Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Black chromate -- what is it?
Q. We are presently calling out Zinc Chromate (black) finish on an assembly. Our Vendor wants to know what is our spec. What specs are needed? I did not know there was anything special that needed to be called out. Also I am not sure of the process used in this finish and a brief explanation would be appreciated.
Donn G. Bellmore- Binghamton, New York
2003
A. The bad news, Donn, is that although the plating shop may be able to help you understand and work through the situation, it's the product designer's job to dictate what finish he wants.
It sounds like you are looking for zinc plating with black chromate. The process involves cleaning, activating, zinc plating, and application of a black chromate conversion coating. But you are the one who has to figure out if that is what you want, and to specify it with more precision, including how thick the plating should be based on the service condition. If the parts rust, it is not the plating shop's fault if the plating was done per spec; and if they turn green instead of staying black it may not be their fault either.
You should find a mil spec, ASTM spec, or ASM spec that covers the zinc plating process and the chromate process, and then pick from that spec what thickness of zinc, etc., that you need. Getting hold of a copy of ASTM B633 would be a very good starting point.
Plating consultants are available to help designers on such issues. But if your company doesn't feel it's urgent enough or important enough to justify hiring a consultant, you could start by getting MFSA's "Quality Guide to Zinc, Zinc Alloy, and Cadmium Coatings" from www.nasf.org for about $10, which will explain things and give you some spec numbers to buy and read. But I think you might be better off specifying zinc-iron alloy plating than zinc plating because the black color will usually be more stable. Best of luck.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003
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Q. In context of BLACK RUST, I know that Fe3O4 is black in color . . . but what if steel has zinc coating with some colored passivation layer? Some local plater told me that Black Rust is nothing but oxidation of passivation layer.(?) - Pune, Maharashtra, India February 10, 2013 A. Hi Adi. Actually, the term "black rust" probably has little to do with your situation. Zinc is usually plated onto steel parts to prevent them from rusting (i.e., from getting "red rust"). The zinc plating is metallic looking (rather like silver, but not as white or as bright). However, the zinc plating itself also has a propensity to corrode, and the corrosion products of zinc are white (this is called "white rust"). To deter white rust, a chromate conversion coating is applied on the zinc plating (some people call this "passivation", but I dislike that term because "passivation" is a very general term and can confuse the issue). The chromate conversion coating can be clear and essentially invisible, or it can be "blue" (a very faint bluish tinge to the metallic color), or it can be gold-ish/brass colored, iridescent yellow, olive drab, or black. Zinc plating is usually not considered a decorative coating so, yes, it is possible that some minor gray or blackish small spotting on a zinc plated and clear chromated finish is a precursor to white rust but not cause for concern. A photo would help a lot! Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey February 10, 2013 |
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