When welding slag occurs, it is important to remove the slag as soon as possible. Slag can cause problems with the weld and lead to corrosion. If left on the surface, it can also form a protective coating that inhibits the weld from sticking to other parts of the metal. There are a few ways to remove slag: Use a wire brush to remove the slag. Use a scrubber to clean the area. Use a pickling solution to dissolve the slag.