Why is lead added to the galvanizing bath?
Lead is not purposely added to galvanizing baths, rather it is an impurity in zinc. Different types of zinc have different levels of lead. The specified lead maximums for the various types of zinc can be found in ASTM B6.
Lead does have the beneficial effect of decreasing the surface tension of zinc, which allows it to flow off the steel easier as it is being removed from the kettle. This creates thinner galvanized coatings that have less runs and drips. Bismuth is sometimes added to galvanizing baths to achieve the same effect as lead has when types of zinc are used that have very low levels of lead.