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Slag inclusion is trapped nonmetallic material inside a stick-welded joint. It usually comes from incomplete cleaning between passes, an incorrect travel angle, poor arc control, or a weld profile that leaves sharp grooves where slag can hide. It is especially common in fillet welds, narrow grooves, and multi-pass joints made with E6013, E7018, or other flux-coated electrodes.

The fix is not simply turning up the amperage. A sound multi-pass weld depends on joint preparation, a controllable bead sequence, complete slag removal, and settings that keep the puddle fluid without making it run ahead of the arc.

Prepare the Joint Before Welding

Remove mill scale, paint, rust, oil, and moisture from the weld area. On thicker carbon steel, clean at least 1 inch on both sides of the joint. A grinder is usually faster than a wire brush for heavy scale, but do not gouge the groove or leave deep grinding marks that trap slag.

Use the correct groove angle and root opening for the thickness and welding procedure. If the groove is too narrow, the electrode cannot reach both sidewalls and slag will remain along the edges. If the root opening is too large, you may be tempted to use excessive heat or a long arc, increasing the chance of undercut and trapped flux.

Fit-up matters in fillet welds as well. A large gap or uneven corner can force you to weave excessively. Keep the pieces aligned and tack weld them securely. Broken or contaminated tack welds should be ground out rather than buried under subsequent passes.

Choose the Electrode and Settings

Electrode choice affects how easily you can prevent and remove slag. E6010 and E6011 have forceful arcs and are useful for root passes and less-than-perfect conditions, but their thin, rapidly freezing slag still requires careful cleaning. E6013 tends to produce a smoother arc and easier starts, but its softer puddle can make slag inclusion likely if you travel too slowly or bury the arc.

E7018 produces strong, low-hydrogen welds and a thick slag covering. It can make excellent multi-pass welds, but the slag must be removed completely and the electrodes must be kept dry according to the manufacturer’s instructions. For work where hydrogen cracking matters, an inexpensive hobby oven is not a substitute for following the electrode storage temperature and exposure limits.

Set amperage within the electrode manufacturer’s range, then adjust for position and joint design. As a rough starting point, a 1/8-inch electrode often runs around 90 to 130 amps, while a 3/32-inch electrode commonly runs around 70 to 100 amps. The correct value depends on electrode type, polarity, position, and machine. Too little current makes the arc stiff and the puddle cold; too much current creates undercut, excessive fluidity, and slag that can flow into the groove ahead of the arc.

Use the specified polarity. Many electrodes run better on DCEP, while some are designed for AC or have different characteristics on DCEN. If your machine has a poor or unstable arc at the selected setting, check polarity and cable connections before changing your technique.

Control the First Pass

The root pass establishes the shape that every later pass must follow. Keep a short arc, generally close to the electrode’s bare-core diameter. A long arc increases spatter and allows the puddle to become broad and difficult to control.

Point the electrode so the arc reaches both sidewalls. In a groove, a slight work angle toward the unwelded portion helps maintain penetration without letting the puddle run too far ahead. In a fillet weld, hold roughly equal attention on both plates and avoid pointing the electrode steeply into one corner.

Use a steady travel speed. A puddle that is too small may leave lack of fusion; a puddle that is too large allows slag to overtake the arc. If a weave is necessary, pause briefly at each sidewall and move across the center without lingering. Wide, decorative weaving is a frequent cause of inclusions. Several narrower beads are usually safer than one oversized pass.

Clean Between Every Pass

Let the weld cool enough that the slag is firm, then chip it with a pointed chipping hammer. Follow with a stiff wire brush. For stubborn E7018 slag, a grinder with a flap disc or small grinding wheel may be necessary, especially in the toes and corners. Do not assume a shiny surface means it is clean: thin glassy slag can remain along the edges.

Inspect the entire pass before depositing another. Pay particular attention to the start and stop areas, crater ends, sidewalls, and the narrow angle at the bottom of a fillet. If you cannot see the joint line clearly, you have not cleaned enough. A welding chipping hammer and wire brush set is adequate for many repairs; a grinder is faster for production-style work.

Grind out any visible wormholes, slag lines, undercut, or lack of fusion before continuing. Burying a defect rarely fixes it and makes later inspection or repair more difficult.

Use a Pass Sequence That Fits the Joint

For a groove weld, place a sound root pass first, then fill with narrow stringer beads or controlled small weaves. Keep each pass tied into clean metal on both sidewalls. On a large joint, alternate sides or use a balanced sequence when practical to reduce distortion, but do not sacrifice access or visibility just to make the sequence symmetrical.

For a fillet weld, avoid stacking beads so that a deep valley forms between them. The next bead should overlap the previous bead enough to fuse with it, while still leaving a visible, clean toe. If the joint requires several layers, stagger the stops and starts rather than creating one vertical column of weak transitions.

Maintain suitable interpass temperature. Welding too cold can leave poor fusion and a sluggish puddle; welding too hot can make the weld pool overly fluid and increase distortion. Use the procedure’s temperature limit when one is specified. Without a formal procedure, an infrared thermometer is a more reliable guide than judging heat by color alone, particularly on thicker steel.

Tools That Help Prevent Inclusions

A basic angle grinder with flap discs makes joint cleaning and defect removal much easier, but it is not mandatory for occasional light fabrication. A chipping hammer, wire brush, and good lighting are enough when the slag breaks cleanly.

For repeated work, a temperature crayon or infrared thermometer helps prevent overheating between passes. Use a stable welding machine with a clear current display, and choose a helmet with a reliable auto-darkening filter so you can keep your eyes on the puddle. The cheapest helmet may be fine for occasional use, but inconsistent darkening or poor optical clarity encourages poor arc placement.

Find the Cause of a Repeated Defect

A straight slag line beside the weld usually points to incomplete sidewall fusion, an incorrect work angle, or a bead that was too wide. Small isolated pockets often indicate leftover slag at a stop-start location. Slag trapped at the root suggests poor access, an unsuitable root profile, or insufficient penetration. Long, irregular inclusions can result from welding over mill scale or from letting slag run ahead of the arc.

When the same defect keeps appearing, make one change at a time: clean more aggressively, reduce the weave width, shorten the arc, adjust travel speed, or change amperage in small increments. For critical structural work, visual inspection is not enough; use the required inspection method, such as dye penetrant, magnetic particle, ultrasonic, or radiographic testing, and follow the applicable welding procedure.