|
Letter 3058
Black Spotting after Baking
-
I'm having some trouble with black spotting on nails after the
hydrogen de-embrittlement step and am looking for suggestions. The
plating process uses zinc-cyanide followed by a clear chromate
finish. The chromate is applied before baking (I realize this
destroys the effectiveness of the coating but on this product it is
an acceptable loss). The nails come off the plating line looking nice
although they are sometimes wet. The black spotting shows up after
approximately 24 hours in the baking oven at 180C. We've seen no
spotting on products that do not go through this baking step. Since
the problem appears to be directly related to the baking step I don't
believe that the plating baths are the source. We've dumped the
chromate bath and rinses on several occations since we started seeing
the problem but that has not solved the problem. The only thing that
seems to make sense is the wetness is reacting with the chromate
finish and the heat to cause the spotting. If anyone has any
suggestions, please let me know. Bob Holderman
Robert Holderman
- Tulsa, Ok
I believe that the spots may be caused by the leaching of some
process solution, during the bake cycle, which has been trapped in
pores in the metal. It may originate in the drawing of the wire for
the nails which, if done poorly, will make long pores in the wire
which can trap solution. (see the gif on this page showing a cross
section example).
If you can see pores in a cross section (at 50X) improve the drawing
process. Reducing the concentration of the most concentrated solution
(a stronger than normal pickle solution, perhaps?) or some hot and
cold rinsing may improve things.
-
-
 |
Save
This Page (why?) - Home - ©1995-2009 finishing.com