Zinc is one of the more common electroplated metals in industry. It is useful because it confers high corrosion resistance while allowing even higher resistance through the use of chromates. A chromate is a gel-like coating of a hex-chrome (Cr+6) complex. It has a high corrosion resistance, and small scratches 'heal' themselves due to it's gel like nature. Different color chromates have differing rust resistance. From worst to best, they are: bluebright, yellow, bronze, green, and black. Chromated zinc parts are used in everything from screws and bolts, to refrigerators, to auto parts.
There are two basic zinc baths, acid and alkaline. The alkaline bath generally produces smooth, yellowish deposits with approximately 100% coverage and is run at room temperature. The acid bath usually produces slightly less smooth deposits, with about 60% coverage, and is run at about 100 degrees, but the deposit is brighter and clearer. The main downside of the alkaline zinc is its smell. The main complexer in most alkaline baths is thiourea. Thiourea reduces over time into various amines, most of which smell like dead fish. The effect walking into a plant can be overwhelming. For this reason, the slightly less stable but brighter and better smelling acid zinc is an area of ongoing research.
The acid zinc bath I helped develop was based in large part on the use of sodium benzoate as a complexer, and nickel wetters and brighteners. With the proper mix of salts, wetters, brighteners, and benzoate, we were able to produce a bath with ~ 90% coverage and a bluish-white deposit. The bath also had the unexpected property of a high cloud point. When normal acid zinc solutions reach a temperature of about 120 degrees, they begin to cloud up. This normally doesn't affect performance (other than the obvious detrimental effects of running a bath too hot), but is undesirable because it makes your product look bad. The bath we developed had a cloud point of around 150 degrees, making it extremely robust in that respect. This bath is currently being run in selected plants around the country, and its composition is still being tweaked.
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This page last updated 11/7/96.