Porosity in MIG welds shows up as pinholes, surface bubbles, or internal voids that weaken the joint. It’s one of the most common—and preventable—issues in shop welding. The root cause is always the same: contamination or inadequate shielding gas coverage.
Key Takeaways
- Porosity is caused by gas coverage failure or contamination
- The most common issues are gas flow, leaks, and dirty material
- Wind, improper nozzle distance, and bad technique can all introduce air
- Fixes are usually simple: clean, adjust flow, check equipment
What’s Causing the Problem
1) Poor Shielding Gas Coverage
- Flow rate too low or too high (turbulence)
- Leaks in hoses or fittings
- Blocked or dirty nozzle/diffuser
2) Contaminated Base Metal
- Oil, rust, mill scale, paint, or moisture
- Galvanized coatings releasing gas during welding
3) Environmental Factors
- Wind or drafts blowing shielding gas away
- Outdoor welding without protection
4) Incorrect Technique
- Stickout too long (reduces gas effectiveness)
- Travel angle pushing gas away from puddle
- Moving too fast for proper shielding
How to Fix It
Step 1: Set Proper Gas Flow
- Typical MIG range: 20–30 CFH (9–14 L/min)
- Indoors: stay near 20–25 CFH
- Outdoors: increase slightly or use wind blocks
Step 2: Check for Leaks
- Inspect all connections from tank to gun
- Listen for hissing or use leak detection fluid
- Replace cracked hoses or worn O-rings
Step 3: Clean the Material
- Grind to bright metal before welding
- Remove coatings, oil, and moisture
- Use acetone if needed (let fully evaporate)
Step 4: Inspect Gun Components
- Clean or replace nozzle and diffuser
- Ensure no spatter is blocking gas flow
- Confirm proper contact tip size
Step 5: Adjust Technique
- Keep stickout around 3/8″–1/2″ (10–12 mm)
- Maintain steady travel speed
- Use correct gun angle (10–15° push or drag)
Common Mistakes to Avoid
- Running gas too high (creates turbulence)
- Welding over dirty or painted surfaces
- Ignoring small gas leaks
- Letting spatter clog the nozzle
- Welding in open air without shielding from wind
Best Settings / Guidelines
| Parameter | Typical Range |
|---|---|
| Gas Flow | 20–30 CFH (9–14 L/min) |
| Stickout | 3/8″–1/2″ (10–12 mm) |
| Travel Angle | 10–15° |
| Wire Speed | Varies by machine/material |
| Voltage | Varies by machine/material |
Always confirm with your machine’s chart or manufacturer specs.
Safety Notes
- Wear proper eye protection (ANSI Z87.1) and welding helmet
- Avoid inhaling fumes—especially on coated or galvanized steel
- Ensure adequate ventilation when cleaning with solvents
- Keep gloves and sleeves dry to prevent contamination and burns
FAQ
What does porosity look like in a weld?
Small holes, bubbles, or pits on the surface or inside the weld bead.
Can porosity be fixed after welding?
Not reliably—grind out the affected area and reweld with proper prep.
Is too much gas flow bad?
Yes—excessive flow can create turbulence and pull in air.
Does wire type affect porosity?
Yes—low-quality or contaminated wire can introduce gas pockets.
Can humidity cause porosity?
Yes—moisture on the material or in the air can contribute.
Sources Checked
- American Welding Society (general guidance on shielding gas and weld quality)
- Lincoln Electric application notes on MIG porosity causes
- Miller Electric troubleshooting resources for shielding gas issues
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