What's inside
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What Slag Coverage Means
With a 7018 stick electrode, slag coverage is the layer of molten flux that floats over the weld puddle and solidifies behind it. It shields the cooling weld metal from the atmosphere, shapes the bead, and slows cooling. Good coverage is continuous and slightly wider than the bead. It should peel or chip away without gouging the steel.
Slag coverage is not the same as burying the weld in flux. A heavy, irregular slag layer can hide undercut, cold lap, or slag inclusions. The goal is a steady puddle with the slag following behind it, not a large pile pushed ahead of the arc.
Set Up the Welder Correctly
Most 7018 electrodes run best on direct current electrode positive (DCEP), although some 7018 products are rated for AC or DC. Check the package rather than assuming. DCEP usually gives a stable arc and good penetration on a standard stick welder.
Use the electrode diameter and joint thickness to choose an initial amperage. The useful range varies by brand and position, but these are practical starting points for flat-position work:
| 7018 diameter | Typical starting range | Common use |
|---|---|---|
| 3/32 inch | 70–100 amps | Thin material, fillets, and out-of-position work |
| 1/8 inch | 95–135 amps | General fabrication and medium plate |
| 5/32 inch | 140–190 amps | Heavy plate and larger fillets |
Turn the current down if the puddle becomes wide and fluid, the edges wash away, or the slag runs ahead of the arc. Turn it up slightly if the electrode sticks repeatedly, the arc sounds weak, or the bead sits tall and narrow. Excessive amperage can produce a thin, fast-moving slag layer that does not protect the puddle consistently.
Clean the joint to bright metal. Remove mill scale, paint, oil, rust, and moisture for at least 1 inch on both sides of the weld. Contamination creates gas and can disrupt the slag blanket, causing porosity or wormholes.
Control Arc Length and Electrode Angle
Arc length has a direct effect on slag coverage. Hold a short arc, roughly equal to the diameter of the bare electrode core. With a 1/8-inch rod, that means an arc around 1/8 inch long. A long arc makes the weld spatter, increases atmospheric exposure, and can produce porous, uneven coverage. It also makes the rod more difficult to control.
Use a drag angle of about 10 to 15 degrees from perpendicular, moving the electrode in the direction of travel. Keep the rod pointed into the leading portion of the puddle, while allowing the slag to trail behind. Too much drag angle can push slag under the arc and trap it in the weld. A nearly vertical rod usually gives better control when you are learning.
Travel fast enough to keep the puddle controlled, but not so fast that the bead becomes narrow and rope-like. A typical 1/8-inch 7018 fillet may travel around 3 to 6 inches per minute, but amperage, joint size, and position change that number. Watch the puddle rather than trying to match a fixed speed.
Use the Right Motion
For a narrow bead, use a straight drag. This is often the easiest way to get continuous slag coverage. For a wider fillet or groove weld, use a small weave with a brief pause at each sidewall. Keep the weave no wider than about two to two and a half electrode diameters unless the procedure calls for more.
Do not whip a 7018 in and out like a fast-freeze cellulosic rod. Whipping can leave thin spots in the slag and create a series of cold overlaps. If the rod starts to stick, do not jerk it sideways. Twist it free or release the holder, then restart on clean metal.
Remove Slag Between Passes
For multi-pass welds, chip and wire-brush every pass before starting the next one. Pay special attention to the toes and the ends of the bead, where slag commonly remains trapped. A welding chipping hammer and wire brush are sufficient for most shop work; a small angle grinder helps when slag is tightly bonded.
Clean starts and stops matter. Restart about 1/2 inch ahead of the crater, run back through it, and continue forward. At the end, pause briefly to fill the crater, then release the arc while maintaining control. A crater crack or unfilled stop can become a leak or fatigue crack even when the rest of the slag coverage looks good.
Recognize Bad Slag Coverage
Good slag is usually dark, glassy, and easy to lift in long pieces. Bad coverage often shows up in the bead before you remove it. Look for excessive spatter, pinholes, a wandering arc, or slag flowing in front of the puddle.
| Problem | Likely cause | Correction |
|---|---|---|
| Slag trapped between passes | Poor cleaning, excessive weave, or slag pushed ahead | Chip and brush fully; shorten the arc and reduce weave width |
| Porosity or pinholes | Long arc, damp rods, dirty steel, or moisture | Use a short arc, clean the joint, and dry or replace the electrodes |
| Slag runs ahead of puddle | Travel too slow, excessive drag angle, or amperage too low | Increase speed slightly, hold nearer vertical, or raise amperage modestly |
| Slag difficult to remove | Undercut, cold lap, or an overly cold arc | Correct amperage and travel; do not rely on grinding to hide the defect |
Store 7018 Electrodes Properly
Low-hydrogen 7018 rods are more sensitive to moisture than ordinary 6013 electrodes. Keep unopened packages sealed. Once opened, store rods in a heated rod oven according to the manufacturer’s instructions. For occasional repair work, a small 7018 welding rod storage oven is useful, but it is not a substitute for following the electrode maker’s rebake limits.
Do not use electrodes with visibly damp flux, swollen coatings, or severe cracking. Moisture can cause porosity and hydrogen-related cracking, especially in thick, restrained, or high-strength joints. For a light-duty bracket made from clean mild steel, a fresh dry package may be enough; critical structural work needs a documented welding procedure and controlled consumable storage.
Protect Yourself While Welding
Wear a properly shaded auto-darkening helmet, flame-resistant clothing, gloves, and leather footwear. Use ventilation or local fume extraction, particularly when welding painted or coated steel. A quality auto-darkening welding helmet makes it easier to watch the puddle and judge slag behavior, but it does not replace proper ventilation or eye protection during chipping and grinding.