Slag — What It Is and Why It Matters
Slag is the glassy, non-metallic residue that forms on the surface of a weld made with flux-bearing processes — stick welding (SMAW), flux-core welding (FCAW), and submerged arc welding (SAW). Slag is a byproduct of the flux chemical reactions and serves a protective purpose during welding: it blankets the solidifying weld metal, shielding it from atmospheric contamination and controlling the cooling rate.
After the weld cools, the slag must be removed by chipping (with a chipping hammer) and wire brushing. Thorough slag removal is critical between passes in multi-pass welds — any slag left between passes becomes a slag inclusion defect trapped inside the weld. On finished welds, slag is removed for inspection, appearance, and to prepare the surface for paint or coating.
Different electrode types produce different slag characteristics. E7018 produces a thick, heavy, self-peeling slag that lifts off easily. E6010 produces a lighter, thinner slag that requires more aggressive chipping. Self-shielded flux-core slag can be particularly tenacious and difficult to remove. Learning the slag behavior of your consumables helps plan cleanup operations.
Frequently Asked Questions
Why does MIG welding not produce slag?
Solid-wire MIG (GMAW) uses shielding gas instead of flux for weld protection. No flux = no slag. This is one of MIG's advantages — no chipping, no slag removal between passes. Flux-core MIG (FCAW) does use internal flux and produces slag that must be removed.
What is the best way to remove slag?
Start with a chipping hammer to break loose the bulk slag. Follow with a stiff wire brush (stainless steel brush for stainless welds, carbon steel brush for carbon steel). For stubborn slag in grooves and tight areas, a needle scaler or small grinder with a flap disc may be needed. Ensure every trace is removed before the next pass.