Welding porosity is the formation of small gas-filled cavities, or pores, inside a weld. It happens when gases like hydrogen, nitrogen, or oxygen dissolve into the molten weld pool during welding and then get trapped as bubbles when the metal solidifies. Think of it like the air pockets you see in a slice of bread, except inside metal, where those voids weaken the joint and can cause a weld to fail inspection or, in severe cases, compromise structural integrity.
How Porosity Forms Inside a Weld
Molten metal can absorb far more gas than solid metal can hold. As the weld pool cools, gas solubility drops sharply, and dissolved gases start coming out of solution, much like carbon dioxide escaping from a soda when you open the cap. If the metal solidifies before those gas bubbles can rise to the surface and escape, they become permanently trapped as pores.
The process follows a predictable sequence. First, the liquid metal gradually absorbs gas from its surroundings. Then, as solidification begins and the metal enters a slushy, partially solid state, the gas concentration in the remaining liquid spikes because there’s less and less liquid left to hold it. Once the gas pressure exceeds the local pressure of the surrounding metal, bubbles nucleate and grow rapidly between the branching crystal structures forming in the cooling weld. A small increase in the solid fraction at this stage produces a disproportionately large jump in porosity: research on aluminum alloys shows that just a 1% increase in the solid fraction can contribute roughly a 15% increase in pore volume.
These bubbles almost always form on existing surfaces rather than spontaneously in the liquid. Oxide particles, inclusions, and tiny imperfections on the base metal all serve as nucleation sites, giving gas molecules a place to gather and grow into a bubble. This is why contamination on the workpiece surface is such a reliable trigger for porosity.
Common Causes
Most porosity traces back to one of two root problems: atmospheric contamination of the weld pool, or gas-producing contaminants on the metal surface.
Poor shielding gas coverage is the single most frequent culprit. The shielding gas exists to keep nitrogen and oxygen in the atmosphere away from the molten pool. As little as 1% air mixing into the shielding gas will produce scattered porosity throughout the weld. Above 1.5% air entrainment, you’ll typically see large pores that break through to the surface. Leaks in the gas line, a flow rate set too high (which creates turbulence and pulls air into the gas stream), drafts in the work area, and a worn or improperly sized nozzle all compromise shielding.
Hydrogen from moisture and surface contamination is the other major source. Hydrogen dissolves readily into molten metal and has a very low solubility in the solid state, making it especially prone to forming pores. It can come from inadequately dried electrodes or flux, moisture on the workpiece, or organic contaminants like grease, oil, paint, and primer coatings on the base metal or filler wire. Rust and residue from previous inspection operations also release gas into the pool.
Types of Porosity
Not all porosity looks the same, and the pattern often points to the cause.
- Distributed (scattered) porosity: Small, roughly spherical pores spread evenly throughout the weld. This pattern typically results from a consistent low-level problem like minor shielding gas contamination or slightly damp electrodes.
- Clustered porosity: A group of pores concentrated in one area of the weld, often at the start or stop point. Localized contamination or a momentary loss of shielding gas coverage is the usual cause.
- Wormholes (elongated porosity): Tubular pores that extend lengthwise through the weld, sometimes perpendicular to the surface. Wormholes indicate a large volume of gas being generated, typically from gross surface contamination or very thick paint and primer coatings. Structural welding codes like AWS D1.1 pay special attention to elongated porosity because it concentrates stress more than round pores do.
- Surface-breaking pores: Pores that open to the weld surface, visible without any special equipment. These usually mean the gas generation rate was high enough that bubbles were still escaping when the surface solidified.
Why Aluminum Is Especially Prone
Aluminum has a uniquely difficult relationship with hydrogen. It is the only gas with meaningful solubility in molten aluminum, and the solubility difference between liquid and solid aluminum is extreme. That means virtually all hydrogen dissolved during welding gets expelled during solidification, and much of it ends up trapped as pores.
Anodized aluminum is even more challenging. The anodized oxide layer, when broken up by the welding arc, creates additional nucleation sites where hydrogen pores can form more easily. This is why aluminum welding demands meticulous surface preparation and dry, clean conditions that might be overkill for steel.
How Porosity Is Detected
Surface porosity is straightforward to spot with a visual inspection, but internal pores require non-destructive testing (NDT). The two primary methods are radiographic testing (X-ray) and ultrasonic testing.
Radiography has traditionally been the go-to for porosity because it produces an image that clearly shows the size and distribution of pores, similar to a medical X-ray. Inspectors compare the image against reference charts in the applicable welding code to decide whether the porosity is within acceptable limits.
Ultrasonic testing can also detect porosity, even though round pores don’t reflect sound waves as cleanly as flat cracks do. Modern ultrasonic equipment can pick up pores as small as 0.5 mm. A B-scan display shows the pattern of varying signal amplitudes and arrival times from individual pores, giving the inspector enough information to characterize the flaw type and estimate its extent. As fitness-for-purpose acceptance criteria become more common in industry, there’s increasing pressure on ultrasonic methods to not just find porosity but accurately size it.
Acceptable Limits
Some degree of porosity is almost inevitable, and welding codes account for that. Under AWS D1.1, the structural welding code most widely used in the United States, the type of porosity matters more than its mere presence. The code focuses specifically on elongated (piping) porosity, defined as pores whose length exceeds their width and that lie roughly perpendicular to the weld face. Scattered round porosity, within reason, is generally not a rejection criterion under D1.1. Other codes and application-specific standards (pressure vessels, pipelines, aerospace) set their own limits, which can be considerably stricter.
Prevention
Preventing porosity is almost entirely about controlling two things: gas shielding and surface cleanliness.
For shielding gas, set your flow rate within the recommended range for your process. For MIG welding and flux-cored arc welding, 20 to 50 cubic feet per hour (cfh) is standard. For TIG welding, 15 to 30 cfh is typical, though gas lenses let you use slightly higher flows with better coverage. The goal is enough gas to protect the pool without so much flow that turbulence pulls surrounding air into the stream. Check all hose connections and fittings for leaks, and avoid welding in drafty areas or use wind screens when working outdoors.
For surface preparation, clean the workpiece and adjacent area thoroughly before welding. Remove oil, grease, rust, paint, primer, and any residue from previous inspection fluids. Use a clean stainless steel brush or solvent wipe appropriate for the base metal. On aluminum, remove the oxide layer immediately before welding and ensure filler wire is stored in dry, clean conditions. If you’re using stick electrodes or flux, follow the manufacturer’s drying and storage recommendations, because moisture absorbed into flux is one of the most common and preventable sources of hydrogen.
Repairing a Porous Weld
When porosity exceeds the acceptance criteria for your code, the affected section of the weld needs to be removed and rewelded. The standard approach is to grind or gouge out the porous area back to sound metal, then re-prepare the surface and re-weld. Before you strike an arc again, identify and correct whatever caused the porosity in the first place. If you simply reweld over the same contamination or with the same shielding gas problem, you’ll get porosity a second time. After the repair, the weld goes through the same inspection it originally failed to confirm the new material is sound.