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Trivalent chromates for zinc alloy castings

I've seen many applications of trivalent chromium in steel parts. But does the tri. chromium fit zinc alloy castings (ASTM B240 #5)? How to avoid the zinc contamination of the bath?

Ricardo Kanitz Damasceno
plating shop (door lock mfgr) - Farroupilha, RS, Brazil
2004

Usually, trivalent chrome plating is used where you need a hardened surface, resistant against wear and with a polished looking aspect of the part plated, having as side effect a high corrosion resistance. Zinc plated parts are usually applied when it's necessary corrosion protection only, being aspect and hardening a low demanded characteristic. Chroming a zinc plated parts is unusual and there is no gain on any other characteristic obtained with the chroming solely, i.e., there will be no advantage by chroming zinc plated surfaces, despite this is technically possible.

Elton Baggio Vieira
- Guarulhos, sp, Brazil
July 8, 2009

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July 9, 2009

Hi, Elton. Thanks you for the interesting reply, but I think that due to a language translation difficulty, the question was misunderstood. Ricardo was asking about plating parts that are made of solid zinc. My answer would be that before the chrome plating step, the parts will have been nickel plated, so the chrome bath sees only a nickel plated surface and doesn't even "know" that they were zinc rather than steel.

Regards,

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