Scale

Mill scale is the oxide layer that forms on metal when it's heated to high temperatures in air. It's a problem when welding because it contaminates the arc and the weld pool, leading to porosity and a dirty weld. Before welding, you have to remove the scale in the weld area, best with a wire brush or grinder, until you see bare metal. If you weld over old scale, you'll see it right away in the weld quality. And after welding, when the weld is still hot, scale forms again – that's normal, you can brush it off afterwards.