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Black dirty deposits at silver plated parts

I do silver plating on different parts used in manufacturers of Electronic components.
While plating silver on some parts like washers, bushes & nuts the edges are black, if I observe under microscope, it has a black color dirty sticking, if I take a pin and scratch that place, I can see brass (base material)
I tried plating the same parts by wiring them & this problem was not there.
We tried changing the barrel, but it didn't work.
I have inspected the parts before plating and after copper plating, the finish is good but the copper is getting removed in silver and we can see there is no silver on the edges of the part.
Please advise me on this problem.

Pavithra Suresh HL
Plating shop employee - Bangalore, Karnataka, India
February 27, 2009

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PROBABLY YOUR CLEANING AND ETCHING PROCESS IS NOT GOOD ENOUGH AND YOU ARE PUTTING TOO MUCH PIECES IN THE TROMEL CAUSING THAT THE INTERNAL PIECES ARE NOT CLEANED PROPERLY.
TRY TO PUT NOT MORE THAN HALF VOLUME OF THE TROMEL, IF THIS SOLVES THE PROBLEM INCREASE SLOWLY THE QUANTITY TO AN OPTIMUM

Ricardo Burstein
Bnei Berak, Israel
March 3, 2009

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Pavithra,
Ricardo is accurate in his response. I just had a few things that come to mind in addition to cleaning and decreasing load size.
1) If possible, decrease the speed of the barrel.
2) Make sure the silver metal and free cyanide are in range.
3) Keep the current density on the low end of the spectrum.
4) Make sure dangler electrodes are as smooth as possible, strip them if necessary. Good Luck to you.

Mark Baker
Fellow Plater - Syracuse, N.Y., USA
March 4, 2009

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April 2, 2009

We have checked on the cleaning process & found it fine & also done the analysis, the metal content & cyanide is in the range.
We tried to inspect some components in between the process. The copper was well covered but after silver we could see that with silver, copper is also getting removed as if there are dents there in that spot.
We were wondering if its time to make a new bath, as this is a five years old bath.
This does not occur in all the lots but only in some of the lots. & once its like this the parts are stripped & replated with more time in copper bath then silver plated. Though the plating is good in the second time& is covered in the dents too, the dents are visible.

Pavithra Suresh HL
Plating shop employee - Bangalore, Karnataka, India

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I think you need to tell us exactly what process you are using.

1. What are the cleaning steps, including conditions?
2. What silver plating are you using? What is the solution analysis like? Have you checked for contaminants, e.g. copper, that may well affect your plating?

You have discounted cleaning/etching as the cause, what investigation did you do to eliminate this as a cause? Have you walked the process, i.e. followed a part through the shop, detailing where it goes and when? If not, I strongly recommend that you do this, you will find it very enlightening. It may be something so minor seeming that is being done wrong, but with a catastrophic knock-on effect.

Let us know how you get on. With this information we may well be able to give you a more constructive answer.

Brian Terry
Aerospace - Yeovil, Somerset, UK
April 3, 2009