Welding slag is a vitreous material produced as a byproduct of some arc welding processes, most specifically shielded metal arc welding (also known as stick welding), submerged arc welding, and flux-cored arc welding). Slag is formed when flux, the solid shielding material used in the welding process, melts in or on top of the weld zone. Slag is the solidified remaining flux after the weld area cools). The thickness of the slag that is produced depends upon the type of flux and the amount of flux in the electrode. The more energy that is used, the more slag is produced, and the arc may jump and create spatter.

Slag does not contribute to the strength or protection of metals after the welding process; it is waste material. Removal of the slag is necessary for several reasons, including the ability to inspect the quality of the weld area, aesthetics or visual appearance, if a second layer or pass of welding is to be made on top of the first, and to clean and clear the surface for other coating processes). If areas of slag become embedded within the solidified metal, they can cause inclusions, which are a form of welding defect. Inclusions may be visible on the surface after cleaning, or they may be completely contained within the metal, in which case they can only be detected on X-rays of the weld, requiring grinding or drilling to remove (followed by re-welding that section) ).

In summary, slag is a byproduct of some arc welding processes that forms when flux melts in or on top of the weld zone. It is waste material that does not contribute to the strength or protection of metals after the welding process. Removal of the slag is necessary for several reasons, including the ability to inspect the quality of the weld area, aesthetics, and to clean and clear the surface for other coating processes.