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topic 46795

Discoloration of black zinc plated parts

A discussion started in 2007 but continuing through 2020


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


affil. link
Zinc Plating
by Herb Geduld
from Abe Books

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

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 [affil. link 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 finishing 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.

46795-1b   46795-1d   46795-1a   46795-1c  

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

affil. link
"Electroplating Engineering Handbook"
by Larry Durney
from Abe Books
info on Amazon

October 5, 2018

A. Hi. I work in a small plating company. It seems to me that the problem could be a poor rinsing after passivation. If they are not rinsed well some solution could remain causing a drip on the piece that will cause discoloration. The overall finish looks good. If it was a chemical problem there would be more discoloration overall on the piece.

John Robert Brodie Perilli
N.T.G s.c.a.r.l. - Casalazzara, Aprilia, Lazio, Italy

February 17, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi, some of my flat parts are having the same lighter patch like the very first few pictures in this thread [these pictures from thread 33784, "Blisters in Nickel Plating with temperature"]:


... but they were black-zinc-ed. Are those patches normal? They don't look good and we are wondering if we could remove them by re-doing the black zinc process differently?

Hank Ling
- livonia, michigan, usa

February 2020

A. Hi Hank. We didn't want to post your question about "light patches in black zinc plating" on our thread about "Blisters in Nickel Plating of die castings when heated" because no one will be able to find anything if we let the threads wander all over -- so we added your question to this black zinc thread, and copied the pics you mentioned here.

Light patches are not normal; and in general, defective parts can be stripped and replated ... but is the substrate zinc die castings or some other material like cold finished steel? Please send pics of your own parts to mooney@finishing.com for posting here because although a picture is worth a thousand words, 'like the other picture except ...' brings us back to the need for a thousand words again :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Aloha -- an idea worth spreading


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