Weld porosity: causes and fixes for pinholes
Porosity is a set of small cavities in the weld, formed by gases (hydrogen, carbon monoxide, nitrogen) that get trapped as the metal freezes. Sometimes the pinholes show on the surface. Sometimes they are inside and only appear when you grind the bead.
The most common causes are dirty metal (paint, oil, rust, galvanizing), poor gas shielding (draft, wrong flow, spatter-clogged nozzle) and damp consumables. The type and location of the pores tell you which one it is.
Diagnosis: each cause leaves a signature
| Cause | How it shows up | Fix |
|---|---|---|
| Moisture in the electrode or consumable | Uniform pores along the bead | Dry the consumables; preheat if needed |
| Contaminated surface (oil, paint, rust) | Pores at the start of the bead | Clean with a non-chlorinated solvent (let it dry fully) and grind to bright metal |
| Insufficient gas shielding (flow, draft) | Pores on the surface | Set the correct flow; block the draft |
| Arc too long (stick) | Scattered round pores | Shorten the arc |
| Current too low | Porosity at the root | Raise the current |
Table 12.1 of the e-book The Welder's Forge.
Porosity in MIG: the checklist
In MIG, the gas is all that protects the puddle. When pores appear, check in this order:
- Is the cylinder open and does it have gas? It sounds obvious, but it explains many "mystery" pores.
- A draft or fan near the gun: any moving air carries the gas away.
- Correct flow: about 17–21 CFH (8–10 L/min) with .030 in wire and 21–25 CFH with .035 in wire. More is not better: it creates turbulence and pulls air in.
- Nozzle full of spatter: it deflects the gas. Clean it and use anti-spatter spray or gel.
- Leaks in the hose, the fittings or the gas diffuser.
- Gun held too far from the work: the gas never reaches the puddle.
- Dirty or galvanized metal: grind to bright metal on both sides of the joint.
Gases and flow rates are in MIG welding gas compared. If the diffuser or the nozzle is clogged, replacement and cleaning are covered in MIG contact tip.
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Porosity with stick electrodes
- Damp electrodes: especially low-hydrogen rods (E7018). Once the sealed container is open, they belong in a rod oven at about 250–300 °F (120–150 °C); time limits depend on the electrode maker and the code (AWS D1.1), so follow the package and the WPS. See 7018 rod storage.
- Long arc: the coating stops protecting the puddle. Keep the arc length close to the rod diameter.
- Dirt at the start of the bead: that is where pores appear first.
Zinc coating causes pores and releases toxic fume. Grind the zinc off the weld area, ventilate well, wear a NIOSH-approved respirator with P100 filters, and never weld galvanized steel in an enclosed space. Paint and other coatings can also give off hazardous fume, so strip them back first.
Porosity with flux-cored wire
Self-shielded flux-core carries its shielding in the wire, so wind matters less. The usual causes are damp wire (a spool left open in the garage), dirty or galvanized metal and slag not removed between passes. With gas-shielded flux-core (E71T-1), use the same checklist as for MIG: the gas still has to reach the puddle. More in gasless MIG with flux-core wire.
A routine that prevents it
- Grind to bright metal on both sides of the joint before welding.
- Store electrodes and wire dry and sealed.
- Clean the nozzle at every break and check the cylinder at the start of the day.
Before you change any setting, run a short bead on clean scrap with the door shut against drafts. If the pores disappear, the problem is the part or the environment, not the machine.
Is porosity a serious defect?
It depends on how much and where. A few small pores in a decorative piece may not matter. In a part that carries load, porosity reduces the effective section and can start cracks. Quality standards such as ISO 5817, AWS D1.1 and ASME Section IX set how much is acceptable in each case. In a structural part, the right move is to grind the area out and weld it again. Other problems are covered in common welding defects.
Frequently asked questions
Why is my MIG weld full of pinholes?
Will turning up the gas fix porosity?
Does welding over rust cause porosity?
How can I tell if there are pores inside the bead?
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