(1) Advantages of adding lead to zinc liquid
Adding lead to zinc liquid shortens the melting time of zinc, reduces the erosion of zinc liquid on the galvanizing pot, facilitates slag removal, and prevents the adhesion of zinc slag to the bottom of the galvanizing pot. Additionally, to obtain an aesthetic appearance on hot-dip galvanized thin steel plates, lead is also added to the zinc liquid.
The use of lead improves the wettability of zinc liquid to iron in galvanized steel, enabling a uniform distribution of zinc liquid on the surface of steel pipes and enhancing the aesthetic appeal and gloss of the 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 liquid is high, i.e., there is a large amount of zinc slag, adding an appropriate amount of lead can also improve the wettability of the zinc liquid.
(2) Disadvantages of adding lead to zinc liquid
Firstly, it reduces the fluidity of the zinc liquid. As shown in Figure 3-19, the fluidity of the zinc liquid is the worst when the lead content is 0.5%, and it only recovers to a level comparable to pure zinc when the lead content reaches 2%.
Secondly, when the lead content is too high, it not only fails to achieve the aforementioned surface finish of the galvanized layer but also turns it into a dark gray color. Therefore, the lead content is generally limited to below 0.5%. The most common lead content is around 0.24%, as seen in the strip steel galvanizing unit of Wuhan Iron and Steel Corporation's cold rolling plant. Additionally, when the lead content in the galvanized layer is excessive, it can affect corrosion resistance; however, some researchers believe that there is little impact.
