It should be that the weld is uneven, and then the convex part is rubbed when rounding, and the zinc layer falls off or sticks to it, resulting in black lines. Just clean this up and spray some galvanized paint.

If there is no black line in the protruding part, the weld may crack, which is troublesome. It needs to be repaired after cracking, and the cracks should be polished before welding, otherwise the quality problem of the pipe will be great. If it has not been rounded, it is necessary to evaluate whether it can be rounded again. In the case of more cracks, the cost of repair welding is too high.