plating, anodizing, & finishing Q&As since 1989
How to Achieve Zinc Rainbow Patina
Q. We are trying to figure out how ti achieve the "rainbow" finish on zinc or any other metal.
- Houston Texas
October 9, 2023
A. Hi Nick,
Although I don't know exactly what product you need, I can explain in general what you are looking at, which may help clarify things for you ...
There are no pigments or patinas involved in this finish. Rather, like carnival glass or an oil slick on a puddle, this is diffraction coloration or interference coloration caused by light bouncing off both the top and bottom of a very thin (partial wavelength) transparent or translucent coating; the two reflections have traveled different distances and are out of sync, amplifying some colors of the light and attenuating others. It's a "rainbow" of colors rather than a single color because the coating is of varying thickness rather than uniform thickness.
This approach has long been used in anodizing of titanium to obtain both solid colors and rainbows depending on whether uniform thicknesses or varying thicknesses are done. Titanium is especially attractive for this application because the coatings have to be very thin to produce this effect, and the anodized layer on titanium is hard and wear resistant. Zinc plating must be chromate conversion coated in order to deter white rust, and I suspect that the finish you are looking at is a very thin and varying thickness of chromate. My guess, but I have no actual experience with it, is that the finish is not very durable because of its thinness. I have seen videos of parts being dipped multiple times into some proprietary formulation to build a similar rainbow finish, and I presume that that coating is some sort of organic clearcoat. So it may be that your zinc sheets have a heavier, clear, non rainbow, chromate on them followed by a thin clearcoat.
If you search the site for 'rainbow color zinc plating', 'rainbow titanium', 'seeking Iridescent Paint or Powder Coating', 'oil slick finish', 'aurora plating', etc. you will see photos and explanations of the finishes I've mentioned.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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