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Neutralization of Cadmium Plating before Chromating

We do cyanide cad plating for the aerospace industry. I am new the the cad plating process. Have done cyanide and non-cyanide zinc plating for many years.

My question concerns the neutralize step after cad plate and before chromate conversion coat. With zinc plate we would neutralize in a 0.25 - 1 % solution of nitric acid. Here, the neutralize step after cad plate is in a solution of chromic acid and sulfuric acid ( 10 % (w/w) chromic acid and 1% (v/v) sulfuric acid ).

Is there any special reason for neutralization in the chromic/sulfuric solution for the cad plate? Can cad plate be neutralized using nitric acid?

Thanks,

Lee Blanton
- Denton, Texas

Sorry, I am not familiar with such a neutralization process, Lee, and admit that it sounded a bit strange to me. But when I looked it up in Lowenheim, I see that cadmium plating is sometimes "bright dipped" in nitric acid or more commonly chromic acid. Many job shops do not neutralize between plating and chromating, and I always assumed that if they did they would use nitric acid, but this "bright dipping" rather than neutralization perhaps explains the use of the chromic acid. Good luck.

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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