58. What is the effect of lead particles in zinc bath on hot-dip galvanizing? Why is lead placed at the bottom of the galvanizing pot?

Lead (Pb) has a cubic crystal structure, appearing gray with an atomic weight of 207.21, melting at 327°C, and boiling at 1540°C. In hot-dip galvanizing, lead content should not exceed 0.3% (equivalent to lead levels in Zn-4 grade zinc) due to its high potential with zinc, which reduces the galvanized layer's corrosion resistance. Exceeding 0.5% not only worsens corrosion but also causes the galvanized layer to become dull and lackluster. Chemical analysis confirms that lead exists exclusively in pure zinc layers, with no presence in iron-zinc alloy layers. Thus, lead as a zinc alloying component has no impact on iron-zinc reactions. Some factories add lead to galvanizing pots, with a 10-30 cm thick base layer. This prevents zinc residue from sticking to the steel base during heating and makes it easier to remove during zinc removal due to lead's low solidification temperature. However, many factories have abandoned lead addition. First, lead is difficult to recycle, increasing costs. Second, most modern galvanizing pots use side heating, where most heat transfers through the upper side walls, making the bottom much cooler and rendering bottom protection ineffective. Third, lead vapor evaporation poses health risks to workers and environmental pollution.