What is Involved in Zinc Plating?

Zinc plating is one of a number of plating processes that utilizes electric current to bond the material to be plated — usually a metal like zinc, chrome or gold — to a workpiece. Careful preparation and strict observance of safety procedures are common to all electroplating processes, as the chemicals involved are both toxic and caustic. Nevertheless, zinc plating involves less hazardous chemicals than most other forms of plating, and is successfully done in small shops and even home metal shops.

One of the most common uses for zinc plating is to provide durable and inexpensive rust protection for steel. An alternative to zinc plating is galvanizing, which involves dipping hot steel in a bath of liquid zinc. A piece that’s been galvanized with zinc will often have a “spangled” looking surface because the zinc crystallizes upon cooling down. Hot dip galvanizing, as it’s called, requires special ovens, vats and rooms for the process, which generally exceed the capabilities of many small shops. An additional consideration is that the fumes given off by molten zinc are toxic. Electroplating, then, is a much more economical way of plating zinc onto a workpiece. Nevertheless, since hot-dip galvanizing applies a much thicker layer of zinc on a work surface — about 50 microns to electroplating’s 3 to 15 microns — if corrosion is a major concern, hot-dip galvanizing is a better choice.

Even small shops can electroplate zinc onto workpieces, such as fasteners like nails, nuts and bolts, as well as other hardware like hinges. Zinc plating is also used for other pieces, both to provide rust protection and also to enhance appearance. Properly applied, zinc plating can be buffed to a finish almost as smooth and lustrous as chrome. The first step in the process, as with all plating jobs, is to clean the workpiece thoroughly. This is a two-step process, beginning with a caustic alkaline detergent bath, followed by “pickling” in an acid bath. The piece can be rinsed with water afterward, but not touched by bare hands; any contamination at all, even microscopic, can interfere with the plating process.

After the workpiece has been cleaned, it’s connected to the negative pole of an electrical source, usually by copper wire, and suspended in a heated and agitated electrolyte bath. The zinc can be dissolved in the bath in the form of zinc salts, or solid zinc plates can be attached to the positive pole of the same electricity source. In either case, starting an electric current triggers a response in the zinc such that it’s attracted to the negatively-charged workpiece — the cathode. Atoms of zinc will migrate through the bath to the workpiece and bond with it. The process requires about 100 milliamps per square inch (6.4516 square centimeters) of workpiece to be plated, and after about an hour, should result in a plate of about 3 microns thickness. A stronger current will speed up the plating process, but the surface won’t be nearly as smooth and will require significant polishing.

It’s difficult to plate metals to a uniform thickness of zinc, and the shape of the piece being plated contributes to that difficulty. Crevices and grooves in the workpiece won’t develop as thick a plate as exposed surfaces and edges. Thus, many platers use multiple anodes and position them in the electrolyte bath to target the problem areas. Some jobs may call for a combination plate, such as nickel-zinc. One way to accomplish this is to wire two anodes to the positive pole, one of zinc and one of nickel. The plate on the workpiece will be a combination of nickel and zinc.

Once the plating process has finished, the workpiece can be removed from the bath and rinsed. Hardware, such as fasteners, hinges or plumbing components, are usually available for immediate use. Workpieces like automotive trim, which require a highly lustrous finish, may require buffing and polishing before they can be put to final use.