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topic 9274

How can I eliminate this plating defect?



I have some powder metal parts that have yellow chromate plating. Parts looked fine after plating, but after 2 or 3 weeks, white spots started to appear everywhere.

Following is the manufacturing process for those parts:

1- Molding iron powder.
2- Sintering.
3- Plastic impregnation.
4- Machining the ID.
5- Heat-treating.
6- Plastic impregnation.
7- Wheelabration.
8- Plating.

What is the root cause of those white spots and how can I eliminate it?


Afshin Afghan
Pacific Sintered Metals - Los Angeles, California, USA


Please can you tell me if the parts are:

1 Zinc plated

2 Plated from a cyanide or alkali bath

If the answer is yes to both the problem could be entrapment of plating solution in the pores of the parts.

The cure is to add a neutralisation step in using chromic acid solution

Martin Trigg-Hogarth
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England

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