Zinc ammonium chloride
Zinc ammonium chloride, also known as diammonium tetrachlorozincate(2-) (IUPAC name), is a zinc salt commonly used as a flux in the process of hot-dip galvanizing. The CAS registry number is 14639-97-5.  
Steel to be galvanized passes through an acidic cleaning process to remove iron oxide mill scale from the surface. After this process, the surface of the steel is very active and oxide layers begin forming immediately upon exposure to the atmosphere. Zinc ammonium chloride flux in aqueous solution is applied to the steel to reduce any oxides that are formed and/or inhibit them from forming altogether. This allows the molten zinc in the proceeding galvanizing step to maximally adhere to and alloy with the surface of the steel.