Mill scale is not the same as the base steel, it doesn’t stretch and it creates poor bonds. It’s rust, not the red type, this type is dark grey. You have to remove the scale before you forge weld, if you don’t you’ll get bad welds. But sometimes you want this type of rust. For example did you ever have a wrench you got from your parent that would never rust (red type) well that’s because it already had. In this picture you can see the cracks in the mill scale and the lighter base steel below.