What Causes Porosity in Stick Welding: Complete Guide and Facts
Porosity in a stick weld is usually caused by gas becoming trapped in the molten weld pool before the metal solidifies. Moisture, surface contamination, a long arc, poor electrode storage, and inadequate shielding are the most common reasons.
Understanding what causes porosity in stick welding helps you correct the problem instead of simply grinding out the visible holes. The appearance may range from small pinholes on the surface to larger internal voids that weaken the weld and can remain hidden beneath the slag.
What causes porosity in stick welding?
Porosity occurs when gas bubbles form in liquid weld metal and cannot escape before solidification. In shielded metal arc welding (SMAW), the electrode coating should create protective gases and slag that keep atmospheric oxygen, nitrogen, and hydrogen away from the molten pool.
If the coating is damaged or damp, the base metal is dirty, or the arc is operated incorrectly, unwanted gas can enter the weld. The bubbles become pores as the weld freezes. Some pores open to the surface, while others remain embedded inside the bead.
The principal causes are:
- Moisture in the electrode coating or on the workpiece
- Oil, grease, paint, rust, mill scale, or dirt on the joint
- Excessive arc length
- Incorrect welding current or travel speed
- Improper electrode angle or manipulation
- Drafts that disturb the shielding gases
- Damaged, contaminated, or improperly stored electrodes
- Insufficient cleaning between weld passes
Moisture and damp electrodes
Moisture is one of the most common causes of porosity in stick welding. Water on the joint or in the electrode coating breaks down in the intense heat of the arc. This releases hydrogen and steam into the weld area. If the gas cannot escape, it forms bubbles in the solidifying metal.
Electrodes can absorb moisture from humid air, especially after the package has been opened. Low-hydrogen electrodes are particularly sensitive to storage conditions. Their coating is designed to limit hydrogen, but that benefit is reduced when the coating becomes damp.
A wet electrode may show several symptoms: excessive spatter, an unstable arc, difficult starts, visible pinholes, or a rough weld surface. However, a dry-looking rod is not necessarily free of absorbed moisture. Proper storage, sealed packaging, and manufacturer-specified rebaking or holding procedures may be necessary for moisture-sensitive electrodes.
Do not use an electrode if its coating is cracked, loose, swollen, oily, or visibly contaminated. The coating is part of the welding system; damage can interfere with both shielding and arc performance.
Dirty base metal and joint surfaces
Contamination on the workpiece can generate gas when it is heated. Oil, grease, paint, plastic residue, cutting fluid, rust, and moisture are common sources. Even a thin coating may produce porosity if it lies directly under the arc or along the joint edges.
Rust and mill scale can also interfere with fusion. As they break down in the arc, they may release gases or trap contaminants between the weld metal and base metal. This is especially problematic at the root of a joint, where gases have fewer paths to escape.
Clean the joint area until bright, sound metal is exposed where practical. Remove grease before using a grinder, because grinding can spread or heat contaminants. Also clean the electrode holder, work clamp connection, and nearby surfaces if dirt or oil could be pulled into the weld area.
Galvanized or zinc-coated steel requires special care. Heating the coating can release hazardous fumes and may contribute to weld defects, including porosity. Coating removal, ventilation, and appropriate respiratory and hot-work controls are essential. Never assume a painted or plated surface is safe to weld without preparation.
Arc length and welding technique
An arc that is too long is a frequent technique-related cause. A long arc exposes the molten pool to more atmosphere and reduces the effectiveness of the electrode’s shielding. It can also increase spatter, produce a wide and irregular bead, and make the arc harder to control.
Maintain the arc length recommended for the specific electrode, generally close to the electrode’s core wire diameter unless the electrode manufacturer specifies otherwise. The correct length depends on electrode type and position, so the package guidance and qualified welding procedure take priority.
Travel speed matters as well. Moving too quickly can leave inadequate shielding over the trailing edge of the pool and may produce an irregular, narrow bead. Moving too slowly can create an oversized, overheated pool that allows gases to remain trapped. A steady travel speed and consistent arc length are more reliable than trying to correct defects after the weld is complete.
Electrode angle affects the direction of the arc force, slag coverage, and gas protection. Excessive dragging, pushing, or weaving can expose part of the pool or allow slag to roll ahead of the weld. Use the technique specified for the electrode classification and welding position instead of applying one motion to every rod.
Incorrect current and poor arc starts
Welding current that is too low can produce a weak, unstable arc and poor fusion. Current that is too high may overheat the coating, increase spatter, and disturb the protective slag. Either condition can contribute to gas entrapment, particularly when combined with an incorrect arc length.
Use the amperage range printed on the electrode package as the starting point. Adjust within that range for position, joint fit-up, material thickness, and the behavior of the arc. A rough or erratic arc is a reason to stop and check the setting rather than continue depositing metal.
Porosity can also appear at arc starts and stops. A poor restart may leave a crater, an unprotected area, or a small pocket at the beginning of the bead. Before restarting, remove the crater and slag as needed, then strike the arc slightly ahead of the previous termination and work back into the bead. Follow the applicable welding procedure for required restart techniques.
Wind, drafts, and lost shielding
Although stick welding does not use a separate shielding-gas cylinder, it still depends on shielding gases produced by the electrode coating. Strong wind, fans, open doors, and cross-drafts can disperse those gases before they protect the molten pool.
Outdoor welding is especially vulnerable. A weld may look acceptable in calm conditions but become porous when air movement reaches the arc. Use a suitable windbreak that does not create a separate safety hazard, and avoid welding in conditions where the shielding cannot remain over the pool.
Do not try to compensate for wind by holding an excessively long arc or changing the current without a controlled reason. Protect the work area first, then maintain the normal technique.
Slag, pass cleaning, and joint fit-up
Slag protects the cooling weld metal, but it must be removed before depositing the next pass. Trapped slag can create inclusions and may also retain contaminants or moisture that contribute to surface defects. Chip and wire-brush each pass until the weld area is clean and sound.
Incomplete cleaning is particularly risky where the next bead overlaps a rough or deeply grooved previous pass. The new molten metal may not properly wash the old surface, leaving pockets at the interface.
Joint fit-up also affects porosity indirectly. Excessive gaps, poor alignment, or an inaccessible root can make it difficult to maintain consistent shielding and arc control. A stable joint allows the electrode to reach the intended location and lets the welder maintain a uniform bead profile.
How to identify the source
Begin with the location and pattern of the pores. Isolated pinholes at the start or end of a bead often point to a poor arc start, restart, or termination. Porosity spread throughout the weld suggests damp electrodes, contamination, excessive arc length, or inadequate shielding.
Surface pores concentrated over one section may indicate local oil, paint, rust, moisture, or a draft. Defects that appear only with one electrode package may indicate storage damage or moisture absorption. If the problem occurs with multiple electrodes and materials, inspect technique, current, joint preparation, and the environment.
Do not rely only on the visible surface. Porosity can be internal even when the cap looks smooth. Depending on the application, inspection may include visual examination, dye penetrant testing for surface-breaking defects, radiographic testing, or other methods required by the governing code or welding procedure.
How to prevent porosity
- Clean the joint thoroughly and remove moisture, oil, paint, rust, and other contaminants.
- Use electrodes that are appropriate for the base metal, position, and procedure.
- Keep unopened electrodes sealed until they are needed.
- Store opened electrodes according to the manufacturer’s temperature and exposure requirements.
- Reject rods with damaged, cracked, loose, or contaminated coatings.
- Set the welding current within the electrode manufacturer’s recommended range.
- Keep the arc short and consistent for the electrode type.
- Maintain the correct angle and a steady travel speed.
- Remove slag completely between passes.
- Shield the work from drafts and wind.
- Inspect starts, stops, and restarts carefully.
If porosity is found in a completed weld, do not simply cover it with another pass. Remove the defective area to sound metal, clean it, and repair it using the approved procedure. For structural, pressure-retaining, lifting, transportation, or code-regulated work, the repair and inspection requirements must be followed by qualified personnel.
FAQ
Can wet welding rods cause porosity?
Yes. Damp electrodes can release hydrogen, steam, and other gases into the arc and weld pool. Those gases may become trapped as pores. Moisture-sensitive electrodes should be stored, handled, and reconditioned only according to the manufacturer’s instructions.
Does a long arc cause porosity in stick welding?
Yes. A long arc weakens the electrode’s protective shielding and exposes the molten pool to atmospheric gases. It commonly causes spatter, an irregular bead, and surface or internal porosity. Maintaining the proper arc length is a basic corrective step.
Can dirty metal cause pinholes in a stick weld?
Yes. Oil, grease, paint, rust, mill scale, moisture, and plating can release gas when heated. Clean the joint to sound metal before welding, and use appropriate precautions when removing galvanized coatings or other hazardous materials.
Why does porosity appear only at the beginning
Start porosity can result from an unstable arc, poor electrode contact, an unclean starting area, or an incorrect restart technique. Clean the start location, establish the arc correctly, and follow the specified method for tying into the previous bead.
Can wind cause porosity even when using stick electrodes?
Yes. Stick electrodes create their own shielding gases, but wind can disperse those gases before they protect the molten pool. Use a suitable windbreak and avoid welding where drafts prevent stable shielding.
Conclusion
The main answer to what causes porosity in stick welding is trapped gas created by moisture, contamination, lost shielding, or inconsistent welding technique. Dry electrodes, clean metal, proper settings, a controlled arc, thorough slag removal, and protection from drafts prevent most cases. When porosity is discovered, remove it to sound metal and repair the weld according to the applicable procedure rather than covering the defect.
