We use a Watts bright nickel plating bath and have recently noticed a grayish color in the low current density areas of our parts. Testing the bath shows that concentrations of all the major bath components are okay. Any suggestions for this problem? N.D.
Your e-mail doesn't give much information to go on, but my first guess would be that the plating bath is contaminated with "tramp" metal ions, with copper probably the most likely suspect. A Watts bright nickel bath can function well with a low concentration of copper (usually less than 10 ppm), but once concentrations get above this number, you can start having problems. Corrosion resistance will decrease, the deposit may darken, and the ductility will decrease.
What to do? The solution is straightforward. The nickel plating solution must be "dummied" on a regular basis. Dummying of a nickel plating bath at 2-5 asf should reduce the copper content of the bath to 5 ppm or less.
Maintaining a good plating bath means not only monitoring what should be in the bath but also monitoring what should not be in the bath.