welding flux may be used in a solid, liquid, or paste form. during welding, the flux will melt and some of it will evaporate. this creates a small pocket of gas around the weld. this pocket of gas prevents oxidation of the metal under weld. melted flux, through a corrosive reaction, cleans contaminants that prevent a proper weld. after welding, the flux solidifies. this layer of solid flux is called slag, and must be removed from the weld. the smaw weld process most commonly uses flux, and is most commonly used on steel.