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Slight discolorations in NiZn plating normal?

We use a NiZn plating process on stamped steel cases for corrosion protection of one of our thermostats. Occasionally some of the parts have small areas of slight discoloration in the finish. The marks appear to be only a cosmetic issue, and inconsequential to the corrosion protection, but management is concerned about the varying appearance of the parts, and the quality level it may infer to our customers.
Can anyone confirm if the discolorations are inherent to the NiZn plating process, or if it is possible to achieve (and reasonable to expect) consistently "clean" results? If the variation is a normal phenomenon, I would appreciate reference to documentation to support this.

Thank you very much.

Andrew Shemo, Engineering Manager
- North Canton, Ohio, USA
2003

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If acid NiZn, what do you see in the Hull Cell? Is it current density related (pH, temp, etc)?

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003

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Robert,

I would like to thank you for your reply. Unfortunately, however, I have absolutely *no* idea what you're talking about ! ;-)

I gather from your response, that if certain parameters are not in balance, then this phenomenon can occur? If so, then the discoloration *is* preventable.
Am I on the right track?

Thanks again.

Andy Shemo
- N. Canton, Ohio
2003

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Robert is suggesting that, in order to make progress in troubleshooting a complex process like electroplating, you need to do some organized troubleshooting by plating test panels in a small device called a Hull Cell and noting the results. You can look up "Hull Cell" and get some papers about such testing. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003