| |
Letter 16315
Yellow Brass turns Green
++
Our Yellow Brass turns Green.
We rack plate steel parts with bright Nickel underplate, followed
by Yellow Brass, ant then a clear lacquer. We have been doing this
for 20 years, recently we have been having a strange problem, some
parts turn from a yellow color to a light green color. We have seen
red or pale color, but never a green color. We are blaming the
lacquer since parts look good but after some time they become green,
but we have tried several different lacquers (nitrocellulose,
acrylic, epoxic) to no avail. Can there be something wrong with the
brass plate solution that is causing this? We haven´t changed
any chemicals, the solution is rather old, since it has never been
changed, could the carbonates be to blame?
Thanks for your help,
Enrique Segovia
- Monterrey, Mexico
++
I am a little confused by your question. Is the brass deposit
green out of the tank? Green after lacquering but immediately after
lacquer? Is it turning green in storage after lacquering? Each would
be a different unique problem. If it is coming out of the tank green,
lower the free cyanide, reduce the amount of ammonium hydroxide used
and allow the pH to come to 9.5 to 10.2.Also check the ratio of zinc
to copper. High carbonates will give you trouble controlling the
brass color by "normal" chemical guidelines. All of the above should
be checked by chemical titration using standard methods of analysis.
If discoloring immediately after lacquering it would be a most
definite reaction to the lacquer or the combination of the lacquer
and the heat of drying. If in storage, the lacquer may not be
formulated for cuprous alloys and may not have an appropirate "anti
tarnish" incorporated in it.
When I mentioned high carbonates, I was talking of Sodium
Carbonate in excess of 10 oz/gal.
Gene Packman
- Syosset, NY
++
Dear Mr Enrique,
The changing alloy compositions in your bath will affect how the
brass behaves after lacquering. If exposed to anything above 80
degrees C, we have seen it change colour.
What you need is to apply an antitarnish after plating and before
lacquering. 5-20 gms / lit of sodium dichromate followed by copious
rinsing, will prevent greening and put healthy rosy red colour on yr
brass!
Dear Reader, please --
- Post a
question on a different subject.
-
- Answer or follow-up on this subject (in non-commercial
fashion).
-
 |
|