Zinc Plating :
Production Process :
Zinc plating is identical to electro galvanizing in principle in that both are electrode position processes. Zinc plating is used for coatings deposited on small parts such as fasteners, crank handles, springs, and other hardware items. The zinc is supplied as an expendable electrode in a cyanide, alkaline non-cyanide, or acid chloride salt solution. Cyanide baths are the most operationally efficient, but they potentially create a pollution and hazardous material problem.
After alkaline or electrolytic cleaning, pickling to remove surface oxides, and rinsing, the parts are loaded into a barrel, rack, or drum and immersed in the plating solution. Various brightening agents may be added to the solution to add luster, but careful control of the bath and brightener is needed to ensure a quality product. Post-plating treatments may be used to passivate the zinc surface and at the same time impart various translucent colors to the coating. These post-plating treatments may be used to provide a desired color or to extend the life of the plated coating.
Coating Characteristics
The normal zinc-plated coating is dull gray in color with a matte finish, although whiter, more lustrous coatings can be produced, depending on the process or agents added to the plating bath or through post-treatments. The coating is thin, ranging up to 1 mil (25 µm), restricting zinc-plated parts to very mild (indoor) exposures. ASTM Specification B 633 lists four classes of zinc plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25. The number indicates the coating thickness in microns. The coating finds application in screws and other light fasteners, light switch plates and other small parts. Materials for use in moderate or severe applications must be chromate conversion coated.
The coating is entirely pure zinc, which has a hardness about one-third to one-half that of most steels. |
