66. Why should steel pipes be tilted at an angle when dipping into molten zinc for plating?

When steel pipes are immersed in molten zinc for galvanizing, they must be tilted at an adequate angle, especially for small-diameter pipes where the central sagging and bending are more pronounced, requiring a steeper incline. What causes this? First, unlike other products, steel pipes are hollow, slender objects with poor rigidity [particularly for small-diameter pipes of 10-25 mm (3/8 "to 1")]. Without proper inclination and rapid zinc immersion, trapped air cannot be expelled. When both ends fill with molten zinc, the central air expands due to heat, causing the zinc to spray out and create hazards. This is especially dangerous when the solvent hasn't dried-evaporating water into steam increases volume by hundreds of times, triggering violent spraying of molten zinc. This leads to coating defects like mottling on the inner pipe walls. Second, when the angle is too small for small-diameter pipes, the central sagging section and ends simultaneously contact the zinc surface. The heated section rapidly elongates, bending into an arc that presses downward. The zinc's resistance prevents further descent, causing the pipe to tumble on the surface. As the galvanizing turntable continues lowering the pipe (as shown in Figure 3-156), the curved pipe's central air expands and sprays out, allowing the inner hole to fill with zinc. If the solvent hasn't dried, the spraying pressure becomes even greater, making the process more hazardous and prone to severe coating defects. Therefore, a steeper inclination angle is required for zinc dipping of steel pipes. The minimum angle must prevent the pipes from rolling after zinc immersion. The optimal configuration, as shown in Figure 3-15c, avoids extensive single-sided contact with molten zinc, thereby minimizing thermal expansion and reducing pipe bending. This design ensures the pipes remain stable during rotary disk immersion, preventing rolling. Simultaneously, gases trapped within pipe holes can escape smoothly. Maintaining uniform zinc coverage across the entire pipe surface effectively prevents leakage. This principle proves particularly effective in step-type rotary disk plating machines with steep dipping angles, whereas continuous rotary systems exhibit more pronounced rolling issues, especially when plating small-diameter pipes. Rolling-induced leakage is especially critical during aluminum-containing zinc plating processes.