Troubleshooting guide › Weld defects › Porosity
Porosity is gas trapped in the weld as it freezes. It shows as small holes on the surface, or as a honeycomb inside when you cut or grind the weld. It is the most common defect in MIG welding and almost always comes from the gas or the metal surface, not from the machine.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
Round pin-holes on the bead surface, sometimes in a line along the weld. In bad cases the bead looks like a sponge. Worm-like tracks (wormholes) point to gas escaping while the pool was freezing.
Illustration, not a photo. Orange marks the defect.
Flow too low, empty cylinder, blocked or spatter-filled nozzle, loose hose, or a draught from a fan or open door blowing the gas away.
Rust, mill scale, oil, paint, galvanising or moisture turn into gas in the arc.
Moisture in the coating (ARC) or in the flux (FCAW) releases hydrogen.
Too long a stick-out pulls the nozzle away from the pool; too steep an angle sucks air into the gas column.
A long arc lets air into the pool.
Argon where CO2 was expected, or a gas cylinder with moisture.
| Process | Typical reason on this process | What to do |
|---|---|---|
| MIG / MAG (CO2 or Ar-CO2) | Gas cover lost: low flow, blocked nozzle, draught, long stick-out. Dirty or primed plate. | Flow 12-18 L/min at the nozzle, clean nozzle hourly, stick-out 10-15 mm, grind the joint bright. |
| Flux-cored (FCAW) | Wire picked up moisture, or gas-shielded wire run without gas. Too long an arc. | Keep spools covered, use the gas the wire needs, shorten the arc. |
| ARC / MMA | Damp electrodes (especially E7018), rusty plate, too long an arc, arc struck outside the joint. | Bake low-hydrogen electrodes, store in a holding oven, keep the arc short, clean 20 mm each side. |
| TIG | Gas disturbed: wrong cup size, flow too high (turbulence) or too low, leaking torch, dirty filler rod. | Argon 6-12 L/min, gas lens on thick tungsten, wipe filler with a clean cloth, check torch O-rings. |
| Submerged arc (SAW) | Damp flux, dirty plate, flux layer too thin so air reaches the arc. | Dry flux as its maker specifies, sieve recovered flux, keep 25-40 mm flux depth. |
| Spot welding | Not porosity in the usual sense; voids inside the nugget come from expulsion or dirty sheets. | Clean sheets, raise electrode force, reduce current one step. |
MIG / MAG, 1.2 mm ER70S-6 solid wire on 6 mm mild steel. A starting point only; confirm on a test piece of the same plate.
| Shielding gas | CO2 or 80/20 Ar/CO2 |
| Gas flow | 12-18 L/min (higher outdoors) |
| Stick-out | 10-15 mm |
| Current / voltage | about 180-220 A / 22-26 V |
| Travel | steady, 30-40 cm/min |
Typical values for mild steel. Your wire, plate condition and machine will move them. Our technical team can set them with your operator on a video call.
Surface porosity is visible by eye. Sub-surface porosity is not, so on a critical weld check it the way an inspector would.
Acceptance limits for porosity size, number and spacing come from the code or specification the job is built to (IS 822, AWS D1.1, ASME Section VIII or B31.3, or the customer drawing) and are checked by the inspection that code calls for. A clean-looking surface is not acceptance by itself: it only tells you the surface has no visible pores. When the job has no code, agree the inspection and the limit with the customer before welding, and record it.
It depends on the code and the service. Scattered small pores on a non-structural fillet are usually acceptable; clustered or linear porosity on a pressure or load-carrying joint is not. Grind out and reweld rather than weld over it.
Gas pre-flow is too short or the nozzle is full of spatter, so the first 10-20 mm of weld has no cover. Set 0.3-0.5 s pre-flow and start on a run-on tab or back-step into the weld.
Zinc boils in the arc and causes porosity and fumes. Grind the coating off 10-15 mm each side of the joint, use a slightly longer stick-out and slower travel, and ventilate well.
Not by itself. CO2 gives a harsher arc and more spatter but shields fine at the right flow. Most CO2 porosity is a flow, draught or wet-cylinder problem.
On an inverter MIG machine with stable arc control, the pool stays calmer and the gas cover is less disturbed, but no machine will cure porosity caused by dirty plate or low gas flow.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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