Porosity is the most common volumetric weld defect: gas that gets trapped in the weld pool and remains as cavities after solidification. A few small pores rarely compromise a statically loaded joint — but porosity above the acceptance limit reduces the effective load-bearing section, acts as an initiation site under fatigue, and is one of the most frequent causes of NDT rejection and rework.
This guide covers what porosity is, the types defined by ISO 6520-1, what actually causes it on the shop floor, the ISO 5817 acceptance limits per quality level, and how to detect and prevent it.
What is porosity in welding?
Porosity (ISO 6520-1 group 2 — cavities) is a gas-filled cavity in the weld metal, formed when gases — hydrogen, nitrogen, oxygen, or shielding gas itself — are absorbed by the molten pool and cannot escape before the metal solidifies.
The main types classified by ISO 6520-1:
| Type | ISO 6520-1 code | Appearance |
|---|---|---|
| Single gas pore | 2011 | Isolated spherical cavity |
| Uniformly distributed porosity | 2012 | Pores spread along the weld |
| Clustered (localized) porosity | 2013 | Group of pores in one zone |
| Linear (aligned) porosity | 2014 | Pores in a line, often along the fusion boundary |
| Elongated cavity / wormhole | 2015 / 2016 | Tubular cavity, often herringbone pattern |
| Surface pore | 2017 | Pore breaking the weld surface |
The distinction matters because acceptance limits differ by type: aligned porosity is treated more severely than scattered pores, since a line of cavities behaves more like a planar discontinuity.
What causes porosity
Practically all porosity comes down to one of three sources:
1. Shielding problems
- Gas flow too low — or too high, causing turbulence that aspirates air
- Drafts in the welding area (doors, fans, ventilation above ~2 m/s)
- Clogged or spatter-loaded nozzle, damaged diffuser
- Wrong torch angle or excessive stick-out increasing air entrainment
2. Contamination
- Moisture, oil, grease, rust, or paint on the joint
- Zinc coating (galvanized steel) vaporizing into the pool
- Damp electrodes or flux — hydrogen from moisture is also the driver of cold cracking
- Condensation on cold plates brought into a warm shop
3. Process parameters
- Arc voltage too high (longer arc = more air entrainment)
- Travel speed too fast — the pool freezes before gas can escape
- Power source dynamics mismatched to transfer mode, producing erratic short circuits
A useful diagnostic rule: scattered porosity across many welds points to gas or consumables; porosity repeating at the same location points to contamination or a draft at that position.
ISO 5817 acceptance limits
ISO 5817 sets limits for porosity per quality level (B = stringent, C = intermediate, D = moderate). The reference values for the two most common cases:
| Imperfection | Code | Level B | Level C | Level D |
|---|---|---|---|---|
| Single pore (max diameter) | 2011 | d ≤ 0.5 mm or 0.05t | d ≤ 0.5 mm or 0.1t | d ≤ 0.5 mm or 0.2t, max 3 mm |
| Uniform porosity (projected area) | 2012 | ≤ 2% | ≤ 4% | ≤ 8% |
t = nominal material thickness, d = pore diameter. Clustered (2013) and linear (2014) porosity carry separate, tighter limits. These values are indicative references — always verify against the current edition of ISO 5817 and any overriding code or customer specification.
For the full picture of how quality levels work and which one your project requires, see the ISO 5817 acceptance criteria guide and the Level B vs C vs D comparison.
How porosity is detected
- Visual testing (ISO 17637) — only catches surface-breaking pores (2017).
- Radiographic testing (ISO 17636) — the reference method: pores appear as rounded dark indications, easy to size and count for the area-percentage limits.
- Ultrasonic testing — detects subsurface porosity but sizing scattered small pores is harder than with RT.
- Real-time process monitoring — thermal imaging and high-speed vision detect the conditions that generate porosity (shielding loss, erratic transfer, pool disturbance) while the weld is being made. See how this works in practice: real-time porosity detection with thermal imaging.
The economics are simple: a pore found by RT after the fact costs an NCR, excavation, and re-weld. A shielding-gas fault flagged in real time costs one interrupted seam. The full comparison of inspection approaches is here: AI weld defect detection — thermal vs vision vs acoustic.
Prevention checklist
- Gas: verify flow at the nozzle (not just the regulator), check for leaks in hoses and fittings, shield from drafts.
- Cleanliness: degrease and remove rust/paint/zinc at least 25 mm from the joint edges.
- Consumables: store dry, respect re-drying procedures for fluxes and basic electrodes.
- Parameters: keep arc length/voltage at WPS values; reduce travel speed if porosity persists.
- Monitor: track the process continuously so shielding or contamination drift is caught on the first weld, not at final NDT — this is exactly the gap in-process monitoring closes.
Porosity is one entry in the broader defect landscape — for the complete map of welding defects and their acceptance criteria, see the welding defects guide.
Frequently Asked Questions
What causes porosity in welding?
Porosity is caused by gas trapped in the solidifying weld pool. The most common sources are inadequate shielding gas coverage (wrong flow rate, drafts, clogged nozzle), surface contamination (moisture, oil, rust, paint, zinc coating), damp or contaminated consumables, excessive arc length or voltage, and travel speed too high for the gas to escape before solidification.
Is porosity acceptable under ISO 5817?
It depends on size, distribution, and the specified quality level. As a reference, ISO 5817 limits a single pore to a diameter proportional to material thickness (stricter at Level B than D), and uniformly distributed porosity to a maximum percentage of the projected cross-section area — roughly 2% at Level B, 4% at C, and 8% at D. Clustered and aligned porosity have tighter, separate limits. Always verify against the current edition of the standard.
How is porosity detected in welds?
Surface-breaking pores are found by visual testing per ISO 17637. Subsurface porosity requires volumetric NDT — radiographic testing (ISO 17636) is the classic method because pores show up as dark rounded indications, with ultrasonic testing as an alternative. In-process thermal and high-speed visual monitoring can flag the process drift that generates porosity in real time, before the seam reaches final inspection.
How do you prevent porosity in MIG/MAG welding?
Verify shielding gas flow (typically 12–18 l/min for MAG), shield the arc from drafts above roughly 2 m/s, clean joint surfaces of oil, rust, moisture and coatings, store consumables dry, keep stick-out consistent, and avoid excessive voltage or travel speed. If porosity recurs at the same point of a robotic cycle, look for a torch orientation or fixture-draft issue at that exact position.
