Porosity is one of the most common welding defects and can quickly turn an otherwise good weld into a failed one. Whether you’re MIG welding, TIG welding or using flux-cored wire, porosity can weaken weld strength, reduce appearance quality and lead to costly rework.
The good news is that porosity is usually easy to identify and even easier to prevent once you understand the root cause.
In this guide, we’ll explain what porosity is, what causes it and how to stop it from ruining your welds.
What Is Porosity?
Porosity occurs when gas becomes trapped inside the weld pool as the molten metal solidifies.
The result is small holes, bubbles or voids within the weld bead.
Porosity may appear:
- On the surface of the weld
- Throughout the weld bead
- As isolated pinholes
- As clusters of holes
Even small amounts of porosity can significantly reduce weld strength and may cause welds to fail inspection.
What Causes Porosity?
Porosity is almost always caused by contamination or poor shielding gas coverage.
Common causes include:
Poor Shielding Gas Coverage
The shielding gas protects the molten weld pool from atmospheric contamination.
If insufficient shielding gas reaches the weld, oxygen, nitrogen and moisture can become trapped in the weld metal.
Possible causes:
- Empty gas cylinder
- Incorrect gas flow rate
- Leaking gas hoses
- Damaged torch components
- Blocked gas diffuser
- Excessive welding drafts
Dirty Material
Oil, grease, paint, rust, mill scale and moisture can all release gases into the weld pool.
Before welding, always clean the joint area thoroughly.
Recommended cleaning methods:
- Stainless steel wire brush
- Acetone or solvent cleaner
- Grinding
- Mechanical preparation
Incorrect Gas Flow Rate
Many welders assume more gas is always better.
This is not true.
Too little gas allows contamination into the weld.
Too much gas creates turbulence which actually draws air into the shielding gas stream.
Typical starting points for gas flow:
MIG Welding
10–15 LPM
TIG Welding
6–12 LPM
Outdoor Welding
Increase slightly as required
Always adjust according to nozzle size and environmental conditions.
Wind and Drafts
Even a small breeze can disrupt shielding gas coverage.
Common sources include:
- Workshop fans
- Open doors
- Extraction systems
- Outdoor working conditions
If welding outside, consider:
- Wind shields
- Temporary screens
- Increasing gas flow slightly
Contaminated Filler Materials
MIG wire, TIG filler rods and flux-cored wire should always be stored correctly.
Contamination from moisture, oil or dirt can introduce gases into the weld pool.
Store consumables in a clean, dry environment.
Discard heavily contaminated filler materials.
Moisture Contamination
Water is a major source of hydrogen contamination.
Moisture can be present on:
- Base materials
- Filler metals
- Gloves
- Workbenches
- Shielding gas systems
Always ensure materials are dry before welding.
How to Identify Porosity
Common signs include:
Surface Pinholes
Small visible holes appearing on the weld surface.
Wormholes
Long narrow cavities often found in flux-cored welding.
Internal Porosity
Not always visible externally and often detected through:
- X-ray inspection
- Ultrasonic testing
- Destructive testing
MIG Welding Porosity Troubleshooting
If you’re experiencing porosity while MIG welding, check:
✓ Gas cylinder contains gas
✓ Regulator is functioning correctly
✓ Flow rate is 10-15 LPM
✓ No leaks in gas hoses
✓ Contact tip and nozzle are clean
✓ Material is free from contamination
✓ Correct torch angle is being used
✓ No strong drafts are present
TIG Welding Porosity Troubleshooting
For TIG welding, also check:
✓ Gas lens condition
✓ Tungsten contamination
✓ Cup size suitability
✓ Post-flow settings
✓ Filler rod cleanliness
✓ Shielding gas purity
TIG welding is particularly sensitive to contamination, especially when welding stainless steel and titanium.
Preventing Porosity Before It Starts
The best welders don’t spend their time fixing porosity—they prevent it.
Follow these simple steps:
- Clean all materials before welding.
- Check shielding gas flow rates.
- Inspect hoses and regulators regularly.
- Keep consumables clean and dry.
- Protect welds from drafts.
- Maintain your welding torch and consumables.
- Verify gas cylinders contain the correct gas.
These simple checks eliminate the majority of porosity issues before they occur.
Final Thoughts
Porosity is one of the easiest welding defects to prevent once you understand its causes.
In most cases, the issue can be traced back to shielding gas problems, contamination or poor welding preparation.
By maintaining clean materials, ensuring proper gas coverage and regularly inspecting your equipment, you’ll produce stronger, cleaner welds and avoid costly rework.
At WeldAssist, we’re committed to helping welders diagnose problems faster and improve weld quality through practical, easy-to-follow guidance.
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