Zinc is one of the most widely used functional sacrificial coatings to inhibit corrosion of ferrous metals. Over the years a variety of Zinc plating and post treatment chemistries (chromates –hexavalent and trivalent– and seals) have been developed to improve the aesthetics and functionality of zinc coatings.
Over the years there have been three types of zinc processes: Cyanide based chemistry – seldom used today because of safety issues, waste treatment and disposal cost—but still it is needed in some applications. These processes were very forgiving and easy to control. Environmental concerns lead to the introduction of alkaline non-cyanide chemistry. After many painful years of development the primary issues of the need for improved cleaning (cyanide is a great cleaner for almost anything) and elimination of delayed blistering have been mostly resolved. The development of improved alkaline cleaners was a necessity for bringing non cyanide zinc into the mainstream. When the industry moved away from solvent degreasing and cyanide based plating, the need for much better cleaning became an imperative. This led to improved cleaners in general and improvements in alkaline non cyanide zinc products in particular. Alkaline non-cyanide zinc plating is most often used in rack applications (but certainly can be used in barrels) for plating larger parts or items with deep recesses because of its’ ability to throw into very low current density areas and produce a more uniform deposit thickness over a wide range of current densities.
And finally there is Acid Zinc which has the ability to deposit on certain types of steel (like cast iron parts) that the other varieties cannot easily plate. Acid Zinc (operating pH 5-6) can be run at very high current densities and higher temperatures (up to 120F+) thus allowing customers to plate a given amount of zinc faster but at the sacrifice of distribution. These baths operate very efficiently. Thus they are attractive for energy savings. But because of the efficiency, zinc metal growth is often an issue. Acid Zinc can be used in the 3 major varieties: non ammoniated baths with potassium chloride only as the electrolyte (easiest to waste treat but sacrifices some throwing power), mixed baths (contains some ammonium chloride and but mostly potassium chloride), and full ammonium chloride baths with optimum throwing. However, full ammonia baths have the most issues in waste treatment because of the chelating features of ammonia. Another advantage of acid plated zinc is the tendency to blister less than alkaline based processes.
Zinc if not post treated will oxidize to a dull gray very quickly. To prevent this discoloration and to extend the rate at which the zinc coating degrades –which allows the base metal to start rusting, chromate conversion coatings and seals are often applied. The older hexavalent chromates have environmental issues and have been phased out in many applications –automotive being the most significant. Trivalent replacements required a painful maturing process because they lacked the film forming and crack self -healing qualities of the old hexavalent chrome products. Trivalent coatings are more expensive to operate, more difficult to control and require additional organic seals to get the corrosion enhancements for the zinc plating. Requirements for longer life functional zinc coatings continue to increase as end users want to maximize the life of the products they make.