Bubbling in welds, known as porosity, happens when gas gets trapped in the molten weld pool and can’t escape before the metal solidifies. The result is tiny holes, craters, or a spongy texture that weakens the joint. The three gases responsible are hydrogen, nitrogen, and oxygen, and they sneak in through a surprisingly short list of causes: contaminated surfaces, shielding gas problems, moisture in your consumables, or environmental conditions like wind.
The good news is that porosity is almost always fixable once you identify which source of gas is the culprit. Here’s how to work through each one.
Dirty Metal Is the Most Common Cause
Oil, grease, paint, rust, and mill scale all release gas when hit by an arc. Hydrocarbons from cutting fluid or fingerprints break down under extreme heat and release hydrogen directly into the weld pool. That hydrogen dissolves easily in liquid metal, but as the puddle cools and solidifies, the gas can no longer stay dissolved. It forms bubbles that get frozen in place.
The fix depends on what you’re welding. For carbon steel, grinding is the most effective way to remove scale and coatings. For stainless steel, copper, and nickel alloys, the recommended sequence is degreasing, wire brushing, then degreasing again. Aluminum requires the most aggressive prep: a chemical clean, followed by wire brushing, degreasing, and scraping the oxide layer immediately before welding. Aluminum oxide re-forms within minutes of being removed, so timing matters.
Don’t overlook your filler wire or rod either. If your MIG wire has been sitting in a dusty shop or a damp garage, it can carry enough surface contamination to cause porosity on its own. Wipe it down or replace it if the spool has been exposed for a long time.
Shielding Gas Problems
Your shielding gas exists to keep nitrogen and oxygen in the atmosphere from reaching the molten puddle. When that gas blanket is disrupted, those atmospheric gases dissolve into the pool and cause porosity. The most common shielding gas issues fall into a few categories.
Flow rate too low: Not enough gas reaches the weld zone, leaving it partially exposed to air. You’ll often see scattered porosity across the entire bead.
Flow rate too high: This one surprises a lot of people. Cranking the flow too high creates turbulence at the nozzle, which actually pulls surrounding air into the gas stream. What matters is the velocity of the gas at the nozzle tip, not just the volume reading on your flowmeter. For most MIG welding, a flow rate between 20 and 30 cubic feet per hour (CFH) works well. If you’re seeing porosity and your flowmeter reads 40 or 50 CFH, try turning it down.
Leaks in the gas line: A cracked hose, loose fitting, or damaged O-ring in the regulator can let air mix with your shielding gas before it ever reaches the torch. If you’ve ruled out other causes, disconnect each fitting and check for cracks. A simple soap-and-water bubble test on connections can reveal small leaks.
Clogged or spattered nozzle: Spatter buildup inside the nozzle restricts and redirects gas flow, creating dead spots in coverage. Clean or replace the nozzle regularly.
Wind and Drafts
The American Welding Society recommends a maximum of 5 mph wind speed for gas-shielded arc welding. That’s barely a breeze. Even an open shop door or an overhead fan can be enough to blow your shielding gas away from the weld zone. If you’re welding outdoors and can’t control the wind, consider using a windscreen or switching to a process that doesn’t rely on external shielding gas, like flux-core wire or stick welding.
Moisture in Electrodes and Flux
For stick welding, moisture is the primary enemy. Water molecules break apart in the arc and release hydrogen directly into the puddle. Low-hydrogen electrodes like 7018 are especially sensitive. Once you open a sealed container, those rods should go into a storage oven held at a minimum of 250°F (120°C). Left sitting out, they absorb moisture from the air, and the AWS standard limits their atmospheric exposure to just 4 hours before they need to be re-baked at a higher temperature. Simply drying them at a low temperature won’t fully remove absorbed moisture.
If you’ve been pulling 7018 rods out of an open box that’s been sitting on the shelf for weeks, that’s very likely your problem. Flux-core wire can absorb moisture too, though it’s less sensitive than bare stick electrodes.
Why Aluminum Is Especially Prone to Bubbles
Aluminum has a unique relationship with hydrogen. It’s the only gas that dissolves in aluminum, and liquid aluminum absorbs roughly 20 times more hydrogen than solid aluminum can hold. During solidification, all that excess hydrogen gets violently rejected. As the metal transitions from liquid to solid, hydrogen concentrations spike in the remaining liquid between the forming crystal structures. Once the gas pressure exceeds the local pressure in the metal, bubbles nucleate and grow.
This means aluminum welding is far less forgiving of any hydrogen source, whether it’s moisture, oil, or a dirty oxide layer. The oxide layer on aluminum is particularly problematic because it’s porous and traps moisture. That’s why aluminum prep is more involved than steel prep: you need to chemically clean the surface, mechanically remove the oxide, degrease, and then scrape it clean right before you strike the arc. Even humidity in the shielding gas can be enough to cause porosity in aluminum welds.
Travel Speed and Joint Fit-Up
Moving too fast doesn’t give trapped gas enough time to rise out of the puddle before the metal solidifies. Slowing down allows bubbles to escape while the pool is still liquid. You’ll notice that porosity from travel speed tends to be evenly distributed along the length of the bead rather than clustered in one spot.
Poor joint fit-up creates its own issues. Gaps between the pieces being welded can trap air pockets that get heated by the arc and forced into the molten pool. Tight, consistent fit-up reduces the volume of air available to contaminate the weld. If your parts don’t fit together cleanly, take the time to grind, clamp, or tack them into proper alignment before running the full bead.
How to Diagnose Your Specific Problem
The pattern of porosity often points to the cause. Scattered, uniform porosity across the whole bead usually indicates a shielding gas or contamination issue. Porosity clustered at the start or end of a weld can mean your gas pre-flow or post-flow timing is too short, leaving the puddle unprotected. A single line of pores along the weld centerline can point to joint fit-up problems or excessive speed.
If you’re seeing porosity only on certain pieces but not others, contamination is the likely cause. Run a test bead on a piece of clean, freshly ground metal with the same settings. If the test bead is clean, your problem is surface prep on the original workpiece. If the test bead still has bubbles, the issue is your gas, consumables, or environment.
For a systematic approach, start with the simplest fixes first: clean the metal thoroughly, check your gas flow and connections, make sure your consumables are dry, and block any drafts. Most porosity problems resolve with one or two of these changes.