What Is Porosity? Weld Defect

Porosity is a weld defect consisting of gas pockets (voids) trapped within the solidified weld metal. These voids form when dissolved gases (hydrogen, nitrogen, or oxygen) cannot escape from the molten weld pool before it solidifies. Porosity reduces the effective cross-sectional area of the weld and, in severe cases, can compromise the pressure integrity of piping joints.

Type	Description	Typical Cause
Scattered porosity	Randomly distributed individual pores throughout the weld	General contamination, moisture in electrode
Clustered porosity	Localized group of pores in one area	Contaminated start/stop area, arc blow
Linear porosity	Pores aligned along the weld axis	Contamination along a joint edge or between passes
Piping porosity (wormholes)	Elongated, tubular voids oriented perpendicular to the weld surface	Excessive moisture, high hydrogen source
Surface porosity	Pores that break the weld surface	Gas shielding loss at the final solidification front

Causes and Prevention

Cause	Prevention
Moisture on base metal	Preheat, clean, and dry the joint area
Contaminated filler metal	Store electrodes in heated ovens per manufacturer specs
Oil, grease, or paint on joint	Solvent-clean and grind surfaces before welding
Inadequate shielding gas	Check gas flow rate (15-25 L/min for GMAW), fix leaks, use wind screens
Excessive arc length	Maintain short arc to keep shielding effective
Rust or mill scale	Grind to bright metal in the weld zone
High humidity	Use low-hydrogen electrodes, increase preheat on carbon steel
Excessive travel speed	Slow down to allow gas to escape before solidification

Acceptance Criteria

Code/Standard	Maximum Allowable Porosity
ASME B31.3	Per acceptance criteria of the applied NDE method (RT per ASME Section V, Article 2)
ASME Section VIII	RT: scattered porosity within charts of Appendix 4 (size and density limits)
AWS D1.1 (static)	Sum of diameters must not exceed 10 mm in any 25 mm of weld
AWS D1.1 (cyclic)	More restrictive; individual pore max 2 mm
API 1104	Individual pore max 3 mm; cluster max 13 mm in any 300 mm
EN ISO 5817 Level B	Max pore diameter: lesser of 0.3 x wall or 3 mm

Pro Tip

If porosity keeps recurring despite clean joint preparation, check the shielding gas system for leaks, contaminated gas cylinders, or a damaged nozzle. Even a small air leak in the gas hose introduces enough nitrogen and oxygen to cause scattered porosity throughout the weld.

Detection

Porosity is detected by radiographic testing (RT), where gas pores appear as round or oval dark spots on the radiograph. Ultrasonic testing (UT) can also detect porosity but is less effective for small, scattered pores. Surface-breaking porosity is identified during visual inspection or by liquid penetrant testing (PT).

Read the full guide to NDT testing