How to Prevent Porosity in Welding: Causes & Fixes

The fillet weld looked perfect when the wire was run over it with a grinder. Then the inspector came back with an X-ray and the bead was riddled with tiny round voids, all clustered right at the stop points. The welder swore nothing was wrong with his gas, his wire, or his technique. The truth was that the gas was fine, but the shielding was being torn apart every time he restarted the arc in the corner of the bracket. Porosity is one of the most common weld defects and one of the most misdiagnosed, because the gas that causes it can come from dozens of places you would never think to check. Here is how to find the real source and keep the bubbles out of your weld metal.

What Porosity Actually Is (And Why It Matters More Than You Think)

At its most basic, porosity is the presence of cavities in the weld metal caused by gas released from the weld pool as it solidifies. These voids are not bits of slag or loose material, but pockets of gas that became trapped before the molten metal had a chance to freeze. With that in mind, porosity is normally classified as a discontinuity rather than an automatic reject. A discontinuity is an interruption to the structure of the weld, while a true defect is only one that fails to meet the acceptance criteria for the specific job.

Before jumping to a fix, it’s important to understand that porosity is a symptom, not a disease. That means the real task isn’t just removing the holes, it’s finding out why gas was allowed to form and get trapped in the first place. Diagnosis should always start with the shape and distribution of those voids. When inspectors follow a systematic approach, industry reporting suggests porosity has an approximate 90 percent prevention rate when the cause is identified and corrected. With more than 200,000 workers in the United States and about 1.8 million worldwide performing welding as all or part of their jobs, that prevention rate isn’t just a shop goal, it’s a safety and reliability goal as well.

The Four Classic Forms of Porosity

Not every cluster of holes points to the same issue. The form of the cavity is often the first clue, and many inspectors refer to AWS B1.11, Guide for the Visual Examination of Welds to infer the probable cause from the type and position of porosity. The four widely recognized forms are as follows:

• Distributed porosity: Small, scattered, round pores spread throughout the weld metal. This is often linked to overall air entrainment in the shielding gas.
• Surface-breaking pores: Pores that open to the face or surface of the weld. These are more exposed to service environments and can initiate pitting corrosion. Based on known gas effects, as little as 1% air entrainment in the shielding gas can cause distributed porosity, while greater than 1.5% air entrainment results in gross surface-breaking pores.
• Wormhole (piping): Elongated, tunnel-like cavities that can run through the weld. These are typically less uniform and can indicate more aggressive gas entrapment.
• Crater pipes: Piped voids that form at the end of a weld, often at the crater of a terminated bead. These are especially common at stop points if the crater isn’t handled correctly.

It’s worth noting that surface-breaking pores can often be seen by visual inspection, while subsurface pores are only found by radiography or ultrasonic testing. For a deeper breakdown of these imperfections, TWI’s porosity guide is a trusted reference for how these cavities behave in practice.

Why Porosity Matters

On the surface, tiny round holes may not look catastrophic, but their effect on a weld can be significant. Porosity reduces the effective cross sectional area of the weld metal, which directly lowers fatigue life under repeated loading. More importantly, these voids create stress concentration points and can act as crack initiation points, especially in higher-strength steels where hydrogen-assisted cracking is a concern. They can also become corrosion initiation sites, with surface-breaking pores being the most vulnerable to this in aggressive environments.

These risks are present across many applications. For example, processes used in aluminum MIG welding can be very sensitive to gas coverage, and even work on components such as alloy wheel weld repairs can be compromised by trapped gas if the conditions aren’t controlled. The key point is this: porosity isn’t just about looks, it’s about long term integrity.

Roadmap For The Rest of This Article

Now that the question of what porosity is has been answered, the following sections will build on this foundation. First, we’ll look at the physics of how gas becomes trapped as the pool solidifies. From there, we’ll break down the three gas families, shielding gas delivery, moisture and contamination, process specific behavior, defect identification, code acceptance, a step by step troubleshooting routine, and finally, a system of daily habits to build a porosity-free shop. Every one of those steps starts with the same principle established here: porosity is a symptom, and its shape and distribution tell you where to begin.

The Physics of a Bubble: How Gas Gets Trapped in Solidifying Steel

Before you can fix porosity, you have to understand the race happening inside every weld pool. Molten steel is constantly trying to release gases, but the second it begins to solidify, that window of escape slams shut. It’s not the gas magically appearing out of nowhere, it’s a matter of solubility, chemistry, and timing.

Solubility: The Tipping Point

At welding temperatures, steel can hold surprisingly large amounts of certain gases in its liquid state. The key, however, is that this ability drops drastically the moment the pool begins to cool. For hydrogen in particular, hydrogen solubility in molten weld metal plummets as temperatures fall. Since the cooling rate of a weld is almost always much faster than hydrogen can diffuse back out of the metal, the weld metal becomes supersaturated with hydrogen. With nowhere to go, that excess hydrogen is forced to come out of solution and form bubbles.

This is also why porosity isn’t random. The faster and more restrictive the cooling path is, the less time those gases have to escape, and the higher the odds of them becoming trapped for good.

Nucleation, Rise, and the Escape Window

Once gases are forced out of solution, tiny bubbles must first nucleate before they can grow. From there, buoyancy takes over. Much like air bubbles in boiling water, these gas pockets want to rise to the surface of the weld pool due to their lower density than the surrounding molten metal.

If a bubble is able to reach the top before the pool solidifies, it will vent out and may show up as surface porosity. If it doesn’t have enough time to travel that distance, it remains suspended within the liquid and becomes internal porosity. The size and travel path of that bubble will determine the difference between a small, scattered pore, or a much larger one left deep within the bead.

Dendrites, Grain Boundaries, and Reaction Porosity

The real locking mechanism comes from the way steel solidifies. As the weld metal cools, it doesn’t freeze as one solid mass, it grows in a tree-like structure known as dendrites. These branching crystals interlock as they advance, leaving behind tight, restrictive channels. At this dendritic solidification point, voids can become entrapped along the grain boundaries between those interlocking crystals, with little to no chance of them ever reaching the surface.

Not all porosity comes from excess gas being forced out of solution either. Reaction porosity can occur when oxides present in the weld pool are chemically reduced by hydrogen or carbon at or just above the melting point, generating new gas in the exact area where solidification is already underway. This is especially relevant when working with certain filler metals, as this is where deoxidizers come into play.

Manganese and silicon, both common deoxidizers found in many filler metals such as alloy steel welding electrodes, act as scavengers. They bind with oxygen in the molten pool to form stable oxides that float out instead of reacting with hydrogen or carbon to create gas. The higher and more balanced the deoxidizer content of a wire or electrode, the more forgiving it is to minor trace amounts of oxygen or surface oxides. A low-deoxidizer filler, on the other hand, has very little room for error.

Turbulence: The Wormhole Problem

Chemistry isn’t the only culprit. The fluid motion of the pool plays a massive role, as highlighted by guidance from Australian Welding Institute defect guidance. Wormhole, or tunnelling porosity, is primarily caused by turbulence in the weld pool, most often from excessively high welding current. Violent agitation pulls air and gas into the molten metal and creates elongated gas tunnels that are incredibly difficult to escape before dendritic solidification locks them in.

This also ties back to process behavior. Due to moisture in the coating and heavier oxidation of the core wire, SMAW tends to be the most porosity-prone. GMAW sits in the middle, with its small-diameter wire carrying a high surface-to-volume ratio that is far more likely to hold oxidation or contaminants. GTAW is typically the least prone, purely due to its clean, flux-free setup, though even it can fall victim to dirty base metals.

Nitrogen, Oxygen, and Hydrogen: The Three Gas Families

While porosity can appear identical to the naked eye, the bubbles trapped inside your weld almost always fall into one of three families: nitrogen, oxygen, or hydrogen. Learning to tell which one you’re up against is the key to fixing the problem, rather than just fighting the symptom. In most cases, nitrogen and oxygen are intruders from the air, whilst hydrogen is a stowaway that was already on your metal or consumables long before you struck an arc.

The Air Intruders: Nitrogen and Oxygen

For nitrogen and oxygen, the route into the weld pool is almost always poor gas shielding. The moment your inert envelope is broken, the atmosphere will find its way in. Some of the most common offenders include:

• Leaks: Damaged O-rings, loose fittings, or cracked hoses can bleed air into your gas line without any visible mist or spray.
• Excessive flow turbulence: More gas isn’t always better. Running your flow rate far higher than needed can create a vortex at the nozzle, actually entraining air back into the shielding stream.
• Draughts: Open bay doors, pedestal fans, or even strong cross-breezes can rip your shield clean off of the pool, especially on outdoor jobs.
• Open roots: When welding from one side with an open root gap, air can be drawn in from the backside with no shielding at all, leaving the root especially vulnerable to these air-based gases.

These two gases are incredibly reactive at welding temperatures, and once absorbed into the molten pool, they have very little time to escape before solidification, leading to those classic, round, spherical pores. This correlation between shielding and atmospheric contamination is backed up by TWI’s breakdown of weld porosity.

The Hidden Culprit: Hydrogen

Unlike nitrogen and oxygen, hydrogen rarely comes from the air itself. It comes from anything that can introduce moisture or hydrocarbons to your weld. This is by far the broadest family, and it’s almost always removable with good housekeeping. The realistic sources to be on the lookout for are:

• Consumables: Moisture in stick electrodes, flux-cored wire, and submerged arc flux. Improperly stored consumables will absorb moisture from the air like a sponge.
• Surface corrosion: Rust forms when iron, water and oxygen combine, and when welded it mixes with weld metal chemistry to generate hydrogen bubbles that may not escape before solidification.
• Oils and grime: Grease, oil, cutting fluid, and even sweat from dirty gloves can transfer hydrocarbons straight into the puddle.
• Coatings: Paint, primer, and anti-spatter gel can all break down and release hydrogen when superheated.
• Zinc galvanizing: One of the worst offenders. Zinc melts at approximately 420 degrees F and changes from solid to gas at welding temperatures far in excess of 2,000 degrees F, producing both fume and trapped gas.

For galvanized steel, grinding off the galvanizing is the recommended fix when it is not prohibited by the job. Otherwise, SMAW and FCAW are more forgiving processes for coated steel due to their flux systems. This is especially important for those working with reactive base metals, such as when utilizing prep methods found within our guide, Aluminum Cleaner for Welding: How to Prep Aluminum Right.

Geometry is Everything

Even with perfect shielding and a spotless joint, geometry can still work against you. The risk of trapping evolved gas is greater in T-joints than in butt joints, and greater still when fillet welds are made on both sides. With double-sided fillet welds, the first pass can create trapped pockets and restrict the escape path for gas on the second pass, drastically increasing the chance for entrapment.

Primed plate and T-fillets are the classic scenario for surface-breaking pores, and this is visible in published weld defect references. With that being said, surface primers do not normally need to be removed, but porosity is likely when primer thickness exceeds the manufacturer’s recommendation, especially on processes other than SMAW.

Ultimately, these three gas families all boil down to the same rule: nitrogen and oxygen are a product of your envelope, hydrogen is a product of your contaminants. Control both of those, and you’ll be tackling the vast majority of gas porosity before it ever has a chance to get trapped.

Shielding Gas Problems: Flow Rate, Leaks, Turbulence and Draughts

Round, evenly scattered pores rarely come from filler metal or wire. They come from the air around the arc. Shielding gas builds an envelope around the molten pool that keeps nitrogen and oxygen away from the puddle. When that envelope is too small, too thin or torn apart, the pool breathes in atmosphere, and atmosphere becomes trapped voids. Gas delivery faults are also the fastest to diagnose in the shop: a regulator, a torch and ten minutes, no X-ray needed.

Too little shielding gas

Low flow or a restricted delivery collapses the envelope and exposes the pool to air. Nitrogen and oxygen dissolve into the molten metal and lock in as pores when it solidifies. Low shielding flow is a listed root cause in the TWI guide to weld defects and porosity and in the Fabricator’s list of possible causes for porosity in welding. Symptoms: even scatter of pores along the bead, a duller or more oxidised surface, and extra porosity at the start of the run as the cylinder empties.

Too much gas: why over-flow causes porosity too

Cranking up the flow does not improve shielding. Above a certain flow the gas stops hugging the arc and starts swirling, forming a turbulent column that shears surrounding air inward and drags it into the pool. You are welding with a wind machine pointed at your puddle. Roughly 60 CFH on a wire process is where most shops start seeing turbulent shielding and the porosity that follows. Turbulence also makes the arc wander, raises spatter and leaves a lumpy bead.

Typical shop flow rates, and why the WPS wins

As a starting point for troubleshooting, not a substitute for your procedure:

Process	Typical range	Watch for
GMAW (solid wire)	30 to 50 CFH	CO2 or mixed; more for stainless and aluminium
FCAW	30 to 50 CFH	Turbulence on wide, flat passes
GTAW	20 to 30 CFH	Argon; pure argon on reactive metals
SAW	10 to 30 CFH	Flux gas does part of the shielding

Your WPS governs: if the qualified procedure calls for 25 CFH and you run 50, the procedure wins, and changing flow to chase porosity means the weld is no longer the one you qualified. Adjust the parameters or requalify. Gas mixes also differ by material and process, which matters when chasing a leak or purity issue: see this guide to shielding gases and cylinder supply and air and argon suppliers.

Nozzle condition, stickout and travel angle

Even perfect flow settings fail if the gas cannot get out of the torch.

• Spatter-blocked nozzle. The most common and most overlooked fault. Spatter partly blocks the gas ports, so the flowmeter still reads fine, the arc still strikes, and shielding is quietly starved. Check the face each shift and during the job, and replace a nozzle with deformed ports.
• Stickout. Long contact tip to work distance throws gas away from the arc. For GMAW short-circuit transfer keep it at 3/8 to 1/2 inch, and 5/8 to 1 inch for spray transfer. Aluminium shifts the numbers again; the practical guide to aluminium MIG welding covers the gun angle and heat control aluminium needs.
• Travel angle. Keep the torch 10 to 20 degrees from perpendicular, or 70 to 80 degrees from the work surface, push or pull. A heavy push exposes the leading edge of the pool; a heavy pull drags ambient air into the back of the shield.

The 60-second leak test

Before blaming the procedure, prove the gas reaches the torch:

1. Open the cylinder handwheel fully and let the system pressurise for 15 to 20 seconds.
2. Close the cylinder hand valve.
3. Watch the regulator outlet gauge. If the pressure coasts down noticeably within a minute or two, you have a leak between the cylinder and the torch.

For a more sensitive check, close the torch valve too. A stable reading over several minutes points to a clean system; a drifting needle means the leak is upstream of the torch valve.

Where leaks actually happen

• Damaged O-rings. The power pin O-ring on a GMAW gun and the cap O-ring on a GTAW torch seal the gas path; a cracked or heat-damaged ring admits enough air to cause porosity.
• Gun not seated. Push it into the feeder bushing and listen for the click.
• Kinked hoses. Gas hoses longer than 20 feet kink easily and can restrict flow with no visible leak. Watch the route around corners, table edges and forklift traffic.
• Cut or burnt hose. Damage between regulator, flowmeter and feeder connection starves the torch, so check the whole run, not just fittings.
• Solenoids and flowmeter. A solenoid that does not open fully, or a flowmeter that reads wrong, starves the torch with no leak in sight. Verify against a known-good gauge.

Draughts and the 5 mph rule

Moving air is the last check. An open bay door, a corridor draught, a wall-mounted exhaust fan or even a welder’s own cooling fan can break up the envelope, and fans have been reported to disrupt welds 25 feet away. Feel it on your face, or watch it with a smoke pencil held near the arc.

Codes set a hard limit: AWS D1.1:D1.1M:2020 prohibits GMAW, GTAW, EGW and FCAW when wind velocity near the weld exceeds 5 mph (8 km/h), per the AWS D1.1 structural welding code. Self-shielded flux-cored wire and SMAW are exempt because they carry their own shielding. Where the draught cannot be controlled, use a windscreen, a curtain or a different process, not more gas. AWS also covers process-specific shielding in its Gas Tungsten Welding guide.

Restarting the arc properly

One of the most reliable porosity sources is not a leak at all. It is a restart: the gas flow stops, the envelope collapses, and a fresh strike drags atmosphere into the puddle before the initial gas burst has purged the corner pocket. That is the classic pattern of pores clustered at stop points. The fix takes two seconds: trigger the gas for one to two seconds with the wire held off the work before striking, so the shield establishes first. The AWS guide on prevention and causes of weld porosity makes the same recommendation.

Work through this list in order and most gas-related porosity disappears without touching a single parameter: confirm flow, prove no leaks, check the nozzle, check stickout and angle, then look for moving air. If pores remain, look elsewhere, most likely at moisture or contamination on the joint.

Moisture and Contamination: The Hydrogen Sources You Can Remove Today

Before you start turning knobs on your machine, look to the metal itself. Water, oil, grease, rust, paint, and shop dirt all carry hydrogen or carbon into the weld pool. At arc temperatures, they break down and release gas, leading directly to porosity. According to the TWI guide to weld porosity, contamination ranks right alongside shielding failure as a primary cause. The difference is that it’s by far the cheapest to fix, with only minutes of prep saving hours of rework.

Clean the Surfaces That Actually Matter

Wiping the face of a plate won’t cut it. The surfaces that need attention are the joint edges, the root area, and the faying surfaces for joints such as laps or flanges. These are where the puddle forms, so they’re also where trapped contaminants will cause issues.

• Degrease: Use a solvent-approved cleaner on these areas, then wipe with a clean, lint-free cloth. Never reuse a dirty rag, as you’ll only spread the grease around.
• Remove oxides: Rust, mill scale, and slugs should be removed mechanically with a rotary wire brush or grinder flap until you reach bright metal.
• Account for fabrication: Parts cut on a saw or drilled on a press often carry cutting oil into tight joints and need extra attention.
• Re-check after handling: Gloves, clamps, or any contact after prep can re-contaminate the joint, so inspect right before you strike an arc.

Oxides, Hands, and Sprays

Mill scale is more than just a rough surface. It’s iron oxide, and when hit by the arc, it decomposes to release oxygen into the pool. This raises oxidation levels, promotes poor wetting, and can lead to cold lap or lack of fusion in addition to porosity. Grinding it back is always faster than chasing repairs.

Not all contamination is visible. In hot shops, sweaty or dirty gloves can transfer grease directly to filler rod, with every pass flashing it into gas. For TIG work, where filler handling is constant, dedicated clean gloves or washed rod are a must. Similarly, anti-spatter sprays and gels can become a problem if overapplied. Puddled gel can drip into the weld pool, a cause that is also flagged by The Fabricator’s list of porosity causes. Keep coats thin and spray the nozzle, not the joint.

Hidden Paths: Liners, Moisture, and Consumables

For GMAW, contamination can be continuously fed to the arc. Gun liners collect grease, oil, dust, and dirt over time, transferring them to the wire with each pass. If the wire feels slick or the gun has a burnt oil smell, the liner should be cleaned or replaced. Stainless and high-nickel wires are especially sensitive to this, a level of discipline that is also expected for aerospace-grade TIG work.

Moisture is just as sneaky. On heavy plate or lap joints in cold weather, condensation can form when steel is below about 50 degrees F (10 degrees C). Preheating to 200 to 225 degrees F (93 to 107 degrees C) will drive it off before welding. This pairs with proper storage for consumables. SMAW electrodes, flux-cored wire, and especially SAW flux all absorb moisture from open air, which is why codes and electrode handling guidance require ovens or dryers for them. Once SAW flux is opened, it acts like a sponge and will need to be baked or discarded if left unprotected.

Filler hygiene is the final piece of the puzzle. Solid wire benefits from a wire wipe and storage off the floor, flux-cored wire should be checked for lot history and wiped, and GTAW filler rod should be cleaned with a dedicated stainless scrubber and kept in a dry tube. For aluminum, the naturally forming oxide layer reforms in seconds, so its prep follows its own strict rules as outlined in how to prep aluminum for welding.

Joint cleanliness, liner upkeep, moisture control, and proper consumable storage are all controls that pay for themselves on the first weld they save. Treat them as required steps, not optional ones, and you’ll eliminate some of the most preventable sources of porosity before they ever reach the pool.

Why Each Welding Process Behaves Differently

Porosity is not spread evenly across processes. The accepted ranking, confirmed by TWI’s guidance on weld porosity and decades of shop experience, puts coated-electrode SMAW highest, MIG/GMAW in the middle and TIG/GTAW lowest. It follows from how much contamination each process lets reach the molten pool.

SMAW: the most porosity-prone process

Stick welding has almost everything working against it. The coating is a hygroscopic powder flux, and cellulose-rich coatings are worst because the organic content decomposes in the arc, releasing hydrogen and water vapour into the pool. It is rolled onto the core wire wet, so rust, oil or grease ends up inside the arc zone. With no external shielding gas, the pool sits exposed to the atmosphere, and a poor view of the puddle hampers arc length control. Fans of a stick welding setup are right about versatility, wrong about defect control.

GMAW: the middle ground

MIG/MAG is cleaner than stick but not clean. The wire is bare, yet its surface is where trouble lives. Small diameters such as 0.030 in and 0.035 in have a high surface-to-volume ratio, so oxidation and absorbed moisture reach a much larger share of the filler metal than in 0.045 in wire. Damp warehouse storage, or a spool handled with greasy gloves, is a real risk. Grease and oil lubricants supply hydrogen and can produce significant porosity, so porosity causes so often trace back to the consumable rather than the gas bottle.

GTAW: the cleanest setup, with caveats

TIG has no flux, no coating and an inert argon envelope over a relatively cool, tightly controlled pool. Filler rod can be cleaned bright with a dedicated stainless brush, and the arc kept short and stable. That makes GTAW the least porosity-prone process for carbon steel, and the reason it dominates where radiographic acceptance is strict, from aerospace TIG weldments to pressure piping.

Clean by default is not the same as immune. GTAW is still vulnerable on dirty or coated aluminum, where the tenacious oxide film is hygroscopic, and on stainless, where an unpurged root oxidizes and picks up oxygen that later shows up as porosity, corrosion initiation and loss of root strength.

FCAW and SAW: porosity hiding in the flux

Both take shielding from flux constituents rather than a bottle, so flux moisture and core wire condition drive the risk in self-shielded and gas-shielded wire alike. Self-shielded FCAW has no shielding gas, so arc length and travel speed control the flux-generated shield and a long, wandering arc invites air. SAW is similar: granular flux is hygroscopic and acts like a sponge once the container is opened, so store it per the manufacturer’s instructions and watch for moisture pickup.

Aluminum and stainless: different dominant drivers

On aluminum, oxide films and moisture dominate, so surface treatment matters more than any other control factor. For aluminum GTAW, clean the oxide with a dedicated stainless wire brush, run AC with balance cleaning to break the film, then weld promptly. Our guide to prepping aluminum for welding covers mechanical and chemical options, and the aluminum MIG guide covers wire and gas settings that keep oxide out.

On stainless steel, the root and backside need inert purging with argon. Tubes and vessels welded without a proper purge oxidize at the root, and back-pickling is often needed to restore corrosion resistance. Purging is part of the procedure, not a luxury.

Process comparison at a glance

Process	Typical gas flow	Main porosity driver	Susceptibility	First checks
SMAW (stick)	None, flux	Coating moisture, cellulose gas, core wire oxidation	Highest	Electrode storage, rod condition, weather
GMAW (steel)	20–30 cfh, 75/25	Wire moisture, grease on spool, turbulence	Moderate	Wire cleanliness, nozzle, flow check
GMAW (aluminum)	25–35 cfh argon	Oxide film and moisture on joint	High if unprepped	Surface prep, spool cover, gun liner
FCAW	0–25 cfh	Flux moisture, core wire, arc length	Moderate	Flux and wire storage, arc control
SAW	None, flux	Hygroscopic flux, moisture in coating	Low when dry	Flux storage, re-drying where allowed
GTAW (TIG)	10–20 cfh argon	Dirty filler rod, unpurged root	Lowest	Argon purity, filler rod cleaning, purge on stainless

Flow rates are typical shop starting points only. Your WPS governs, and a procedure qualified for a different flow, nozzle size or position can behave very differently in the field.

Reading the Bead: Telling Porosity Apart from Other Defects

By the time porosity reaches inspection, the gas is already long gone. The only way to undo that mistake is to read the evidence left behind. The shape, distribution, and location of each indication will tell you exactly where to start looking, and more often than not, it will point you to the source faster than changing your entire procedure.

Reading Porosity by Its Shape and Pattern

Not all gas voids look the same. According to TWI’s breakdown of weld porosity, the pattern is the biggest clue to the root cause.

Uniformly distributed spherical porosity means the issue was present for most of the weld. These are small, round, and smooth voids scattered throughout the bead, and they almost always point to a shielding or gas purity problem.

Clustered spherical porosity on the other hand, appears where the process changed. You’ll typically see tight groups of these round pores at stop/start points, crater fills, or unpurged restarts.

Linear groups of spherical pores confined to the very beginning of the weld are a red flag for contamination. Dirt, grease, or residual mill scale on the joint edge will often only release gases for the first few inches before being burned away. This is especially prevalent when working with reactive metals, such as those seen in aluminum MIG welding, where any leftover film can cause issues.

Piping porosity (also known as wormhole porosity) is far more severe. These are tear-drop shaped voids with a linear distribution, and they’re most easily identified on a radiograph by their streamlined shape. Herringbone, or fishbone porosity, is a close relative. Seen mostly in automatic arc welding, it forms regularly spaced, tear-drop shapes caused by air contamination being pulled into the shielding envelope. Both wormhole and piping porosity almost invariably indicate severe moisture contamination, organic coatings, or galvanised surfaces, and are rarely conditionally acceptable.

Porosity vs. Its Look-Alikes

The biggest mistake made on the shop floor is mistaking a crack for a pore. A pore is round and smooth, whereas a crack has sharp, jagged, and linear edges. The two have completely different structural consequences, so cracks must never be brushed off as gas voids.

There are plenty of other defects that can trick an untrained eye as well. Lack of fusion appears as an unfused, rope-like line between weld passes or the toe, with no smooth, rounded cavity. Undercut is a groove melted into the base metal at the toe, not a hole left in the weld metal. Slag inclusions are typically angular or irregular in shape, often with a darker, glassy appearance, and are most common with processes that utilize slag, unlike the rounded voids from gas. Tungsten inclusions are dense, bright specks seen on radiographs, and are only found when tungsten from a GTAW electrode has been transferred into the weld.

With reactive materials, this distinction is even more important. Proper joint prep is vital, and utilizing the correct aluminum cleaner for welding can prevent the exact type of contamination that is often misdiagnosed as something else entirely. These visual differences are echoed by resources such as Welding Australia’s fabrication defects guide.

Your Three-Step Diagnostic Routine

With so many possibilities, you don’t need to guess. Stick to this simple, repeatable order to track the source down before you scrap another joint:

• Bead inspection first. Look at the weld as a whole. Is the porosity uniform, clustered at a stop, linear at the start, or in a herringbone pattern? Map the location before touching anything else.
• Joint inspection second. Look at the faying surfaces, edges, and back side. Check for oil, rust, moisture, or coatings, especially on open roots where air can be drawn from behind.
• Equipment inspection last. Only after the weld and joint have been ruled out should you move to regulators, hoses, nozzles, liners, wire, and consumable storage. More often than not, the answer is found in the first two steps.

In the end, porosity doesn’t need to be a mystery. By learning to read the bead, you’ll be diagnosing the defect by its history, not by trial and error. That’s the difference between chasing bubbles, and keeping them out for good.

Finding and Measuring Porosity: NDT and Code Acceptance Limits

Prevention is half the job. Even a clean shop can trap voids inside a weld, and nobody knows until a film shows them. Then the question is “does this weld pass?”

Visual: the first look at surface-breaking pores

Visual inspection (VT) is the cheapest test you run. Surface-breaking pores show as round craters 0.5 to 3 mm across, pits clustered where the arc stopped or restarted, a sugary or frosted bead texture, and black specks that rub out as open pores.

Every structural code makes VT mandatory. VT also reveals the classic porosity patterns that hint at root cause.

Liquid penetrant: finding the pores that break through

Liquid penetrant testing (PT) is a surface method, far more sensitive than the eye for defects open to the surface. It works best on non-porous materials with clean contrast; on rough, oxidized, scaled or dirty surfaces background noise can be worse than the indication, and on dirty unpainted or corroded steel it is largely useless, because the penetrant soak

Finding and Measuring Porosity: NDT and Code Acceptance Limits

Prevention is half the job. Even a clean shop can trap voids inside a weld, and nobody knows until a film shows them. Then the question is “does this weld pass?”

Visual: the first look at surface-breaking pores

Visual inspection (VT) is the cheapest test you run. Surface-breaking pores show as round craters 0.5 to 3 mm across, pits clustered where the arc stopped or restarted, a sugary or frosted bead texture, and black specks that rub out as open pores.

Every structural code makes VT mandatory. VT also reveals the classic porosity patterns that hint at root cause.

Liquid penetrant: finding the pores that break through

Liquid penetrant testing (PT) is a surface method, far more sensitive than the eye for defects open to the surface. It works best on non-porous materials with clean contrast; on rough, oxidized, scaled or dirty surfaces background noise can be worse than the indication, and on dirty unpainted or corroded steel it is largely useless, because the penetrant soaks in everywhere. Where it works it outlines the pore, telling cluster from isolated indication.

Radiography: the reference method for volumetric defects

Radiographic testing (RT) is the standard way to find porosity inside a weld. A film or digital detector sits behind the joint, the source fires through, and density differences show as contrast. Gas voids are less dense than steel, so pores appear as rounded dark spots on a lighter background.

RT works on any material at any thickness and is the only method giving a true two-dimensional picture of pore size, position and spacing, which is why codes use it to accept volumetric defects. The TWI notes on weld porosity agree: radiography is the natural choice for gas porosity. It needs shielding, a qualified operator, certified film or detectors, time and skilled interpretation. Very thick plate needs high-energy gamma sources.

Ultrasonic testing: excellent for cracks, poor for pores

Ultrasonic testing (UT) uses sound waves rather than radiation, and it is more sensitive for planar defects like cracks and lack of fusion when they sit favorably to the beam. For porosity it is limited: a small spherical pore is a poor reflector, gives a weak and unreliable signal, and is easily lost in grain-structure and surface-roughness noise. Radiography detects volumetric discontinuities, while UT is far more reliable for cracks; a “high sensitivity” setup still finds planar defects, not pores. Complementary techniques such as eddy current and magnetic particle testing cover surface and near-surface indications.

Code acceptance limits: ISO 5817, ASME VIII, API 1104 and AWS D1.1

• ISO 5817, level B (the most widely used international weld quality standard, and the stringent level many codes cite; butt welds unless noted): gas pores, max projected area 1% single-layer and 2% multi-layer and uniformly distributed; single pore, d ≤ 0.2s, max 3 mm (s = weld thickness); clustered porosity, dA ≤ 15 mm or dA max ≤ Wp/2 (dA = sum of pore diameters, Wp = cluster length); linear porosity, h ≤ 0.2s max 2 mm, l ≤ s max 25 mm.
• ASME Section VIII Division 1 Appendix 4 (pressure vessels and boilers; UT acceptance of volumetric indications): rounded indications 1/4t or 4 mm, whichever is smaller (t = plate thickness); isolated indications 25 mm or more apart, 1/3t or 6 mm, whichever is smaller; aligned rounded indications, sum of diameters less than t within 12t; welds under 3 mm thick, max 12 indications in 150 mm.
• API 1104 (pipeline welds): a pore is a defect above 3 mm or 25% of the thinner nominal wall thickness, whichever governs; finish-pass porosity clustered, max 13 mm, the sum of individual pore diameters.
• AWS D1.1 (structural and piping): no visible piping porosity in a complete joint penetration butt weld transverse to the computed tensile stress; other groove and fillet welds, 3/8 inch summed diameter per linear inch and 3/4 inch per 12 inches of weld. The zero-tolerance rule is fatigue-driven.

Putting the limits to work

Before work starts, note which code governs, which NDT method and extent apply, and who interprets the test.

Keep the baseline: a solid, well-supported electrode choice and a clean joint still matter, since passing an RT on a weld full of scattered small pores can still be a near-failure. On aluminium, cleaning and purge discipline changes everything, covered in our guide to prepping aluminium for welding.

Stop the gas entering the pool, then prove it did not.

A Step-by-Step Troubleshooting Routine and the Mistakes That Cause Repeat Porosity

Prevention gets you most of the way, but failed welds need a repeatable procedure, not a guessing game. Work down an ordered list, cheap checks first.

Work Down the List in Order

1. Confirm cylinder level. A cylinder near empty delivers gas at falling pressure, and the shielding collapses partway through a bead. It reads full at the start of shift and empty by lunch.
2. Run the regulator leak test. Open the valve, pressurize, shut the cylinder off and watch the gauge. If it falls, you have a leak, and no flow setting will save you. Leaks are often invisible: the back of the gun may not be fully seated in the receiving bushing, so it looks connected and still bleeds air. Gun assembly leak hunting starts there.
3. Verify flow rate and nozzle condition. Set the flow you actually get at the nozzle, not at the regulator. A spatter-blocked nozzle chokes the shield cone and lets air in at the edges. Check the liner, contact tip and gas diffuser.
4. Check contact tip to work distance and travel angle. Keep the nozzle close enough that the gas cone still covers the pool. Long stickout and a wandering nozzle are common causes of scattered porosity.
5. Inspect wire, liner and consumables. Look for rust, oil, fingerprints, a kinked liner or a whipping cable turned into a grease trap. Contamination travels into the pool.
6. Examine joint cleanliness and faying surfaces. Mill scale and rust create decomposition gases, oxidation and the risk of cold lapping and lack of fusion at the weld toe. Clean to bright metal just before welding.
7. Review the WPS parameters. High current, high travel speed or a long arc raise the gas the pool must absorb. Over-welding a joint that needed less heat adds porosity, not strength.
8. Only then suspect gas purity and dew point. If the first seven checks are clean, look at the supplier. Moisture pickup in long gas lines and cylinders stored in a damp shed are real causes, but the least common.

Cold Starts, Restarts and Crater Fill

Two of the most frustrating porosity patterns show up at the start and end of welds rather than the middle. Clustered porosity at weld starts and stops is a recurring symptom of cold arc starts and inadequate shielding purge, and no mid-weld technique fixes it.

Start a weld in a tight corner and the burst of shielding gas at ignition is seldom enough to purge the pocket before the pool forms, so the first millimetre solidifies in air in a cluster of pores. After a break or lunch the same happens: purging the gas line alone gives a gas-free start, because that purge was line pressure, not a live shield at the nozzle. Trigger the gun for a second or two with the wire off before striking.

At the other end of the bead the problem is crater fill. The arc breaks, the pool sits open, and the shrinking pool sucks air in. The result is a cone or teardrop shape at the stop, often called piped porosity. The fix: hold the arc a little longer than you think you need, feed filler into the crater, then move the heat slowly away rather than snapping the gun off.

The Repeat Offenders

Most shops that fight porosity year after year hit the same short list.

• Too high a flow rate. Turbulence at the nozzle edge entrains air instead of shielding the pool.
• An open root drawing air from the back side. Unprotected molten metal absorbs air easily, and an open root joint plus awkward nozzle position at an outside corner are recognized GMAW porosity triggers. Plug the root, back-burn it, or run a backing bar and seal the gap.
• Nozzle trouble, too far away or caked with spatter. At a distance the gas cone cannot follow the pool, and a caked nozzle acts as a diffuser that shreds the gas flow. Clean it on a schedule.
• Excessive push or pull angle. Steep angles cause porosity, lack of penetration and excess spatter, which is why the 10 to 20 degree travel angle rule exists.
• Unpurged restart. The most common cause of porosity clustered at the start of a bead.
• Not cleaning the joint immediately before welding. A joint degreased at the start of shift is dirty again by lunch. Clean where the welder stands.

Make It a Habit

Run the list the same way every time. A short, fixed routine surfaces the answer in minutes instead of after a rejected part. Log what you find. The long list of possible porosity causes is manageable only because each weld adds a data point, and repeat clusters at the same start point or nozzle position become a fixable instruction for the shop. The AWS B1.10M guide for welding castings is a reminder that welding quality is largely procedure discipline done consistently.

Building a Porosity-Free Shop: Daily Habits, Safety and Your Prevention Checklist

Porosity is won or lost in the boring parts of a welding operation: the cylinder check, the leak test, the oven temperature, the degreaser left by the fit-up bench. The shops that rarely find porosity are the ones where the routine is written down and repeated, not the ones with better welders.

The Start-of-Shift Routine That Prevents Most Porosity

Give every welder and fitter a five-item check that takes ten minutes, ordered from cheapest to most expensive to replace.

1. Cylinder check. Upright, chained, enough gas for the shift. A 300 cubic foot cylinder at 200 psi has plenty left; a 40 cubic foot cylinder at 80 psi does not.
2. Regulator leak test. Set the pressure, close the cylinder valve, watch the delivery gauge. A falling needle means a downstream leak, and most leaks are in the hose.
3. Flow check at the nozzle. Use a flowmeter on the nozzle, not the flow control dial. A long hose or cracked liner changes what the arc sees.
4. Consumable storage. Electrodes and flux-cored wire as intended, SAW flux dry, filler rods bagged, oven temperature checked.
5. Joint cleanliness. Clean and dry at the moment of welding. Degrease edges and faying surfaces again if touched.

Training Topics That Actually Move the Needle

Porosity training needs different emphasis than metallurgy and technique: gas flow too high or too low, reading a bead for indication shape and distribution, cleaning a joint, storing consumables, leak testing, and code acceptance criteria. Then train the people who are not welders. Fitters control joint cleanliness and fit-up gap, paint crews decide what coating is on the steel, and shipping decides where flux-cored wire lives. Plenty of porosity starts in departments that do not own a welding machine. Aluminum prep rules differ enough to earn their own session, and the guides on prepping aluminum for welding and aluminum MIG welding help.

WPS Discipline and Filler Metal Choice

Audit the WPS against the known porosity drivers: excessive current for the joint thickness, travel speed too slow for the amperage, arc length too long, gas flow outside the qualified range. Filler metal matters too. Deoxidizers such as manganese and silicon tie up dissolved oxygen, so some wires tolerate a dirty joint better. Where the WPS allows it, the higher deoxidizer wire wins. Same logic for all-position rod selection: match the rod to the position, then match the storage to the rod.

Trend the Defects, Do Not Just Count Them

Log every porosity rejection: date, welder, joint, WPS number, cylinder number, position and shape of the indication. Read the log monthly, not annually. Three pores at the same stop point on three jobs means one restart habit, not three gas problems. Pores that always land on one shift point to a different cylinder, wire lot or pre-weld clean. Use the defect reference from The Fabricator and the technical summary from TWI to build that cause list.

The Safety Side: What Porosity Prevention Costs You in Fume

The coatings you grind off before welding are often the reason the fume is dangerous, and removing them is a control measure OSHA recommends. The OSHA welding safety fact sheet spells it out: surfaces must be cleaned of any coating that could create toxic exposure, including solvent residue and paint, and hexavalent chromium is the headline constituent.

• Hexavalent chromium limits are far apart. OSHA’s permissible exposure limit is 5 micrograms per cubic meter over an 8-hour time-weighted average. NIOSH recommends 0.2 micrograms per cubic meter, 25 times lower, and the CDC hexavalent chromium page covers the health evidence. Meet the NIOSH number where you can.
• Welding fume causes real disease. Prolonged exposure may cause lung damage and cancers of the lung, larynx and urinary tract, plus metal fume fever, stomach ulcers, kidney damage and nervous system damage. Metal fume fever shows up as flu-like symptoms the evening after a heavy welding day.
• Manganese has its own signature. Long-term exposure to manganese fume can cause Parkinson-like symptoms, so control matters most on stainless and hardfacing work.
• Shielding gas is an asphyxiation hazard. Argon, helium and carbon dioxide displace oxygen, and carbon monoxide can form during welding. Confined space welding needs ventilation, monitoring and a permit. See 29 CFR 1910.134 for respirators, 1910.1200 for hazard communication, and 29 CFR 1910 Subpart Q for general welding rules. Construction work falls under 1926 Subpart J.

Nickel and manganese sit alongside hexavalent chromium as the principal hazardous constituents of welding fume, with iron, carbon monoxide and ozone. That is the argument for controlling the plume instead of tolerating it.

Ventilation and Process Substitution

Local exhaust works best when the hood, fume extractor gun or vacuum nozzle stays close to the plume source, so move the nozzle as the arc moves. Where the job allows it, substitute a less toxic process or consumable: GMAW generally produces less fume than stick welding on coated or galvanized material, and low fume or low hydrogen consumables can change a whole shift of exposure. Substitution removes the hazard instead of filtering it.

The Printable Porosity Prevention Checklist

• Cylinder upright, chained, full enough for the shift
• Regulator holds pressure with the outlet closed
• Flow verified at the nozzle, not at the machine
• Nozzle clean, gas holes clear, hoses unkinked
• Wire, liner and filler rod clean and dry
• Electrodes, flux-cored wire and SAW flux dry and closed
• Edges and faying surfaces degreased and dry before welding
• Paint, primer, rust, mill scale and cutting fluid removed
• Amperage, travel speed and arc length inside the WPS limits
• Crater filled, restarts purged with gas flowing
• Porosity logged and trended monthly
• Local exhaust at the plume, confined space permit in place

Conclusion

Porosity is controllable, and the control is never exotic. Control the gas, the moisture and the contamination, then check the equipment before you blame the procedure. The bubbles were always coming from somewhere. Find the source and they stop.

Frequently Asked Questions

Is porosity a defect that always has to be repaired?

Not always. Codes permit limited amounts of porosity, so a small isolated pore in a statically loaded joint may pass inspection under ISO 5817, ASME Section VIII or AWS D1.1. Porosity that is clustered, linear, on a cyclically loaded joint, or associated with a crack must be excavated and rewelded.

Why does my weld get porous only at the start and stop points?

Clustered pores at starts and stops almost always point to a cold arc start or a loss of shielding during restart, not to a bad gas cylinder. Purge the gun for a second or two with the wire off, establish the pool before moving, and make sure the corner pocket is cleared of air before the arc starts.

Is increasing the shielding gas flow rate the answer to porosity?

Only if the flow is genuinely too low. Excess flow creates turbulence at the nozzle that pulls room air into the weld zone and makes porosity worse. Stay within the range on your WPS, typically around 30 to 50 CFH for GMAW and 20 to 30 CFH for GTAW, and fix leaks or draughts instead of opening the valve.

What is the most common cause of porosity in a fab shop?

In most cases it is a shielding problem rather than a chemical one: a leak in the gas delivery system, a spatter-blocked nozzle, or an air current disturbing the shield. Shop experience consistently ranks empty cylinders, draughts and blocked nozzles at the top of the list, with hydrogen from contamination and moisture behind them.

Can I weld over galvanized steel without getting porosity?

You can, but you need to manage the zinc. Zinc melts near 420 degrees F and vaporizes at welding temperatures, producing both fume and trapped gas. Grinding the coating off before welding is the most reliable fix when the job allows it; otherwise use a more forgiving process such as SMAW or FCAW, and expect extra fume extraction.

How do I stop hydrogen cracking, which often starts at porosity?

Porosity gives cracks a starting point, so control the hydrogen in the first place. Dry and store electrodes, flux and wire properly, clean and degrease the joint immediately before welding, remove rust and mill scale, and use low-hydrogen consumables and preheat on higher-strength steels such as A514, A588 and A709.

Does preheating the metal really help with porosity?

It helps with a specific cause: moisture. Preheating to roughly 200 to 225 degrees F drives off condensation from plate and faying surfaces, which is a real risk on heavy sections, lap joints and cold mornings below 50 degrees F. Preheating will not fix shielding or contamination problems.

Can weld porosity be detected without cutting the weld open?

Surface-breaking pores can be found by visual inspection and confirmed by liquid penetrant testing, while internal porosity requires radiography or ultrasonic testing. Radiography is the reference method for volumetric pores because a round pore shows as a distinct dark round indication on the film, but acceptance criteria must be specified in the contract documents.

Why does porosity look worse in GTAW than in GMAW even though TIG is cleaner?

GTAW is generally the least porosity-prone process because there is no flux and the setup is clean. When it does appear, suspect the surface: aluminum oxide films, moisture, or dirty filler rod and gloves transferring grease. On stainless tube, also check whether the root was purged properly.

How do I know if the porosity is a shielding problem or a contamination problem?

Look at the pattern. Fine, evenly distributed pores across the whole bead point to shielding or gas purity. Pores in a line that follows the joint edge point to surface contamination. Pores clustered at starts and stops point to cold starts and shielding interruption. Elongated wormholes point to heavy moisture, organic coatings or galvanized steel.