Stick welding defects: slag, arc blow and porosity

October 31, 2026  ·  Indumor

On the left, in the cross-section of a multipass joint: 1 slag trapped between passes, 2 porosity, 3 undercut. On the right, 4 the arc deflected by arc blow near the end of the plate.

Slag inclusion: a non-metallic particle trapped inside the weld. In stick welding, the defect that best gives away a rush job. Together with arc blow and start porosity, it makes up the trio of problems that keeps coming back with this process. Why so many? Because the electrode leaves slag, depends on the hand and often works on DC and outdoors. The list below sets them out with their cause and cure.

• Trapped slag. It shows up when passes are not cleaned well, when the previous bead left grooves where slag hides, with too little current or with the rod tilted so far that the slag runs ahead of the pool. Cure: chip and brush every pass, grind out the grooves, enough current and a moderate drag angle.
• Arc blow. On DC, the arc's magnetic field gets distorted near plate ends, edges and the work clamp, and the arc is pushed to one side as if someone were blowing on it. Cure: switch to AC, move the clamp, add tack welds, tilt the rod against the blow and weld toward heavy tacks or a finished weld.
• Porosity. It comes from damp electrodes, especially low-hydrogen ones, from a long arc, or from surfaces with rust, oil or paint. On low-hydrogen rods it often appears at the start, from striking on the cold crater. Cure: dry rods, a short arc and striking ahead of the crater.
• Arc strikes. Marks outside the joint where the rod was struck. Cure: always strike inside the joint and, if any are left, grind them out and check them.
• Undercut. A groove at the toe, from too much current or arc, or an angle that points at the plate. Cure: lower the current, shorten the arc and correct the angle.
• Lack of fusion. Too little current, too fast a travel, or a pool running ahead. Cure: proper current and speed, with the arc on the leading edge of the pool.
• Spatter. Too much current, a long arc or arc blow. With low-hydrogen rods some coarse spatter is normal; with rutile rods, heavy spatter means something is wrong. Cure: current within the range, a short arc and, if there is arc blow, its own cures.
• Hydrogen cracks. Damp low-hydrogen rods, high-strength steels without preheat or very rigid joints. Cure: dry rods, preheat when required and inspection after some time.

The list can go on the wall at the station, but half the cures start before striking the arc: dry rods, a clean joint and a well-placed clamp. The other half is craft, and it is earned on test plates.

The slag also reports back. If it peels off by itself on a rutile rod, the settings are right; if it sticks along the edges, there is usually undercut hiding underneath, worth checking before the next pass.

In practice, one habit sums up the lesson: a pass left uncleaned is a pass done twice. If you have joints that must pass inspection, Indumor welds them with that care, and the quote costs nothing.

Self-test

Think of the answer before you open it.

1. What causes slag inclusions between passes?

Poor cleaning between passes, grooves that hide slag, too little current or an angle that lets the slag run ahead of the pool.

2. What is arc blow, and how is it corrected?

The deflection of the arc by its magnetic field on DC, near edges, plate ends and the clamp. It is corrected by switching to AC, moving the clamp, tacking and tilting the rod against it.

3. Why does porosity appear at the start with low-hydrogen rods?

From striking on the cold crater; it is avoided by striking ahead of the crater and coming back.

4. What must be done with an arc strike outside the joint?

Grind it out and check it, because it leaves a hard spot with microcracks.