Why is acid pickling necessary for steel tube billets?

The purpose of acid pickling steel tube billets is to remove the oxide scale formed on the steel tube itself and during welding. Even though some oxide scale may peel off when the strip billet is being shaped, the exposed steel substrate will quickly form rust with oxygen and moisture in the air. If these two types of oxides are not removed before hot-dip galvanizing, they will prevent the reaction between the steel substrate and the molten zinc. Consequently, the area will not be coated with zinc, resulting in missed galvanizing and black spots.