Chemical formula for chromating solutions
Q. Can anyone suggest the chemical formulation for plain yellow, olive drab and black passivation on zinc plating?Rajesh Kumar
A. All of these passivations are based on hexavalent chromium. Hex chrome can be written as Cr+6. Hexavalent chrome is the main active chemical, but there are other additives. Specific chromate bath formulations are closely guarded proprietary information. There are many plating chemical suppliers who are in the business of researching better passivate bath formulations and then selling them. The major difference in the different colors is the thickness of the passivate layer. If you want a different color, sometimes all you need to do is dip your parts longer in the passivate bath.
Rochester Hills, Michigan
September 3, 2014
Q. Dear sir,
Please give me Dark but shiny yellow passivation making formulation.
- Ahmednagar Maharashtra India
A. Hi Suryawanshi. Chromate conversion coating chemistry has been proprietary for more than 60 years, so you'll have to go back awfully far. My 1976 "Finishing Handbook and Directory" only says that they contain hexavalent chromium and a mineral acid and sometimes an organic acid. But N.V.Parthasaradhy's "Practical Electroplating Handbook" =>
says 200 g Potassium Dichromate and 4 ml sulfuric acid to 1 liter of water. Note that this will not be RoHS complaint.
If you'd like to really understand chromate conversion coatings, Biestek & Weber's "Electrolytic and Chemical Conversion Coatings" has a 125-page chapter on the subject, but even this is probably dated because of the shift to trivalent conversion coating products. Good luck.
Ted Mooney, P.E.
Pine Beach, New Jersey
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