Troubleshooting Porosity in MIG Welds
Identify and fix the common causes of porosity (gas pockets) in MIG welds.
Why does porosity happen?
Porosity forms when gas gets trapped in the weld puddle as it solidifies. The molten metal can absorb gases from contamination, atmosphere, or the shielding gas itself. As the puddle freezes, these gases try to escape but get trapped as bubbles.
Why it matters: Porosity reduces the effective cross-section of the weld, concentrating stress at the voids. A weld with significant porosity may only have 60-70% of its intended strength. In structural or pressure applications, this can lead to failure.
Porosity appears as small holes or voids in the weld bead, either on the surface or internally. It weakens the weld and often indicates a process problem that needs correction.
What Porosity Looks Like
- Surface porosity: visible pinholes or craters on the bead surface
- Subsurface porosity: only visible when grinding or cutting the weld
- Wormhole porosity: elongated tunnels through the weld
Common Causes
- Contaminated base metal (oil, rust, paint, moisture)
- Insufficient shielding gas coverage
- Contaminated or wet shielding gas
- Dirty or damaged consumables
- Excessive stick-out
- Wind or drafts in work area
Check Shielding Gas Flow
Verify regulator shows 25-30 CFH. Trigger gun and feel for gas flow at nozzle. Check all connections for leaks using soapy water - bubbles indicate a leak.
Pro Tip
A cheap flow meter at the gun tells you what's actually reaching the weld, not just what's leaving the bottle.
Inspect Consumables
Remove and inspect contact tip for wear or buildup. Check nozzle for spatter accumulation. Examine diffuser for damage. Replace any suspect components.
Pro Tip
Contact tips are consumables - replace them before they cause problems, not after.
Clean the Base Metal
Grind to bright metal removing all rust, scale, paint, and oil. Use acetone or brake cleaner for stubborn contamination. Ensure surface is completely dry before welding.
Warning
Some cleaners are flammable - let them fully evaporate before striking an arc.
Pro Tip
If you can see any discoloration, it's not clean enough.
Check for Drafts
Identify any air movement in the work area from fans, open doors, or HVAC. Block drafts with welding screens or cardboard. Increase gas flow slightly (30-35 CFH) if drafts can't be eliminated.
Pro Tip
A piece of tissue paper taped near the joint will show you air movement you can't feel.
Verify Stick-Out Distance
Maintain 3/8" to 1/2" stick-out (wire extending past contact tip). Excessive stick-out reduces gas coverage at the arc. Adjust technique or trim wire as needed.
Pro Tip
Mark your nozzle with a paint pen at the correct distance as a visual reference.
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The "pop test" - Strike an arc on clean scrap. If you hear popping or see spatter flying everywhere, you have a gas problem before you even start on the real work.
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Check your hoses - Old rubber hoses absorb moisture and contaminate your gas. Replace them every few years, or upgrade to braided stainless lines.
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Morning condensation - If your gas bottle sits outside overnight, moisture can form inside the regulator. Crack the valve and purge for 10 seconds before connecting.
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The acetone trick - For really stubborn contamination, wipe the joint with acetone before welding. Let it fully evaporate (30 seconds) before striking an arc.
Thinking more gas = better protection Cranking gas flow to 50+ CFH actually creates turbulence that pulls in atmospheric contamination. Stick to 25-35 CFH for most applications.
Ignoring the spatter buildup Spatter inside your nozzle restricts gas flow and creates turbulence. Clean it out regularly with anti-spatter compound.
Welding over primer or galvanizing These coatings vaporize and contaminate the weld. Always grind to bare metal, even if it takes extra time.