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topic 46795

Discoloration of black zinc plated parts

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A discussion started in 2007 but continuing through 2018


Q. Recently I am receiving parts which are Black Zinc plated from my supplier which had discolored. Some quantity of them had "Brownish black" and "Greenish Black" appearance instead of the "Black color" that we usually see. My supplier is claiming that this is a process limitation in Black Zinc plating as that chromate contains 'Silver Nitrate' which is light sensitive and will discolor when exposed to UV.
Question:1) Is this normal in Black Zinc plating process?
2) What is the factor contributing to this change of color? Is it true as that silver nitrate will caused discoloration of black zinc parts?
3) Is there any form of certificate like MSDS that actually spelt out this limitation and potential discoloration?
4) Or is it actually my supplier poor process control? And what parameters should I get them to monitor?

Please advise

Elvin Tan
Engineer - Singapore


A. I would suggest that you speak to your designers and your plating shop about the possibility of switching to zinc-iron alloy plating. This alloy not only offers better corrosion resistance than zinc plating but, presumably because of the iron content, it can be more reliably black chromated without light-sensitive silver-bearing chemicals. Good luck.

Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

July 9, 2012

A. Silver nitrate IS what makes the chromate black.
UV might damage the black zinc finish but often the discoloration is due to a poor chemistry (--too much silver: dull, powdery finish; --too little silver: copper to brown color, poor coverage or blotchy; --not enough zinc plating: odd colors; --wrong pH).
The greenish-rainbowy flash over a nice black is OK, it is the chromate showing.

Joanna Yu
- Vancouver, BC, Canada

January 9, 2018

Q. We have several CRS fabricated parts which we have finished to ASTM B633 [link by ed. to spec at TechStreet] SC2 Type II black zinc chromate, and for years all of our parts (no matter what vendor we used) looked and functioned perfectly (mostly local fabricators and amadeya42 shops). Now we are receiving parts with major discoloration and we can not accept them but I have no idea why this is happening and would very much like to aid my vendors in discovering what the cause is.

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Justin Hascup
Design Engineer - Lawrence Massachusetts

February 5, 2018

A. Hi Justin!

Did you measure thickness where you have this shading? It seems to be very low or none.

Maybe it's the photo, but I would check it.

Keep posting!

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

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