After a certain process time of air cooling, hot-dip galvanized steel pipes must immediately undergo water cooling. During air cooling, the temperature of hot-dip galvanized steel pipes decreases slowly, allowing zinc atoms to continue diffusing towards the steel substrate and iron atoms to diffuse towards the zinc layer. As a result, the pure zinc layer is gradually consumed until it is completely depleted. At this point, the surface of the hot-dip galvanized steel pipe is no longer covered by a shiny pure zinc layer but instead by a gray-black iron-zinc alloy layer. To achieve both an appropriately thick iron-zinc alloy layer and a shiny pure zinc layer surface, it is necessary to prevent the continued diffusion of iron and zinc atoms. This is the rationale behind immediately performing water cooling after the specified process time.
Another benefit of water cooling is to prevent high-temperature hot-dip galvanized steel pipes from entering the passivation solution and affecting its effectiveness. The temperature of the passivation solution should not exceed approximately 50°C.
