Flux in the galvanizing process is an organic compound coating applied to the steel surface prior to galvanizing, which prohibits the steel from oxidizing and improves galvanizing quality. 

Flux application is a necessary step in flux type galvanizing processes. Here, the liquid is applied to a clean (pre-treated by washing and acid pickling) steel surface just before the steel object is immersed into molten zinc. In batch hot-dip galvanizing, flux is often applied by immersion into a bath of flux. However, some manufacturers prefer the alternative method of pouring flux on top of the molten zinc bath. With this method, the flux will burn away quickly, but the steel object requires one less tank of immersion, which improves production output.

Flux is a compound of organic and inorganic surfactants, wetting agents, chloride salts and is liquid at relatively low temperatures. It acts as a sealant to air, similar like oil, and protects the steel from oxidizing between the pre-treatment of the surface and the galvanizing stage. Zinc ammonium chloride (ZnCl4) as the basis for flux is most widely used and has proven to be most effective in both inhibiting oxidation and improving galvanizing quality. 

The alternative method of galvanizing with flux is by using nitrogen (Sendzimir method).
Both methods have advantages and disadvantages for different production setups and specifications.

References: Wikipedia - Zinc Ammonium Chloride