Six defects account for the vast majority of stick welding failures: slag inclusion, porosity, undercut, lack of fusion, excessive spatter, and arc blow. Each has specific causes, visible indicators, and proven fixes. Knowing what to look for and why it happens turns a bad weld into a learning opportunity instead of a mystery.
Every defect on this list is preventable. If you’re seeing any of them consistently, something in your technique, settings, or preparation needs adjusting. Here’s how to diagnose and correct each one.
Slag Inclusion
Slag inclusion is non-metallic material (hardened flux) trapped inside the weld metal. It weakens the weld by creating discontinuities in the metal structure and causes rejection on X-ray and UT (ultrasonic) inspection.
How to Identify
Visual: On the surface, slag inclusions sometimes appear as dark spots or lines in the weld bead where slag has been trapped at or near the surface. Between passes, you may see slag that won’t chip out because it’s embedded in the weld face.
X-ray/RT: Shows up as irregular dark spots or lines, distinguishable from porosity by their non-round, angular shape.
After grinding: If you grind into a weld and see dark, glassy streaks or pockets, that’s trapped slag.
Causes
Inadequate slag removal between passes. This is the number one cause. Every pass of stick welding produces a slag coating over the bead. If you don’t chip and brush it completely before the next pass, the new weld metal traps the old slag.
Weave too wide. On a wide weave bead, the puddle flows ahead of the arc at the edges. Slag pools in the center or at the toes where the metal is cooler. When you bring the arc back, you weld over that slag instead of clean metal.
Amperage too low. Low amperage lets the slag float ahead of the puddle. Instead of the arc burning through the slag, the slag leads the puddle and gets trapped under the advancing bead.
Improper bead profile. A bead with sharp, undercut toes creates valleys where slag collects. The next pass welds over these valleys and traps the slag.
Wrong rod angle. Pointing the rod too far backward (excessive drag angle) pushes slag forward under the puddle.
Fixes
- Chip every pass completely with a chipping hammer. Wire-brush the entire bead surface. Inspect visually and by feel before welding the next pass.
- Narrow your weave to 2-3 rod widths maximum. Use stringer beads instead of wide weaves on joints where slag trapping is a problem.
- Increase amperage by 5-10 amps so the arc stays ahead of the slag.
- Maintain a smooth, slightly convex bead profile that doesn’t create slag-trapping valleys.
- Use a slight push or perpendicular rod angle rather than an excessive drag angle.
Porosity
Porosity is gas pockets trapped in the solidified weld metal. Individual pores are spherical or elongated voids, ranging from pinhole size to 1/8" or larger.
How to Identify
Surface porosity: Visible pinholes on the weld surface after slag removal. They look like tiny craters or holes in the bead face.
Subsurface porosity: Only visible on X-ray, where pores appear as round, dark spots. Sometimes detected during bend testing when pores near the surface open up as cracks.
Wormholes: Elongated pores that follow the direction of welding. They appear as dark tubes on X-ray. Caused by gas escaping upward through the solidifying metal.
Causes
Moisture in the electrode coating. The primary cause, especially with E7018. Water in the flux decomposes to hydrogen and oxygen in the arc, and the gas gets trapped in the solidifying weld. See 7018 Rod Storage and Handling.
Base metal contamination. Rust, oil, grease, paint, galvanized coating, and moisture on the base metal surface all produce gas when heated by the arc.
Arc too long. A long arc length weakens the gas shield, allowing atmospheric nitrogen and oxygen to reach the molten pool. These gases dissolve in the liquid metal and form pores during solidification.
Wind. Outdoor welding in wind disrupts the shielding gas from the flux coating. See Stick Welding in Wind.
Contaminated filler metal. Rods with damaged coatings (chipped, cracked, or flaking) expose the core wire and compromise shielding.
Welding too fast. Extremely high travel speed doesn’t give gas bubbles time to escape the puddle before solidification.
Fixes
- Store E7018 in a rod oven at 250-300F. Recondition exposed rods at 700-800F for one hour.
- Clean the base metal. Grind rust, mill scale, and paint within 1" of the joint. Degrease with acetone or contact cleaner.
- Shorten the arc to one rod-core-wire diameter.
- Use wind screens outdoors, or switch to E6010/E6011 which tolerate wind better.
- Discard rods with damaged or chipped coatings.
- Slow down travel speed to give gas time to escape the puddle.
Undercut
Undercut is a groove melted into the base metal along the toe of the weld that isn’t filled by weld metal. It reduces the cross-section of the base metal at the most stressed point (the toe) and acts as a stress concentrator that can initiate cracking.
How to Identify
Visual: Run your fingernail along the toe of the weld where it meets the base metal. Undercut feels like a groove or channel. It’s often visible as a dark line along one or both sides of the bead.
Measurable: AWS D1.1 limits undercut to 1/32" depth for most structural work. Anything deeper requires repair.
Causes
Amperage too high. Excess heat melts a groove in the base metal at the bead toe, but the filler metal flows toward the center of the bead instead of filling the groove.
Travel speed too fast. The arc melts the base metal at the toe, but the filler metal can’t flow out to fill it before the arc moves on.
Incorrect rod angle. Aiming the arc at the base metal instead of into the joint concentrates heat on one side, melting a groove.
Insufficient pause at weave toes. During a weave, the arc must pause at each edge long enough for filler metal to flow out and fill the toe. Rushing through the edges causes undercut.
Arc too long. A long arc spreads heat and reduces the arc force that pushes filler into the toes.
Fixes
- Reduce amperage 5-10 amps.
- Slow travel speed so the puddle has time to fill the toes.
- Angle the rod into the joint, not at the base metal surface.
- On weave beads, pause 1-2 seconds at each toe.
- Shorten the arc to concentrate heat and filler deposition.
- If undercut has already formed, grind it smooth and add a cosmetic pass or repair bead.
Lack of Fusion
Lack of fusion (also called incomplete fusion) means the weld metal didn’t fully bond to the base metal or to a previous weld pass. The weld appears bonded visually but has an unfused interface that acts like a crack under load.
How to Identify
Visual (sometimes): A “cold lap” visible at the weld toe where the bead sits on top of the base metal rather than fusing into it. The toe looks like a ridge rather than a smooth transition.
Bend test: Lack of fusion opens up during bend testing as a crack along the fusion line.
X-ray/UT: Shows as a linear indication along the weld fusion line or between passes.
Causes
Amperage too low. The arc doesn’t generate enough heat to melt the base metal surface or the previous pass.
Travel speed too fast. The arc passes over the base metal before it has time to melt.
Wrong rod angle. The arc isn’t directed at the base metal surface. It’s depositing filler on top of unmelted base metal.
Insufficient interpass cleaning. Slag or oxide on the surface of a previous pass prevents fusion with the next pass.
Cold start. The beginning of a bead where the base metal is still cold may lack fusion until enough heat builds up.
Excessive weave width. On wide weave beads, the edges of the oscillation may not receive enough direct heat for fusion.
Fixes
- Increase amperage by 5-10 amps.
- Slow travel speed.
- Direct the arc at the joint surfaces, not into the air above the puddle.
- Clean every pass completely before the next.
- On restarts, overlap the previous bead by 1/4" to 3/8" and ensure the arc melts into the existing weld.
- Narrow the weave and spend more time at the edges.
Excessive Spatter
Spatter is the droplets of molten metal that fly off the arc and stick to the base metal surface around the weld. Some spatter is normal, especially with cellulosic rods (6010, 6011). Excessive spatter wastes filler metal, requires cleanup, and indicates unstable arc conditions.
Causes
Arc too long. The most common cause. A long arc increases the electromagnetic force that flings molten droplets away from the puddle.
Amperage too high. Excess current creates an explosive transfer of metal from the electrode to the workpiece.
Wrong polarity. Running a rod on the wrong polarity destabilizes the arc transfer. E6010 on AC or 7018 on DCEN both produce excessive spatter.
Moisture in the coating. Moisture creates micro-explosions in the arc as water flash-boils, flinging metal outward.
Magnetic arc blow. The wandering arc transfers metal erratically, sending droplets in all directions.
Wind. Wind disrupts the arc and causes irregular metal transfer.
Fixes
- Shorten the arc to one rod-core-wire diameter.
- Reduce amperage by 5-10 amps.
- Verify correct polarity for the rod type.
- Check electrode storage conditions.
- Switch to AC if arc blow is the cause.
- Apply anti-spatter compound to the work area to make cleanup easier. This doesn’t fix the cause but reduces cleanup time.
Arc Blow
Arc blow is the deflection of the welding arc by magnetic fields in the workpiece. The arc wanders off to one side of the joint despite correct rod angle, or it feels like an invisible force is pushing the rod away from where you’re aiming.
How to Identify
- Arc deflects consistently to one side
- Spatter accumulates heavily on one side of the bead
- Undercut appears on one side only
- Bead profile is asymmetric (built up on one side, washed out on the other)
- Problem is worst at the ends of joints or near edges of the plate
- Problem disappears or reduces when switching to AC
Causes
DC current in ferromagnetic material. The unidirectional current creates a persistent magnetic field. Where the field is asymmetric (at edges, ends, near ground clamps), it deflects the arc.
Magnetized workpiece. Steel can retain magnetism from previous contact with magnets, magnetic chucks, or simply from sitting in one position for years. This residual magnetism deflects DC arcs.
Ground clamp position. The ground clamp creates a magnetic field distortion at its connection point. Arc blow tends to push the arc away from the ground clamp.
Welding near edges or corners. The magnetic field is asymmetric at the edges of a plate because it can’t circulate through the steel on one side.
Fixes
| Fix | Effectiveness | How to Do It |
|---|---|---|
| Switch to AC | High (usually eliminates arc blow) | Change machine output or switch to an AC machine |
| Move ground clamp | Moderate to high | Place ground at the end of the joint you're welding toward |
| Weld toward tack welds | Moderate | Place heavy tacks at the far end of the joint |
| Shorten arc length | Moderate | A shorter arc is harder for magnetic fields to deflect |
| Wrap ground cable | Moderate | Wrap the ground lead around the workpiece several times |
| Reduce amperage | Low to moderate | Lower current produces weaker magnetic field |
| Demagnetize workpiece | High (for residual magnetism) | Use a demagnetizing coil before welding |
For a full discussion of polarity and arc blow, see AC vs DC Stick Welding.
Other Common Issues
Crater Cracking
A crack that forms in the crater (the depression left when you break the arc at the end of a bead). The crater solidifies last and shrinks inward, creating tensile stresses that cause a star-shaped crack.
Fix: Fill the crater before breaking the arc. At the end of each bead, pause briefly, shorten the arc slightly, and let the crater fill with filler metal. Then break the arc by pulling quickly upward. Some welders reverse travel direction for 1/4" at the end to fill the crater.
Overlap (Cold Lap at Surface)
Weld metal that flows over the base metal surface without fusing to it. Unlike lack of fusion (which is at the joint interface), overlap occurs at the surface toe. It creates a stress concentration and doesn’t meet workmanship standards.
Fix: Increase amperage, slow travel speed, or adjust rod angle so the arc heats the base metal at the toe. This is often a travel-speed issue: the puddle is too cold when it reaches the toe.
Incomplete Penetration
The weld doesn’t extend through the full joint thickness. On groove welds, the root bead doesn’t reach the back side of the joint.
Fix: Open the root gap, reduce the root face thickness, increase amperage for the root pass, use a smaller diameter rod that can access the root, or switch to a more penetrating rod (E6010).
Defect Prevention Checklist
Before starting any stick weld, run through this quick checklist:
- Base metal clean? Ground to bright metal at the joint, wire-brushed for 1" on each side.
- Rod in good condition? Coating intact, properly stored, correct type for the application.
- Polarity correct? Check the rod’s rated polarity and verify lead connections.
- Amperage set? Start at the middle of the rod’s range for the position you’re welding.
- Fit-up right? Proper gap, root face, bevel angle, and tack welds.
- Previous pass cleaned? All slag removed, surface wire-brushed and inspected.
- Arc length right? One rod-core-wire diameter. Adjust tighter for overhead, wind, or thin material.
If you check these seven items before every weld and pass, you’ll eliminate the majority of defects before they happen.