(1) Advantages of Adding Lead to Zinc Bath
Adding lead to the zinc bath shortens the melting time of zinc, reduces the erosion of the galvanizing pot by the zinc bath, facilitates slag removal, and prevents the adhesion of zinc slag to the bottom of the galvanizing pot. Moreover, to obtain an aesthetically pleasing exterior pattern on hot-dip galvanized thin steel sheets, lead is also added to the zinc bath.
The use of lead improves the wettability of the zinc bath to iron in galvanized steel, enabling the zinc bath to distribute evenly on the surface of the steel tube. This results in a more aesthetically pleasing and glossy galvanized layer (within the specified lead content range of the process, typically 0.2%-0.25%, equivalent to the natural lead content of Zn-4 grade). When the iron content in the zinc bath is high, indicating a high amount of zinc slag, adding an appropriate amount of lead can also enhance the wettability of the zinc bath.
(2) Disadvantages of Adding Lead to Zinc Bath
Firstly, it reduces the fluidity of the zinc bath. As shown in Figure 3-19, the fluidity of the zinc bath is worst when the lead content is 0.5%. It only recovers to a level comparable to pure zinc when the lead content reaches 2%.
Secondly, excessively high lead content not only fails to achieve the aforementioned smoothness on the galvanized layer surface but instead turns it dark gray. Therefore, the lead content is generally limited to below 0.5%, with the most common range being around 0.24%, as seen in the strip galvanizing unit of Wuhan Iron and Steel Corporation's Cold Rolling Plant. In addition, an excessive amount of lead in the galvanized layer can also affect corrosion resistance; however, some researchers believe there is little to no impact.
