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Black chrome buildup

I am a polisher at a gun refinishing shop that is having trouble with black chrome plating. Sometimes when we black chrome a gun it looks like it has been sprayed with truck bed liner paint (very heavy buildup) and does not throw very well over the entire gun. We do our black chrome in a matte finish, and have tried plating over nickel and hard chrome. We use a 55 gallon drum for plating and plate at room temperature. The small parts seem to plate very well but the large parts (frames, slides, etc..) turn out very rough. We have sent out samples for analysis but the company can not tell us what the chemical levels should be. Can anyone give me any suggestions. What temperature should the solution be, how many amps should be used for plating and what would be the reason for the heavy buildup?

Any help in this matter would be appreciated.

Harold Dean Burgess
- Brooksville, Florida
2002

Black chrome is a strange animal. It should be run at below room temp as low as 60. Keep the concentration as high as possible without exceeding the solubility limit. Make regular adds of barium to remove any sulphate that may appear. Black chrome is a bad conductor, so to avoid rough chaulky build up, keep the plating times short.

Steve Delaney
- Ontario
2002

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2002

Harold,

Your supplier owes you info regarding the recommended concentration! It is his duty. A lot of things can go wrong in black chromium solution . Did you make sure that you have no sulphates in solution? If not, add barium carbonate (approx. 3-5 g/l)
Temperature - 16-18 C.(61-65F)
Current density - 15-25 amper/dc2
Chromium oxide concentration - approx. 400 g/l

Sara Michaeli
Tel-Aviv-Yafo, Israel