Why are there more missed galvanizing spots inside and at the ends of galvanized steel pipes?

Steel pipes are coating pieces with a significant difference in length-to-diameter ratio, especially when it comes to galvanizing the inner wall of the steel pipe, where conditions are much harsher than those on the outer surface. During acid pickling, the inner wall cannot be thoroughly cleaned like the outer wall, leading to the presence of iron oxide scale and subsequently missed galvanizing. Even if the inner wall is completely pickled of iron oxide scale like the outer wall, there is still a chance for iron salts to adhere to the pipe wall, which cannot be washed or rinsed off as easily as the outer wall, resulting in missed galvanizing spots. When using a radiant drying oven to dry steel pipes, internal moisture will escape towards the ends upon heating. Due to the faster heat dissipation at the ends of the steel pipe, the temperature there is lower, making it easier for moisture to condense. This dilutes the applied solvent and damages it due to moisture entering the zinc bath, causing a "popping" phenomenon. This leads to missed galvanizing. During zinc dipping, one end of the steel pipe is submerged first, and then the inclination of the steel pipe is gradually reduced until it is fully immersed in the zinc bath horizontally. This operation allows zinc ash, moisture, and other impurities in the inner bore of the steel pipe to be smoothly discharged. If the inclination angle is small and both ends of the steel pipe are submerged 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 "popping," leading to missed galvanizing spots. Phenomena such as the steel pipe rolling on the zinc bath surface can also result in missed galvanizing spots in the middle.