The Influence of Lead (Pb) in Molten Zinc on Hot-Dip Galvanizing

Lead (Pb), with a cubic lattice structure and a gray appearance, has an atomic weight of 207.21, a melting point of 327°C, and a boiling point of 1540°C. In hot-dip galvanizing, lead has limited significance to the galvanized coating, and its content is preferably not to exceed 0.3% (equivalent to the lead content in Zn-4 grade zinc), as it exhibits a higher electrical potential when combined with zinc, thereby reducing the corrosion resistance of the galvanized coating. When the lead content exceeds 0.5%, it not only exacerbates the adverse effects on corrosion but also causes the galvanized coating to appear dull and lifeless. Laboratory tests have confirmed that lead only exists in the pure zinc layer phase and is absent from the iron-zinc alloy layer. Therefore, lead as an alloy component of zinc has no significant impact on the iron-zinc reaction. Some factories introduce lead into the galvanizing pot, forming a layer as thick as 10-30 centimeters at the bottom. The purpose of this is to prevent zinc slag from settling and adhering to the steel bottom due to heat, and to facilitate the removal of zinc slag during skimming due to lead's low solidification temperature. However, some factories have ceased using lead for several reasons. Firstly, lead cannot be effectively recovered, increasing costs. Secondly, the majority of galvanizing pots now employ side heating. A significant amount of heat is exchanged at the upper part of the side wall of the galvanizing pot, making the temperature at the bottom of the pot much lower, rendering bottom protection unnecessary. Thirdly, the emission of lead vapor poses health risks to operators and contaminates the environment.