After galvanized steel pipes are removed from the molten zinc, there is always an air-cooling period, whose duration depends on the production process. From practical production, it is known that a certain period of air cooling for galvanized steel pipes is necessary, benefiting the quality of the galvanized layer, production output, and the lifespan of the galvanizing pot. Therefore, the typical air-cooling duration ranges from 1 to 2 minutes.
A longer air-cooling period can compensate for insufficient zinc dipping time to form the required thickness of the iron-zinc alloy layer, thereby increasing production output. Meanwhile, after a certain period of air cooling, the temperature of the galvanized steel pipe drops from the original range of 460–480°C to 300–350°C, and then it is immersed in 80°C cooling water. Water cooling under a smaller temperature gradient minimizes the cracking pattern on the galvanized layer's surface and the curvature of the galvanized steel pipe. Without the air-cooling step and directly quenching the galvanized steel pipe at 460–480°C in water, significant cracking will occur on the galvanized layer's surface due to the different coefficient of linear expansion between zinc and the steel pipe substrate, reducing corrosion resistance.
Using a lower molten zinc temperature and shorter zinc dipping time for hot-dip galvanizing cannot form the required iron-zinc alloy layer, and longer air cooling is generally adopted as a remedy. Therefore, producing hot-dip galvanized steel pipes can be achieved using a lower molten zinc temperature and longer air cooling. In this way, with a lower molten zinc temperature, the lifespan of the steel galvanizing pot is correspondingly extended.
