Porosity is caused when gas becomes trapped in the weld as it solidifies, leaving small holes or cavities. The most common causes are contaminated parent material, moisture, incorrect shielding gas flow, damp consumables and poor welding technique. Clean, dry material, the correct shielding gas and flow rate, properly stored consumables and protection from draughts prevent the vast majority of porosity.
This guide answers the most common porosity questions — what causes it, why MIG and TIG welds go porous, why it appears at the end of a weld, and how to repair a porous weld properly.
What causes porosity in welding?
Porosity is caused when gas becomes trapped in the weld as it solidifies, leaving small holes or cavities. Common causes include contaminated parent material, moisture, incorrect shielding gas flow, damp consumables or poor welding technique.
To prevent porosity, ensure the material is clean and dry, use the correct shielding gas and flow rate, store consumables correctly and protect the weld area from wind or draughts. Regularly inspect your equipment for gas leaks or damaged hoses. Consistent welding consumables and correct preparation are essential for producing sound, porosity-free welds.
Why is my MIG weld porous?
Porosity in MIG welding is usually caused by poor shielding gas coverage. This may result from low gas flow, excessive gas flow causing turbulence, wind, blocked gas nozzles or leaks in the gas supply. Contaminated steel — rust, paint, oil or moisture — can also introduce gas into the weld pool.
Check your gas flow, inspect the torch and consumables, clean the parent material thoroughly and ensure the correct welding parameters are being used. High-quality MIG wire and properly maintained equipment help produce consistent, defect-free welds.
Why is my TIG weld porous?
TIG weld porosity is often caused by contamination. Oil, grease, moisture, oxide layers or inadequate shielding gas can all introduce gas into the molten weld pool.
Ensure the parent material and filler rod are clean, use pure Argon where appropriate and avoid excessive tungsten stick-out. Check that the gas nozzle, collet and hoses are in good condition. Maintaining clean working practices is one of the most effective ways to eliminate porosity in TIG welding.
Why does porosity appear at the end of my weld?
End crater porosity usually occurs because shielding gas is removed too quickly after the arc stops. As the weld cools, the molten pool can absorb atmospheric contamination if gas protection ends prematurely.
Many modern welding machines include post-flow settings that continue shielding gas after welding has finished. Maintaining the torch position until the weld has cooled and using the correct post-flow time helps prevent end crater porosity.
How do I stop porosity?
Preventing porosity starts with good preparation. Clean the parent material, remove rust, oil, paint and moisture, and use clean filler materials. Check shielding gas flow, inspect hoses and regulators for leaks, protect the weld from wind and use quality consumables that have been stored correctly.
If porosity continues, review your welding parameters and equipment to identify the source of contamination.
Can shielding gas cause porosity?
Yes. Incorrect shielding gas selection, inadequate flow or gas leaks can all cause porosity. Too little gas allows atmospheric contamination into the weld pool, while excessive gas flow may create turbulence that draws air into the weld.
Always use the recommended shielding gas for the material being welded and verify gas flow before starting work.
Can damp welding wire cause porosity?
Yes. Moisture on welding wire or other consumables can introduce hydrogen and other gases into the weld, increasing the risk of porosity.
Store welding wire in clean, dry conditions and keep spools protected from humidity and contamination when not in use. Correct storage helps maintain consistent welding performance.
Can contaminated material cause porosity?
Yes. Rust, mill scale, paint, oil, grease and moisture can all generate gas during welding, causing porosity. Cleaning the weld area before welding is one of the simplest and most effective ways to improve weld quality.
Use dedicated stainless steel brushes when welding stainless steel to avoid cross-contamination.
Can wind cause porosity?
Yes. Wind can disperse shielding gas before it protects the weld pool, allowing oxygen and nitrogen to contaminate the weld. This is particularly common during outdoor MIG and TIG welding.
Where possible, use welding screens, increase protection from draughts or select a welding process better suited to outdoor conditions, such as MMA or self-shielded flux-cored welding.
How do I repair porous welds?
The defective section should normally be removed by grinding or gouging before rewelding. Simply welding over porosity rarely eliminates the problem and may trap defects beneath the new weld.
Identify and correct the cause of the porosity before carrying out the repair to prevent the defect from recurring.
Prevention starts with clean material and quality consumables
Almost every porosity cause comes back to contamination, moisture or shielding. Clean, dry material, leak-free gas equipment, correct flow and properly stored, quality consumables remove most of the risk before you strike an arc. Where damp or inconsistent wire is the culprit, switching to a reliably stored, quality MIG wire often resolves recurring porosity.
For consumable advice, explore the welding consumables guide or contact the Weldability SIF technical team.