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topic 0764

Rainbow Passivation solution for zinc post treatment


Q. When you make "Rainbow Passivation solution for zinc post treatment" with chromic acid and Sulphuric acid , what else you should add to enhance bright rainbow colors?


Srimath Dias Perera


A. I believe you may be referring to "yellow iridescent" or yellow chromate. If you get a nice coating, it will give you a rainbow type effect. The spec for yellow chromate allows for a yellow to this iridescent color. I would suggest that you purchase the prepared chromate chemical from a proprietary house (see our supplier directory). The formulas are a little complicated, and much research goes into making a good chromate.

tom pullizzi monitor
Tom Pullizzi
Falls Township,


A. Generally, the longer you immerse the parts in the yellow chromate solution, the more pronounced the iridescence will be ( although too long of a dip time will cause the chromate to rub off. Also, subjecting the parts to heat, such as a heated forced air dryer, will magnify the iridescence. Finally, using a chloride zinc system will generally give brighter parts than cyanide or alkaline non-cyanide based zinc.

Keith Rosenblum
plating shop - St. Paul, Minnesota

April 2018

thumbs up sign Readers: Keith's response was very insightful and seemed to answer Srimath's inquiry well. But you can use the search engine to find a dozen newer threads on rainbow passivation of zinc if this didn't answer the question you had in mind :-)


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Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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