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Contrast stains from a nickel sulfamate plating bath

adv.  

A brief explanation of my process is as follows: We clean and polish a glass substrate, which is then dipped in an electroless nickel plating bath to plate a very thin coating of nickel. The nickel coated glass is then processed through a nickel sulfamate bath to make flat metal parts. I passivate with potassium permanganate [on eBay or Amazon] solution on subsequent metal parts.

The problem I am experiencing is that some of the metal parts have a contrast change. Sometimes it looks dark sometimes light, they are in 3 basic patterns: circular, circular/rake, or just a line across the part.

I use a sodium hydroxide [affil links] cleaning solution heated to approx. 110 °F and D.I. water rinse.

Dennis Ehret
music - Pitman new jersey
2000

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Hi Dennis. Many years ago I saw a similar electroforming problem. The shop called it "hazing". In that case the nickel electroform was used to stamp/mold an end product which was used in projection, so the "hazing" was actually visible in the projected image as a subtle unwanted shading or graying. What made you pull your hair out was that this obviously was caused by some subtle microscopic difference in the finish in those broad areas . . . it was the ultimate "you can't see the forest for the trees" problem; you could study the surface with a microscope for a lifetime without ever even being sure you were looking at a surface feature which was causing the problem.

In that case there was a manual cleaning operation with pumice, and we theorized that something unusual about the pumice cleaning operation at some times, or the brush, or the operator's mood was causing this subtle variation which could be seen as a macro effect but was never actually traced to a cause-and-effect micro defect :-)

We eliminated the scrubbing operation and the incidences of the defect were very greatly reduced. So I would suggest studying your process for any possible abrasive, polishing, buffing, scrubbing, wiping or handling step and trying to eliminate it.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 1, 2013