Why is Zinc Plating changing colors?
Q. Gang - I'm a QA Manager and I need your assist to figure out what is going on with our zinc coated products. Color is not consistent. Parts returned from our plating vendor that were coated with yellow zinc chromate are not very bright and greenish in tint, unlike previous batches that had a bright yellow/copper appearance. What caused the change in color? I'd like some technical knowledge before I respond to our vendor.
- Madison, South Dakota, USA
A. Yellow chromate will vary widely in color. It really is not usually yellow. By the ASTM definition it is yellow with red, blue and green iridescence. Dullness is something separate. Color matching of chromated parts is not usually considered a part of the spec since it can vary from lot to lot not to mention day to day.Gene Packman
process supplier - Great Neck, New York
The yellow passivation film on a bright zinc surface is usually of very low thickness, of the order of max 0.5 microns. The light falling on this transparent film is interfered with in different frequencies of visible range giving different colors,depending on the local thickness of the chromate film.Passivation colors belong to the category of "interference colors". The iridescence may give rise to 2 or more colors depending on the chromate film thickness. Variation in this is unavoidable and is not a reason for rejection, if salt-spray test passes. Thanks.
Bangalore, Karnataka, India
June 16, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. We have recently started a zinc plating plant. After plating, we found the plating, passivation and top coat everything was ok. But after one day, we found that the colour was not uniform . At some places, it was blue whereas at some places it was reddish. I suppose this is a problem with passivation because after plating everything was uniform. Please help to avoid this issue.Swayanjeet Sorte
- Virar, Maharashtra, India
A. Hi Swayanjeet. As Subramanian notes, your zinc plating and chromate conversion coating are appearing to change colors, but the colors are probably not "real"; they are interference patterns because light is bouncing off of the top and bottom of a transparent/translucent thin film of partial wavelength thickness.
It really should not be a cause for failure, any more than a CD / DVD displaying such a rainbow coloration. But if your customer will not accept it, you will need to change the top coat or the chromate to build a thicker or thinner or colored film. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
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