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How can I eliminate this plating defect?

2001

Hi:

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?

Thanks,

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

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2001

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
surface treatment shop - Stroud, Glos, England