73. Why is there more leakage in the interior and exterior of galvanized steel pipes?

Steel pipes are galvanized components with a significant length-to-diameter ratio. The inner walls require zinc plating under far harsher conditions than the outer surfaces. During pickling, the inner walls cannot be treated as thoroughly as the outer surfaces, leaving iron oxide scale that causes missed coating. Even when the inner walls are pickled as thoroughly as the outer surfaces, iron salts may adhere to the pipe walls and cannot be removed through washing or rinsing, resulting in missed coating spots. When drying steel pipes in radiant drying furnaces, internal moisture escapes to both ends when heated. Since the pipe ends cool faster, lower temperatures cause condensation in these areas, diluting the coating solvent. Additionally, moisture carried into the zinc bath may trigger "sparking" reactions that destroy the solvent, leading to missed coating. During zinc immersion, the pipe is first submerged at one end, then gradually tilted until fully submerged. This method allows effective drainage of zinc ash and moisture from the inner cavity. If the tilt angle is too small and both ends are immersed simultaneously, residual zinc ash and moisture cannot drain properly, potentially damaging the solvent or causing "sparking" that results in missed coating spots. Rolling the pipe on the zinc bath surface may also lead to missed coating spots in the middle.