In the November issue, a reader asked a question regarding the bluish appearance of black nickel when plated over brass. One of our readers, Larry Durney, was kind enough to point out a couple of other factors that affect the appearance of a black nickel deposit. Here is the original question and my response.
We plate black nickel over small brass components. Quite often, the parts come out of the plating bath with a bluish caste to them. The black nickel plating solution that we use contains nickel sulfate, nickel ammonium sulfate, zinc sulfate and sodium thiocyanate. Do you have any suggestions for getting rid of the bluish coloration? K. B.
Investigate the thickness of the black nickel coating. If the layer is not thick enough you will tend to get a bluish color on the finished parts. If the color is not consistent over the entire surface, you should also look at your cleaning steps prior to plating.
Larry points out that if the current density is too high, you will also run into this problem. The trick here is to use a very low current density, usually 1.5 –2.0 asf. Another parameter is controlling the concentration of sodium thiocyanate carefully. A concentration of no more than two ounces per gallon is ideal. Last but not least, the ammonium sulfate concentration should not be any higher than four ounces per gallon.