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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 occasions 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, Oklahoma
1999

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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.

  
Tom Pullizzi
Falls Township, Pennsylvania