Reasons for Frequent Occurrences of Missing Galvanization Inside and at the Ends of Galvanized Steel Pipes

Steel pipes are galvanized components with a significant difference between their length and diameter ratio. In particular, the inner walls of steel pipes also require a zinc coating, and the conditions inside are much harsher than those on the outer surface. This is because during pickling treatment, the inner wall cannot be pickled as thoroughly as the outer wall, leaving behind oxide scale, which can lead to missing galvanization. Even if the inner wall has all oxide scale removed through pickling, like the outer wall, there is still a chance for iron salts to adhere to the pipe wall, which cannot be cleaned or rinsed off as easily as the outer wall, also resulting in missed galvanization spots. When using a radiant dryer to dry steel pipes, the internal moisture will escape towards the ends when heated. Due to the faster heat dissipation at the ends of the steel pipes, the temperature is lower, making it easy for moisture to condense there, diluting the applied solvent. At the same time, immersing with moisture into the zinc bath can cause "explosion" and damage the solvent, leading to missing galvanization. When immersing in zinc, one end of the steel pipe is dipped first, and then the inclination of the steel pipe is gradually reduced until it is fully submerged horizontally in the zinc bath. This operation method allows zinc ash and moisture inside the steel pipe bore to be smoothly discharged. If the inclination angle is small and both ends of the steel pipe are immersed in the zinc bath simultaneously, the zinc ash and moisture inside the steel pipe cannot be discharged, which can easily burn or dilute the solvent and cause "explosion," resulting in missed galvanization spots. Phenomena such as the steel pipe rolling on the zinc bath surface can also lead to the possibility of missed galvanization spots in the middle.