67. What are the advantages and disadvantages of adding lead to zinc liquid?

67. What are the advantages and disadvantages of adding lead to zinc liquid? (1) The benefits of adding lead to zinc liquid include reducing melting time, minimizing corrosion of galvanized pots, facilitating slag removal, and preventing adhesion between zinc slag and pot bottoms. Additionally, this method is also employed on hot-dip galvanized thin steel plates to achieve aesthetically pleasing surface patterns.
The application of lead enhances the zinc bath's wettability for iron in galvanized steel, ensuring uniform distribution across the pipe surface. This process improves the appearance and gloss of the galvanized coating (within the standard lead content range of 0.2%-0.25%, equivalent to the natural lead content of Zn-4 grade). When the zinc bath contains high iron levels (i.e., significant zinc sludge), adding lead in appropriate amounts can further boost its wettability.
(2) The drawbacks of adding lead to zinc bath: Firstly, it reduces the fluidity of the zinc bath. As shown in Figure 3-19, the zinc bath exhibits the poorest fluidity when lead content is 0.5%, and only when lead content reaches 2% does the fluidity recover to that of pure zinc.
Secondly, excessive lead content not only fails to achieve the smooth surface finish of the galvanized layer as mentioned earlier, but instead causes it to turn dark gray. Therefore, the lead content is generally limited to below 0.5%, with the most common level being around 0.24%. This is exemplified by the galvanizing unit for strip steel at Wuhan Iron and Steel Company's cold-rolling plant. Additionally, higher lead content in the galvanized layer may affect corrosion resistance; however, some researchers argue that the impact is negligible.