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What are electrolytic black and yellow chromate bath solution for zinc?



I am a University student that study in corrosion course. I am researching about electrolytic chromating on acid zinc plated steels but unfortunately in my country there is not enough source for this subject. I have some problem,what is composition of electrolytic chromate bath solution if black & yellow colors are desirable? what are conditions of working?(PH,C.D,TEMP) and what is single dichromating? Please help me with your experience.

Thank you in advance.

Shahram Mirkhani
-Tehran, Iran

Electrolytic chromating is unusual compared to simply dipping the articles in a chromate conversion chemical these days, Shahram. I am somewhat familiar with yellow electrolytic chromating, since we installed it in one plating plant I designed, but I am not familiar with black electrolytic chromating. Presumably it involves silver salts, since black chromates traditionally do.

As I recall, the operating conditions for the electrolytic yellow chromate were similar to standard immersion chromates. The pH would be about 1.0 to 2.0, ambient temperature. The composition would include chromic acid at about 1 percent and probably a little sulfuric acid and nitric acid. But there would also be minor amounts of accelerators, stabilizers, and misc. other additives. Here is the USA users never formulate chromating solutions; rather the suppliers develop efficient chemistries and hold the formulations as proprietary.

It's hard to know precisely what you mean by 'single dichromating'. Chromating is sometimes called dichromating because the solution contains both chromate Cr04= and dichromate Cr2O7=. It is usually just a single dip process but clear chromates can be obtained either by dipping into a "single dip" clear product, or by dipping into a yellow chromate followed by a leaching/bleaching step. Good luck.

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

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