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Golden Color of Zinc Electroplating Fades on Castings

I am doing electroplating of zinc on sg 420/12 castings of around 1 kg.

1. I need complete theory of electroplating and specially for zinc electroplating.

2. I dip the pieces in golden passivation but the golden color is not very stable and it changes or fades after a few weeks or months.What can I do to avoid this.

Faisal Mansoor Khan
Rastgar Engg company. - Islamabad, Pakistan
2000

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Hello Faisal.

Successful industrial electroplating is pretty complicated, so we are happy to try to address individual questions but unfortunately no one can give you the complete theory of zinc electroplating in a few paragraphs. There are whole books written just on zinc plating, such as Geduld's "Zinc Plating" [on Amazon, AbeBooks affil links], but a reasonably available book that might be a starting point for your studies is the Metal Finishing Guidebook. You can see our "must-have book list" for additional suggestions. As your run into specific problems please get back to us with details.

Proprietary chromates are commercially available that are formulated to deliver a more stable color than generic mixes, so that would be one avenue to explore. Best of luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2000

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Your part is most likely porous. The parts are exposed to the plating chemistry longer than any other process and if the parts are porous they will absorb plating solution into the pores of the part.The Plating chemistry which the part sees for approx. 20 minutes is leaching your chromate.

Things to try: Take a casting (part) and after plating rinse it in boiling water for 60 seconds. then rinse in cold water. repeat 2 more times and Chromate the part with your yellow (gold) chromate rinse and dry.

If this reduces the color fading or bleed out the problem is in the porosity of the casting. The parts may need to be sealed prior to plating. The sealing process is quite involved. I believe Loctite sells the chemistry, but it requires special equipment.

Danny Tynes
used electroplating equipment
- Birmingham, Alabama
2000