After a certain process time of air cooling, hot-dip galvanized steel pipes must be immediately subjected to 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 continue diffusing 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 will no longer have a bright pure zinc layer but instead will be covered by a gray-black iron-zinc alloy layer. In order to obtain both an appropriately thick iron-zinc alloy layer and a bright 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 hot-dip galvanized steel pipes with high temperatures from entering the passivation solution and affecting its effectiveness. The temperature of the passivation solution should not exceed approximately 50°C.
