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What chemical is used to make zinc plating black?

My work now is testing quality in Zinc product for garden. Our products are Zinc Topiary, Zinc Rectangular that have the color coat. The surface of product is black color due to oxide by chemical occur on the surface of product. I would like to know what are chemicals (name, ratio and concentrate of chemical) used to treat for these product to make the color change to black? and I would like to know how to test quality of the finishing for these product?. Would be high appreciative with your early answer.

Mandy Linh
Quality controller - Ho Chi Minh , South, Viet Nam
2003

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Hello Mandy. The chemical is a black chromate conversion coating. If it is applied to conventional zinc plating, and it is a rich black color; it is probably a silver-based chromate. Often, a zinc-iron alloy plating will be used instead of plain zinc, because the iron content allows the formation of stable black chromates that do not contain silver.

Here in the USA, shops do not formulate their own chromate conversion recipes though; rather they buy it from suppliers who have perfected proprietary formulations. In this age of RoHS, where chromates usually must be trivalent rather than hexavalent, home-brew chemistry is even more problematic.

There are many appropriate QA tests that are applicable to zinc plating. A great starting point is the Metal Finishing Suppliers Association (NASF) booklet "Quality Metal finishing Guide to Zinc and Cadmium Plating" which costs only about $15.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003