Definition - What does Weld Scale mean?
Weld scale is a layer of material that forms on top of a weld bead and sometimes on the adjacent base metal after a weld cools. Weld scale is typically a different color than the underlying weld and the surrounding base material. Weld scale can be removed through a variety of means, as it frequently is for aesthetic and corrosion protection purposes.
Corrosionpedia explains Weld Scale
Weld scale should not be confused with weld slag. Weld slag forms on top of a weld bead after the weld cools when using a welding process that uses a flux. Weld slag is inherent to processes such as shielded metal arc welding, flux cored arc welding and submerged arc welding. Weld scale, on the other hand, is not the result of solidified flux. Weld scale is the result of oxidation.
Weld scale is generally thought of as unappealing. For this reason, weld scale is often removed to improve the appearance of the welded component. Another concern with leaving weld scale on a welded part or structure is corrosion. If the weld scale is not removed before a coating is applied then the coating may adhere to the weld scale and not the base material. The weld scale is likely to fall off over time, taking with it the coating that was meant to protect the base metal.