What Causes Weld Porosity?

Porosity is one of the most common weld defects we see cause real problems on the job: failed inspections, costly rework, and delays that ripple through an entire project schedule. At LWS Manufacturing & Welding, understanding what causes weld porosity isn’t just textbook knowledge. It’s the difference between a weld that passes and one that has to be ground out and redone.

Learn all about the role of surface preparation in achieving quality welds.

The Root Cause: Trapped Gas in the Weld Pool

Porosity happens when gas gets trapped in the molten weld pool before it solidifies. Those voids weaken the weld, create stress concentration points, and show up fast under inspection. The gas has to come from somewhere, and it almost always traces back to one of four sources.

Contaminated Base Material

When there’s anything on the metal surface that can vaporize under heat, it will. That gas gets drawn into the weld pool and has nowhere to go. Common offenders include:

• Moisture from humid storage or wet conditions
• Rust, mill scale, and surface oxides
• Oil, grease, paint, or any applied coating
• Residue from handling

Surface prep is the first line of defense. Grinding, solvent wiping, and wire brushing before the arc strikes eliminates the majority of porosity problems before they start. A welder who skips prep to save ten minutes creates a rework job that takes hours.

Shielding Gas Failures

The shielding gas, whether that’s argon, CO2, or a mix, exists to keep the atmosphere away from the molten pool. When that protection breaks down, nitrogen and oxygen get in, and porosity follows.

Flow Rate Problems

• Flow rate too low: the pool isn’t adequately covered
• Flow rate too high: turbulence draws ambient air into the shielding envelope

Supply and Equipment Issues

• Contaminated supply: moisture in the lines or a cylinder running low
• Wind and drafts: a shop door, fan, or outdoor environment can disrupt coverage entirely
• Damaged nozzle: spatter buildup or a cracked nozzle changes the gas delivery pattern

For MIG welding, flow rate matters. Too far in either direction and you’re welding unprotected. Checking equipment setup before every job isn’t overcautious. It’s standard practice.

Technique Errors

Clean material and good gas coverage won’t save a weld if the technique is off. Porosity from operator error often shows up in clusters or distributed through the length of a run.

Key technique issues include:

• Arc length too long: reduces shielding gas effectiveness at the pool
• Travel speed too fast: gases don’t have time to escape before solidification
• Travel speed too slow: overheating causes excessive gas absorption
• Wrong electrode angle: disrupts gas coverage and pool behaviour
• Inconsistent positioning: especially problematic in vertical or overhead welds

Rushed work under deadline pressure is where technique problems concentrate. When a welder is pushing to make up time, these are the corners that get cut, and they show up in the finished weld.

Wrong Filler Metal or Parameters

Using the wrong consumable for the base material, or running incorrect voltage and amperage, introduces chemical imbalances that produce gas during the weld. In stick welding, electrodes with damaged or moisture-absorbed flux coating are a direct porosity risk. In MIG welding, wire that’s been stored improperly or left on the spool too long can cause the same problem.

Parameter settings matter too. Running too hot doesn’t just cause distortion. It accelerates gas entrapment and spatter that contaminates the pool.

Learn how to choose the right filler material for your welding project.

Why Causes Compound Each Other

This is what most generic troubleshooting guides miss: porosity is rarely a single-cause problem. A slightly contaminated surface combined with a marginal flow rate and a welder pushing travel speed creates a defect that none of those factors alone might have caused. That’s why process control across the whole job, covering prep, setup, and execution, matters more than fixing one variable in isolation.

When a weld fails inspection, the question isn’t just “what went wrong.” It’s “where in the process did control break down, and who owns each step?” A shop that manages the full chain, from material handling through equipment setup to welder execution, catches problems before they become rework.

Getting It Right the First Time

On deadline work, there’s no room for a weld that has to come back. If you’re dealing with repeated porosity issues or vetting a fabrication partner on quality standards, the questions to ask are about process control: How is material prepped? How is equipment checked before a run? What happens when a welder identifies a problem mid-job? If you want to talk through a project or have questions about what tight quality control looks like on complex fabrication work, reach out to us: 604-854-1277.