11.3 Porosity & Gas-Related Imperfections

11.3 Porosity & Gas-Related Imperfections

Quick Answer: Porosity is a cavity group imperfection (ISO 6520): gas trapped in solidifying weld metal. Forms include single gas pores, uniformly distributed, clustered, linear, elongated cavities, and wormholes (piping). Causes include moisture, surface contamination, poor shielding gas control, wrong arc length, and solidification conditions. Prevention is process hygiene and parameter control. Under ISO 5817, pores are usually size- and density-limited by quality level—not automatically zero-tolerance like cracks.

Module WI1.7 and ISO 6520 cavity classification require IWI-S candidates to name porosity types, list shop-floor causes, prescribe prevention checks, and apply acceptance as limits, not slogans.

What Porosity Is (and Is Not)

A gas pore is a cavity formed by gas entrapped during solidification. Pores may be:

• Spherical or nearly spherical (classic gas pores).
• Elongated when gas tries to escape along solidification fronts.
• Wormholes / piping — elongated tubular cavities, often from root regions or layered solidification paths.
• Crater pipes — shrinkage/gas cavities in the crater (related but described carefully).
• Surface-breaking (visible as pinholes) or embedded (found by RT/UT).

Porosity is not a crack: tips are not sharp planar ruptures. Severity is usually lower than cracks of similar length, which is why acceptance standards allow limited porosity but reject cracks. Still, dense porosity reduces cross-section, can mask other flaws on RT, and may connect into leak paths in pressure equipment.

Common Morphologies Inspectors Must Name

Use precise names in reports: “clustered gas pores in weld metal, mid-run” is better than “bad porosity.”

Primary Causes

1. Moisture and hydrogenous contamination

Moisture on plate, in electrode coatings, in flux, or on wire produces gas in the arc and weld pool. Rust with absorbed moisture, condensation on cold plate brought into a warm shop, and damp consumables are classic sources. Cellulosic electrodes intentionally generate gas—but uncontrolled moisture on “low-hydrogen” processes is a defect generator.

2. Surface contamination

Oil, grease, cutting fluids, paint, primers, markers, and heavy mill scale in the weld zone decompose or trap gas. Shop primers may be allowed only to procedures that define removal width and approved primer types.

3. Shielding gas problems (GMAW/GTAW/FCAW gas-shielded)

• Wrong gas or mix for the process/material.
• Inadequate flow (porosity) or excessive flow (turbulence aspirating air).
• Leaks in hoses, loose fittings, contaminated gas lines.
• Draughts blowing shield away (fans, open doors).
• Spattered or damaged nozzles; excessive contact-tip-to-work distance.
• Damaged or wet gas lenses / poor torch maintenance on TIG.

4. Arc length and parameters

Excessive arc length increases atmospheric pickup risk on many processes and destabilises the pool. Travel speed too high can freeze gas in place; heat input too low can worsen gas escape. On MMA, wrong electrode angle and arc length are everyday porosity causes.

5. Joint design and position effects

Deep narrow preparations can trap gas. Overhead and vertical positions change pool behaviour. Root runs in pipe can show linear root porosity when purge or root cleanliness fails (TIG root with poor purge is a classic stainless/pipe story).

6. Material and consumable factors

Some alloys and consumables are more sensitive (e.g. certain aluminium practices with hydrated oxides). Wet aluminium oxide films are notorious hydrogen sources—link to Chapter 8 aluminium notes.

7. Solidification and crater effects

Rapid freezes trap bubbles. Crater pipes combine shrinkage and gas. Poor crater fill leaves surface cavities that may be shaped imperfections plus cavity imperfections.

Prevention Controls

Systems thinking: clustered porosity at every stop/start points to technique and restart hygiene; uniform porosity along a whole seam points to gas supply, consumable moisture, or base cleanliness system failures.

Detection

• VT — surface pinholes, crater pipes, some wormhole openings.
• RT — classic method for volumetric porosity pattern (rounded dark spots on film/digital); excellent for distribution assessment.
• UT — detects some cavities but pore fields can be harder to characterise than planar flaws; still used per procedure.
• PT — only surface-breaking pores; will not find embedded porosity.

For acceptance, size, spacing, and projected area / density rules in ISO 5817 (and product standards) matter—so NDT must support measurement, not only “porosity present.”

Acceptance Philosophy (ISO 5817 Context)

Unlike cracks:

• Single pores have maximum dimension limits that tighten from D → C → B.
• Clustered and localised porosity have area/density style limits.
• Linear porosity and certain elongated forms may have special treatment or stricter treatment depending on tables and joint.
• Surface pores may interact with shape and corrosion/fatigue concerns even when small.

IWI-S exam stance: porosity is often conditionally acceptable; the inspector must measure and compare, not auto-reject every pore or auto-accept every pore field. Contracted quality level and product standard rule.

Also watch combined imperfections: porosity plus lack of fusion, or porosity masking crack-like linear indications on RT, requires careful interpretation and sometimes secondary NDT.

Process-Specific Quick Notes

MMA/SMAW — damp electrodes, long arc, contaminated prep; basic electrodes left out of dry storage.

GMAW/MAG — gas flow extremes, draughts, spatter-blocked nozzle, oily wire, mill scale.

FCAW — self-shielded vs gas-shielded differences; moisture in flux core; stick-out length.

GTAW/TIG — poor purge on stainless pipe roots; contaminated tungsten/wire; draughts; moisture on aluminium.

SAW — damp flux, contaminated joint, improper flux depth; less “draught” issue but moisture in flux is critical.

Inspector Response to Porosity Findings

1. Classify morphology (single, clustered, linear, wormhole).
2. Locate (root, fill, cap, stop/start).
3. Quantify against acceptance tables.
4. If non-conforming, hold and investigate cause category (moisture vs gas vs technique).
5. Correct the system (baking, gas leak, cleaning) before mass repair.
6. Repair with approved methods (typically remove to sound metal and re-weld with corrected controls); re-inspect.

Blindly grinding and re-welding without fixing damp flux or a leaking gas hose reproduces the same pores on the next joint.

Link Forward

Section 11.4 moves to lack of fusion and lack of penetration—planar imperfections that, like cracks, are typically not permitted. Chapter 12 covers slag inclusions that RT beginners sometimes confuse with aligned porosity. Chapter 14 details RT film quality so pore sizing on radiographs remains trustworthy—an IWI-S duty is verifying film/image quality, not performing full radiographic interpretation as an RT Level III substitute.