Porosity is trapped gas in solidified weld metal. It shows up as round or elongated voids on the weld surface or inside the weld cross-section. The gas comes from contamination, inadequate shielding, moisture, or improper settings. Fix the gas source and porosity stops.
Most porosity traces back to one of four problems: the joint wasn’t clean, the shielding gas wasn’t protecting the puddle, the consumables were wet or contaminated, or the settings were wrong. Identifying which type of porosity you have tells you which cause to chase.
Types of Porosity
Scattered Porosity
Small, round pores distributed randomly throughout the weld bead. No pattern, no concentration in one area.
What it means: A general gas coverage problem. The shielding is inadequate across the whole weld, but not completely absent.
Typical causes:
- Gas flow rate too low (under 20 CFH for most MIG applications)
- Minor contamination across the joint (light oil film, mill scale)
- Slight moisture on the base metal
- Gas flow rate too high (over 50 CFH creates turbulence that sucks in air)
Clustered Porosity
A group of pores concentrated in one spot. The surrounding weld may be clean.
What it means: A localized problem at that specific location. Something was different at that spot.
Typical causes:
- A spot of grease, paint, or heavy contamination
- A gap in shielding gas coverage at that location (nozzle blockage, gas leak, draft from one direction)
- Tack weld with contamination that wasn’t cleaned before the production weld
- Arc start/stop where gas pre-flow or post-flow was insufficient
Linear Porosity
Pores arranged in a line, usually along the weld axis or along one toe.
What it means: The gas problem follows the joint. Something about the joint itself is generating gas.
Typical causes:
- Contamination in the root (rust, oil, paint, galvanizing on the back side of the joint)
- Root opening too wide, allowing atmospheric gas to enter from the back side
- Moisture trapped in the joint fit-up
- Improper tack welds releasing gas as they’re consumed by the production weld
Piping Porosity (Wormholes)
Elongated, tubular pores that extend from the interior toward the surface. They look like worm tracks in a cross-section.
What it means: Gas is being generated within the weld pool itself and trying to escape as the metal solidifies.
Typical causes:
- Wet stick electrodes (moisture in the flux coating decomposes into hydrogen)
- Heavy rust or scale on the base metal (iron oxide reacts with carbon in the weld to produce CO gas)
- Galvanized steel (zinc coating vaporizes at welding temperature)
- Excessive moisture in flux (SAW, FCAW)
Causes and Fixes
| Cause | Process | Evidence | Fix |
|---|---|---|---|
| Contaminated base metal | All | Porosity concentrated near joint surfaces | Clean within 1" of joint: grind, solvent wipe, wire brush |
| Low gas flow rate | MIG, TIG | Scattered porosity across the bead | Set flow to 20-25 CFH (MIG) or 15-20 CFH (TIG) |
| Excessive gas flow rate | MIG, TIG | Scattered porosity, especially on edges | Reduce to 25-35 CFH maximum. Turbulence draws in air. |
| Wind/drafts | MIG, TIG | Porosity on windward side of bead | Wind screens, switch to FCAW-S, reposition |
| Gas hose leak | MIG, TIG | Inconsistent porosity, worse at some times | Check all connections with leak solution. Replace damaged hose. |
| Wrong gas | MIG, TIG | Heavy porosity, unstable arc | Verify gas type. 100% CO2 on aluminum = severe porosity. |
| Wet electrodes | Stick, FCAW | Piping porosity, wormholes | Store E7018 in rod oven (250-300F). Re-bake if exposed. |
| Moisture on base metal | All | Scattered or piping porosity | Preheat to evaporate moisture (200-250F) |
| Dirty MIG wire | MIG | Scattered porosity, erratic arc | Replace wire, check wire feeder for contamination |
| Clogged gas nozzle | MIG | Porosity that gets worse over time | Clean spatter from inside the nozzle. Replace if damaged. |
| Excessive arc length | Stick | Scattered porosity across bead | Shorten arc to one rod diameter |
| Travel speed too fast | All | Porosity in the trailing edge of the bead | Slow down to let gas escape before solidification |
| Galvanized coating | All | Heavy piping porosity, white zinc oxide fumes | Grind zinc from joint area, use E6010/E6011 for root, slow travel |
AWS D1.1 Porosity Acceptance Criteria
AWS D1.1 Table 6.1 sets limits for both visual and NDE-detected porosity:
Visual Porosity Limits (Non-Tubular Connections)
- Statically loaded: Sum of visible piping porosity diameters shall not exceed 3/8 inch in any linear inch of weld, or 3/4 inch in any 12-inch weld length.
- Cyclically loaded: Same limits but also requires that no single pore exceed 3/32 inch diameter.
Radiographic/UT Porosity Limits
Internal porosity detected by RT or UT has separate acceptance criteria based on pore size relative to the effective throat and the frequency of pores in a given length of weld.
Prevention Checklist
Use this checklist before welding to prevent porosity:
- Clean the joint. Grind, wire-brush, or solvent-wipe within 1 inch of both edges. Remove all oil, paint, rust, scale, and moisture.
- Check gas flow. Set the flowmeter to 20-25 CFH for MIG, 15-20 CFH for TIG. Check for leaks at all connections.
- Inspect the gun/nozzle. Clean spatter from the MIG nozzle. Replace damaged diffusers or O-rings. Check TIG cup for cracks.
- Verify consumables. E7018 stored at 250-300F in a rod oven. MIG wire clean and dry. TIG filler clean.
- Check for wind. If outdoors, set up wind screens on all sides. If the gas plume visibly wavers, you’ll get porosity.
- Pre-flow and post-flow. Set gas pre-flow to 1-2 seconds (purge the line before striking the arc). Post-flow to 5-10 seconds (protect the crater as it cools).
- Maintain proper arc length. Stick: one electrode diameter. MIG: 3/8 to 1/2 inch stickout. TIG: 1/8 inch or less.
Repairing Porosity
If porosity exceeds acceptance limits:
- Mark the defective area
- Grind or gouge out the porous weld metal back to sound metal
- Verify the cavity edges are clean and free of porosity
- Identify and fix the root cause before re-welding
- Re-weld per the WPS
- Re-inspect the repair
Don’t try to “burn through” porosity with subsequent passes. The gas pockets don’t disappear when you weld over them. They get buried and show up on NDE.
Porosity by Welding Process
MIG (GMAW) Porosity
MIG is the most sensitive process to shielding gas problems. Any disruption to the gas envelope (wind, turbulence from high flow, nozzle blockage, leaks) produces immediate porosity. The gas-free alternative is switching to flux-core (FCAW-S), which carries its own shielding.
MIG-specific checks: Verify gas flow with a flowmeter at the nozzle (not just at the regulator). Check for kinks in the gas hose, cracked O-rings at the cable connection, and spatter buildup inside the diffuser that can redirect gas flow away from the arc.
Stick (SMAW) Porosity
Stick electrodes generate their own shielding gas from the flux coating, so wind is less of a factor. Porosity in stick welding almost always comes from moisture in the flux (wet rods), excessive arc length, or contaminated base metal.
Stick-specific checks: E7018 must be stored at 250-300F in a rod oven. Once exposed to ambient air for more than 4 hours (for most specifications), E7018 must be re-baked at 700-800F for 1 hour before use. E6010 and E6011 are not low-hydrogen electrodes and don’t require rod oven storage, but they shouldn’t be soaking wet either.
TIG (GTAW) Porosity
TIG porosity is rare when the equipment is working properly. When it does occur, check for:
- Contaminated tungsten (dipped in puddle, pick up base metal)
- Contaminated filler rod (fingerprints, oil, cutting fluid residue)
- Gas lens or cup damage allowing air entrainment
- Moisture condensation inside the gas lines
Flux-Core (FCAW) Porosity
Gas-shielded flux-core (FCAW-G) is sensitive to the same gas coverage issues as MIG. Self-shielded flux-core (FCAW-S) generates its own shielding and tolerates wind, but wet or damaged wire can still produce porosity. Store flux-core wire in a dry location and use it within the manufacturer’s shelf life recommendations.
Common Mistakes
Blaming the machine. Porosity is almost never a machine problem. It’s contamination, gas coverage, or consumable condition. Check those first.
Cranking up the gas flow. If 25 CFH produces porosity, 50 CFH won’t fix it. Excessive flow creates turbulence that pulls air into the gas envelope. Find the real cause.
Not checking the back side of the joint. Rust, paint, or galvanizing on the inside surface of a joint vaporizes during welding and causes porosity from the root up. Clean both sides.
Storing E7018 on the shop floor. Low-hydrogen electrodes absorb moisture in hours if left exposed. Rod oven at 250-300F. Period.
Ignoring pre-flow and post-flow settings. A dead gas line takes 1-2 seconds to purge air. A cooling crater needs 5-10 seconds of gas coverage. Skipping these causes arc-start and crater porosity.
For more on weld defects, see the undercut in welding, lack of fusion, and weld spatter guides. For inspection methods that detect porosity, visit the weld inspection category. Return to weld defects or the welding techniques pillar.