black stains on zinc plated brass parts

The part on the left is the worst, sorry for the poor lighting.
These parts were made 5 years ago for stock and they have gradually gotten worse with the black stains.
Parts were silver soldered together then tumbled to remove the flux residue. I used a compound with vinegar to brighten the finish.
Most of the black is showing up near the solder joint so I think the flux or the vinegar is the cause. Any ideas on what would remove the black? How would you avoid the problem in the future? I talked to the plater and was going to pay to have the parts stripped and replated but there new minimum invoice amount is $1000.00! When I had these plated we did about 2500 pcs at somewhere around $.30 each. 1k for 200 pcs is a bit much.
Any recommendations on a different plater in Texas? rack plating is desired as barrel tumbling really beats these up.

The first question is were the parts simply zinc plate or was it with a dichromate conversion coating or other surface protection? Zinc by itself will oxidize and normally turns white and powdery. I would take a sample to a chemist examine them. Check this out.

Zinc Plated Brass turns black in some environments

Tom

Is it possible that it's the silver?

We play hell trying to keep silver from tarnishing.

Try some Happich simichrome polish.

ThunderJet, pretty unlikely to be silver when I paid for zinc. We have other zinc plated parts in plastic bags on the shelf and they are still bright.

Tom , I will have something else to ask the next time I send out parts for plating. Since the black color is principally around the solder joint the cause could be flux coming out of the joint with time, the vinegar and soap compound I used during tumbling or a chemical the plater used in preparation for plating leaking out of said joint. I may bake them in the oven before sending them out if we do these again.
Thanks,
Fred

Thunderjet said:

Is it possible that it's the silver?

We play hell trying to keep silver from tarnishing.

Try some Happich simichrome polish.

Click to expand...

Just putting the parts away till the customer orders some more and I noticed one of the worst with black stains has the best solder with a fillet all the way around, so no voids for any liquid to migrate out of. Maybe some molecules from the silver solder are replated to the surrounding area and the problem is silver tarnishing after all?