Porosity in welds: causes and how to stop it

Gas that was dissolved in the molten pool and did not get out before the metal froze. The most common defect there is, and the one whose cause can usually be read off the pattern of the holes.

Read the pattern first

Evenly scattered along the whole weld. The shielding is inadequate everywhere: flow too low, a leaking hose, a nozzle clogged with spatter, or a draught across the bench. This is the most common case by a wide margin.

In clusters. Something local on the surface — a patch of paint, oil, primer or rust that was not removed. Clusters follow contamination, so they appear where the contamination was.

In a line along the root. The root gap is drawing air in from the other side, or the backing is contaminated. On pipe this usually means the purge failed.

Wormholes — long tubular pores. Gas escaping through metal that is already freezing, typically because the pool is too cold or is freezing too fast. Often accompanied by moisture in the consumable.

The five sources

Shielding gas. Flow rate, leaks, blocked nozzle, wind. The threshold for wind is low: about 2 m/s strips the shield, which is a light draught through an open door.

Moisture. In basic electrodes above all, but also in flux core and on a cold plate that has condensation on it. Basic electrodes want re-baking at 300–350 °C and then a heated quiver.

Surface contamination. Paint, primer, oil, cutting fluid, rust and — worst of all — galvanising. Zinc boils at 907 °C, well below the temperature of the pool, so it gasses off straight into the weld. Grind the coating back 25 mm each side of the joint.

Arc length. A long arc pulls air in past the shield, in MMA as much as in MIG. This is why porosity often disappears when a beginner is told simply to weld closer.

Nitrogen from air. The end product of every failure above: nitrogen is far more soluble in molten steel than in solid steel, so it comes out of solution as the weld freezes and has nowhere to go.

Working through it

Take them in order of cost. Look at the joint surface and clean it properly. Look at the nozzle and the gas hose. Put a flowmeter on the torch rather than trusting the regulator. Screen the bench. Only then start changing settings — and if the consumables have been open in a damp store, deal with that before anything else.

On aluminium the rules are stricter still, because hydrogen dissolves readily in molten aluminium and hardly at all in solid aluminium: see welding aluminium.

Updated: 14 August 2026