What Causes Porosity in Welds and How Do You Fix It?

Porosity is one of the most common welding defects, and it happens when gas becomes trapped inside the weld pool as it solidifies, leaving small voids or holes in the finished weld. The good news is that most porosity problems come down to a handful of preventable causes — contamination, poor shielding gas coverage, or incorrect technique — and once you identify the culprit, the fix is usually straightforward.

What Is Porosity and Why Does It Matter?

Porosity refers to gas pockets or voids within a weld bead. These can appear on the surface (visible porosity) or be hidden inside the weld (subsurface porosity). Either way, porosity weakens the weld by reducing the effective cross-section of the joint and creating stress concentration points. In structural or load-bearing applications, a porous weld can fail under stress — so it's not just a cosmetic issue.

The Main Causes of Porosity

1. Contaminated Base Metal or Filler

This is the number one cause. Oil, grease, paint, rust, mill scale, moisture, and even fingerprints on the base metal or filler wire can all introduce hydrogen and other gases into the weld pool.

• Fix: Thoroughly clean the base metal with a wire brush and degrease with acetone or a suitable solvent before welding. Store filler rods and wire in a dry location and keep them clean.

2. Inadequate Shielding Gas Coverage

In MIG (GMAW) and TIG (GTAW) welding, the shielding gas protects the molten weld pool from atmospheric oxygen and nitrogen. If coverage is poor, these gases dissolve into the pool and cause porosity.

Common shielding gas problems include:

• Low flow rate — too little gas reaching the weld zone

• High flow rate — turbulence can actually draw in air

• Drafts and wind — even a light breeze can blow shielding gas away

• Damaged or blocked gas lines and nozzles

• Spatter-clogged MIG nozzle

• Fix: Check your flow rate (typically 10–15 L/min for MIG on mild steel, but always check your machine and consumable manufacturer's recommendations). Shield the work area from wind. Clean or replace your MIG nozzle regularly and use anti-spatter spray.

3. Damp or Incorrect Electrodes (Stick/SMAW Welding)

With stick welding, low-hydrogen electrodes (such as E7018) are particularly sensitive to moisture. Damp electrodes release hydrogen into the weld pool, causing porosity and potentially hydrogen-induced cracking.

• Fix: Store low-hydrogen electrodes in a rod oven at the manufacturer's recommended temperature. If electrodes have been exposed to humidity, re-dry them per the manufacturer's instructions before use.

4. Incorrect Welding Technique

Too fast a travel speed, incorrect torch angle, or holding the arc too long can all disrupt shielding gas coverage or allow the pool to cool too quickly, trapping gas.

• Fix: Maintain a consistent travel speed and keep the torch or electrode at the correct angle (typically 5–15° drag angle for MIG). Practice on scrap material to dial in your technique.

5. Wrong Voltage or Wire Feed Speed (MIG)

Incorrect machine settings can cause an unstable arc, which disrupts the shielding gas envelope and leads to porosity.

• Fix: Refer to the wire manufacturer's data sheet and your welder's settings chart. Start with the recommended parameters for your wire diameter and material thickness, then fine-tune.

6. Galvanised, Coated, or Painted Steel

Welding through zinc galvanising, paint, or other coatings releases gases and fumes that cause severe porosity — and can also be a serious health hazard.

• Fix: Grind or strip coatings from the weld area before welding. Always work in a well-ventilated space. WorkSafe NZ has guidance on welding fume hazards — zinc fume fever is a real risk with galvanised steel.

How to Fix an Already-Porous Weld

If you've already laid a porous weld, the repair process depends on severity:

1. Grind out the defect — Use an angle grinder or die grinder to remove all porous material back to sound metal.
2. Clean thoroughly — Degrease and wire-brush the area.
3. Address the root cause — Fix whatever caused the porosity before re-welding.
4. Re-weld — Fill the prepared area with a fresh, clean weld pass.

For critical or structural welds, non-destructive testing (NDT) such as dye penetrant testing or ultrasonic testing may be needed to confirm the repair is sound.

Preventing Porosity: Quick Reference

Safety First

Welding produces UV radiation, intense heat, sparks, and fumes. Always wear appropriate PPE — welding helmet, gloves, and protective clothing. Ensure good ventilation, especially when welding coated or galvanised materials. Review WorkSafe NZ guidance on welding safety before starting any project.

When to Call a Professional

Some welding work in NZ is best left to — or legally requires — a qualified professional:

• Structural steelwork covered by NZS 3404 or requiring building consent must be carried out by a suitably qualified welder, and may need inspection. Check with your local council.
• Pressure vessels and pipework have strict compliance requirements under the Health and Safety at Work Act and relevant standards.
• If you're unsure whether your repair is structurally sound, have it inspected by a certified welding inspector or structural engineer.
• Gas pipework welding is restricted work under the Gas Act — only a licensed gasfitter registered with the Plumbers, Gasfitters and Drainlayers Board may carry out this work.

If you need a qualified welder for a structural, automotive, or fabrication job, you can find local professionals on tradedirectory.co.nz.

Sources & Further Reading

• WorkSafe NZ — Welding Hazards
• BRANZ — Building Research
• MBIE Building Performance — Building Code
• Standards NZ — NZS 3404 Steel Structures
• Plumbers, Gasfitters and Drainlayers Board