Once the steel has been completely cleaned, it is ready for immersion in the liquid zinc. The galvanizing kettle contains zinc specified to ASTM B 6, a document that specifies any one of three different grades of zinc that are each at least 98% pure. Sometimes other metals may be added to the zinc melt in order to promote certain desirable properties in the galvanized coating.
The galvanizing kettle, like the one seen in Figure 5, is typically operated at a temperature ranging from 820-860 F (438-460 C), at which point the zinc is in its liquid state. The steel products are immersed into the galvanizing kettle and remain in the kettle until the temperature of the steel has reached the temperature required to form a hot-dip galvanized coating. Once the interdiffusion reaction of iron and zinc is completed, the steel product is withdrawn from the zinc kettle. The entire dip usually lasts less than ten minutes, depending upon the thickness of the steel.
The coating, as seen in the micrograph in Figure 6, is typical for low silicon steels with silicon impurities less than 0.04% and where the thickness of the coating is limited by the interdiffusion of iron and zinc.
Post-Treatment
When the steel is removed from the galvanizing kettle, it may receive a post-treatment to enhance the galvanized coating. One of the most commonly used treatments is quenching. The quench tank contains mostly water but may also have chemicals added to create a passivation layer that protects the galvanized steel during storage and transportation. Other finishing steps include removal of zinc drips, or icicles, by grinding them off.
