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Impurities in a Nickel Plating Bath

What causes gray coloration in the low-current-density areas of parts after nickel plating?
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Q. 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 of the bath shows that concentrations of all the major bath components are right on the money! Any suggestions as to what the problem is? –H.J.N.

A. Your email doesn’t give much to go on but my first guess would be that the plating bath is contaminated with “tramp” metal ions with copper and/or zinc being the most likely suspects. A Watts bright nickel bath can function well with 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 to this problem is straightforward. The nickel plating solution must be “dummied” on a regular scheduled basis. Dummying of a nickel plating bath at 2–5 ASF should reduce the copper or zinc content of the bath to 5 ppm or less. 

Keep in mind that maintaining a good plating bath means monitoring what should be in the bath and monitoring what should not be in the bath.

 

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