, and filler storage.
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Cluster porosity: Pores are grouped in one area. Look for a dirty spot, rust pocket, trapped oil, or a brief loss of gas coverage.
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Uniform porosity: Pores are spread through the weld. Look for a repeated setup issue, such as poor gas coverage, a leak, long stickout, or dirty material.
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Wormhole porosity: Long, tube-like voids appear in the weld. Treat this seriously, especially on load-bearing or code-controlled work.
Source: https://www.youtube.com/watch?v=aSeiMj1JOSc
What Causes Porosity in Welding?
Most porosity starts with dirty metal, moisture, or poor shielding gas. In real shop work, it is usually a mix of small issues. Check metal, filler, gas coverage, and machine setup together, not just one setting.
Dirty or Contaminated Metal
Dirty base metal is one of the most common causes of porosity. Oil, paint, rust, mill scale, grease, cutting fluid, and residue release gas under arc heat. If the gas is trapped in the weld pool, porosity forms.
A quick wipe is not enough when contamination is heavy or spread near the joint. Clean beyond the weld area, not just the weld line.
Poor Shielding Around the Weld Pool
Shielding gas protects the molten weld pool from air. When coverage breaks down, oxygen and nitrogen can enter the arc and cause porosity.
Common causes include wind, drafts, incorrect flow rate, clogged nozzle, loose fittings, leaks, long stickout, or incorrect gun angle. A full gas bottle does not help if the puddle is not actually being shielded.
Moisture in the Metal or Filler
Moisture can introduce gas into the weld pool. Damp base metal, wet electrodes, poor storage of filler rods, or moisture in flux-core wire all contribute. At welding temperatures, moisture breaks down and releases gas. If the gas cannot escape before the weld solidifies, pores remain.
For flux-core welding, also check wire storage, polarity, and slag coverage. A clean joint can still become porous if the wire is damp or the setup is wrong.
Source: https://www.youtube.com/shorts/UCNHhcIuf8g
Is Weld Porosity a Serious Defect?
Porosity can be serious depending on its size, quantity, location, and the purpose of the weld.
Small pores on a practice weld are very different from porosity in structural, lifting, pressure, or load-bearing joints. Large pores, clustered porosity, wormholes, or porosity near the root or fusion line deserve closer attention. Welds exposed to vibration, impact, bending, or repeated loading also require higher standards.
For critical welds, appearance is not enough. Use the required inspection standard or get a qualified welding professional involved.
In most shop cases, porosity is usually linked to shielding gas issues or contamination rather than machine failure.
Source: https://www.youtube.com/watch?v=IJKn4pUOYgs
Porosity vs Other Weld Defects
Porosity is caused by gas trapped during solidification. Defects such as crater formation, burn-through, and slag inclusion have different mechanical or process-related causes.
| Issue | Looks like | Key difference |
|---|---|---|
| Porosity | Small holes | Gas trapped inside weld |
| Crater | Large hole at end | Occurs when arc stops |
| Burn-through | Collapsed metal | Heat damage, not gas issue |
| Slag inclusion | Irregular voids | Solid material trapped in weld |
How to Prevent Porosity in Welding
The best way to avoid porosity is to keep gas out of the weld pool. Start with the basics before blaming the machine or the material.
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Clean the joint thoroughly. Remove oil, paint, rust, heavy mill scale, moisture, cutting fluid, and residue. Contamination can release gas during welding and cause porosity.
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Protect shielding gas coverage. Use the correct flow rate. Keep the nozzle clean. Check for leaks. Block wind or drafts. Poor shielding allows air to enter the arc zone.
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Keep filler materials dry. Store electrodes, rods, flux, and wire properly. Do not expose moisture-sensitive materials to humidity. Moisture can generate gas during welding.
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Control stickout and gun angle. Excessive stickout reduces shielding. Poor torch angle can reduce shielding effectiveness and increase the risk of air contamination.
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Dial in welding parameters. Adjust voltage, amperage, wire speed, travel speed, and electrode angle. Stable settings help maintain a consistent arc.
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Check polarity, especially in flux-core welding. Incorrect polarity destabilizes the arc. It can lead to weld defects.
If porosity appears suddenly, stop and check the gas flow, nozzle, drafts, stickout, and filler condition before continuing. A few minutes of checking can save a lot of grinding and rework.
Source: https://www.youtube.com/watch?v=82LQ2cMoE5c
Conclusion
Porosity may appear as holes on the bead, or it may stay hidden below the surface.
Small defects in practice welds can help you learn. On structural, pressure, lifting, or safety-related welds, porosity should be taken seriously. Clean the joint, protect the puddle, keep filler dry, and use steady settings to prevent most porosity before it starts.
🧐 What Is Porosity in Welding FAQ
1. Can porosity be hidden inside a weld?
Yes. It can remain subsurface and only become visible after grinding, cutting, or inspection.
2. Why does porosity appear after grinding?
Grinding exposes subsurface voids that were already present inside the weld.
3. Can porosity happen with shielding gas?
Yes. Poor coverage due to wind, leaks, or incorrect setup can still allow air contamination.
4. Is one pinhole enough to reject a weld?
It depends on the application and inspection standard. Critical welds have stricter acceptance criteria.
5. Can flux-core welds get porosity too?
Yes. Porosity can still occur in flux-core welding. Damp wire, wrong polarity, poor travel angle, or welding over dirty metal can trap gas in the weld and cause porosity.
👏 You may be interested in the following:
- Most Common MIG Welding Defects
- Common FCAW Welding Mistakes And Defects
- Common TIG Welding Defects And How To Deal With Them
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