Zinc Plating

Zinc is extremely electronegative and thus barely plateable at all from water solution. Therefore, zinc plating baths are most susceptible to the presence of metallic impurities. To prevent the buildup of these impurities it is customary to maintain a small excess of sulfide or polysulfide ions so that metals that contaminate, such as lead, cadmium and other insoluble sulfides are continuously precipitated.

Zinc plating baths are of three major types: alkaline cyanide, alkaline noncyanide, and acid. The conventional cyanide bath was the preferred method for general zinc plating because of its good throwing power, ease of control, relatively wide range of satisfactory operating conditions, and many years of experience with its use. With the advent of stricter limits on effluents, its high cyanide content presents a problem. There are well known methods for destroying cyanide but they are very expensive.
With the development of improved brightener systems, it finally became possible to formulate alkaline zinc baths containing no cyanide. These baths are similar to those of the conventional cyanide type, with added limitations that satisfactory deposits depend to a great extent on the additives used, and that they are more critical and harder to control.

Zinc may be plated from several types of acid solutions, based primarily on either zinc sulfate or zinc chloride. These baths with the proper additives are capable of producing bright deposits. The most common additives to give smooth deposits are dextrin, licorice, glucose and gelatin.
Unfortunately, there is the question of whether substituting a strong chelating agent for cyanide is an advantage. Since it is necessary to remove zinc from the effluent as well, the chealating agents render this more difficult.