After galvanized steel pipes are removed from the zinc bath, there is always an air cooling period, the duration of which depends on the production process. Practical production experience shows that a certain period of air cooling is necessary for galvanized steel pipes, benefiting the quality of the galvanized layer, production output, and the lifespan of the galvanizing pot. Therefore, the typical air cooling time ranges from 1 to 2 minutes.
A longer air cooling period can compensate for insufficient immersion time in the zinc bath, allowing for the formation of an iron-zinc alloy layer of the desired thickness, thereby increasing production output. Meanwhile, after a certain period of air cooling, the temperature of the galvanized steel pipes drops from the original range of 460–480°C to 300–350°C. They are then immersed in cooling water at 80°C. Water cooling under a smaller temperature gradient minimizes the checking (cracking) on the galvanized layer's surface and the curvature of the galvanized steel pipes. Without the air cooling step, directly quenching the galvanized steel pipes at 460–480°C in water would result in severe checking on the galvanized layer's surface due to the difference in coefficient of linear expansion between zinc and the steel substrate, thereby reducing corrosion resistance.
Hot-dip galvanizing with a lower zinc bath temperature and shorter immersion time cannot produce the necessary iron-zinc alloy layer. Generally, a longer air cooling period is adopted as a remedy. Therefore, galvanized steel pipes can be produced using a lower zinc bath temperature combined with a longer air cooling period. This reduces the temperature of the zinc bath and correspondingly extends the lifespan of the steel galvanizing pot.
