Causes of porosity in a structural weld

Porosity appears as small round or elongated cavities within or at the surface of a weld. Gas becomes trapped in the molten weld pool and remains as the metal solidifies. A few pores may be permitted within limits for a particular weld category, but excessive or improperly located porosity can reduce weld quality and require repair.

Why gas becomes trapped

The two common contributors are inadequate shielding and contamination at the joint. A shielding-gas flow that is too low, disrupted by drafts, or otherwise unsuitable may fail to protect molten metal from the atmosphere. Oil, moisture, rust, paint, coatings, mill scale, or dirt on the base metal can release gas or interfere with sound fusion. Contaminated flux or incorrect consumable storage can also contribute, depending on the welding process.

Prevention starts before the arc

• Remove oil, moisture, rust, paint, coatings, and other contamination from the joint area as required by the procedure.
• Store and handle electrodes, flux, and filler materials according to their specifications.
• Verify shielding-gas type, cylinder condition, flow rate, hoses, and connections.
• Protect the arc and weld pool from drafts or wind that can disrupt shielding.
• Use the qualified welding procedure and correct process settings.
• Inspect the completed weld using the specified acceptance criteria and method.

Inspection and repair

Visual inspection may reveal surface-breaking pores, but internal porosity can require nondestructive examination specified for the work. Acceptance depends on the governing construction code, weld type, loading, location, and project specification. If a weld exceeds the permitted limit, remove the unacceptable material and repair it using the approved procedure; do not cover the defect with another pass without evaluating it.

Do not confuse porosity with other defects

Porosity is gas-related cavity formation. Slag inclusions, lack of fusion, cracks, and undercut have different causes and inspection implications. A chain of small surface pits may suggest shielding or contamination issues, while an elongated cavity may prompt a different examination. Identify the discontinuity before deciding what process correction is needed.

Key takeaway

Porosity points to trapped gas, often from poor shielding or a contaminated joint. Correct surface preparation and stable shielding, then judge the weld against the specified code rather than a generic pore-count rule.

Read the pore pattern as a process clue

The appearance and location of porosity can help an inspector decide what to investigate, but appearance alone does not establish acceptance or root cause. Scattered rounded pores may point toward gas entrapment, while a line of pores can suggest a recurring shielding or surface-preparation problem. Surface pits reveal only discontinuities open to the surface; internal pores may require the examination method specified for the work. Record where the indication occurs, the weld process, joint position, material, and relevant procedure variables.

A single weld may have more than one contributing condition. A cylinder can be full while a leaking hose, blocked nozzle, incorrect regulator setting, or turbulent flow still disrupts shielding. A clean plate can become contaminated by condensation, dirty gloves, cutting fluid, or storage against a wet surface. Check the full process chain instead of immediately changing one setting.

Troubleshoot systematically

First stop and identify the extent of the affected work. Follow the quality plan for marking, inspection, and disposition; do not continue production in a way that repeats a suspected defect. Verify the welding procedure and confirm that the process, filler material, shielding gas, joint preparation, and environmental limits match it. Inspect the gas supply and delivery components, including connections and the torch or nozzle, then check for drafts or wind at the arc.

Next review surface preparation and consumable handling. Remove contaminants using an approved method without introducing new residue. Check that electrodes or flux were stored and conditioned as specified. Confirm fit-up and arc technique against the procedure, and have a qualified welding professional make any process adjustment. After the cause is addressed, make a test weld or resume only under the applicable quality controls, then inspect using the required acceptance criteria.

Repair requires an approved disposition

Porosity is a discontinuity; whether it is rejectable depends on the governing code and the project’s acceptance criteria. The inspector or responsible quality authority determines whether the indication is acceptable, needs further examination, or must be repaired. A repair should identify the affected region, remove the discontinuity to sound material as required, and use the approved repair procedure. Adding a cosmetic pass over a pore can conceal the indication without correcting it.

After repair, perform the inspection required by the code and quality plan, which may include visual examination or nondestructive testing. Keep traceable records of the weld identification, indication, repair, welder or procedure where required, and reinspection result. If similar porosity appears in multiple welds, treat it as a process issue and check other work made under the same conditions.

Worked example: porosity appears after a move

A welder moves from a sheltered shop area to an exposed site and begins seeing pores in otherwise similar welds. The material and consumables may be unchanged, but wind can disturb shielding at the arc. The team should protect the welding area as permitted, verify gas delivery and procedure limits, and inspect affected welds under the project plan. Increasing gas flow without checking the cause may create turbulence and worsen shielding. The correction should be verified before production resumes.

Separate cause correction from acceptance

A process correction helps prevent new discontinuities; it does not decide whether an existing indication is acceptable. Keep those decisions separate. The inspection authority applies the contract documents and governing code to the affected weld, while the welding team investigates the source and verifies the corrected process. This prevents an apparently successful adjustment from being used to waive a required repair or examination.

For exam questions, follow the sequence: recognize porosity as gas-related, check shielding and contamination, follow the qualified procedure, and use the specified inspection and acceptance criteria. Avoid memorizing a universal number of allowable pores because limits depend on the applicable code and weld category.