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Inconsistent Brass Plated Color
2004
Q. Brass plating: We use an outside source to plate our stainless steel parts in brass. After plating, we clear coat them (clear powder). The color changes after the powder coat.
We are having a hard time getting a consistent brass plated color to match real brass substrate that is clear coated.
The plated part should match a real brass substrate part as they are assembled together.
We use a color eye to test color before clear coat and after clear coat on both brass substrate and plated brass. Unfortunately, the colors of the substrate and the colors of the plated brass do not match....and when we are lucky enough to come up with a plated color that matches the substrate.......visually......the color numbers do not match...they may be close, but not where we can say 'use only the color numbers from a spectrophotometer' ...as we have tried this and when they match, the visual is not a match. We check the *L, *a, and *b values.
The outside plater also has issues as the brass bath is not temperature controlled and when it's very hot, we get a different color than on cooler days.
I think we may have confused the plater as we were having problems with poor quality stainless steel with lots of pits and porosity...we asked the plater to help us so they used heavy copper plating, buffed the copper around to fill the pits and porosity, put on nickel, satined the nickel, then brass plated.......this got rid of the outgassing that was occurring during and after the clear powder process, but totally changed the color numbers we had used prior to the outgassing problems.
Any suggestions on consistent color in brass plating on stainless steel. Any comments and thoughts about using a spectrophotometer to measure color on plated finishes? Thanks for the help.
Gloria SchwedlerCaptive Plating Shop - Indianapolis, Indiana, USA
adv (affil link): Color matching spectrophotometers on eBay and Amazon
A. You already know the answer to the portion of the problem that has to do with the colors changing in a tank that isn't temperature controlled.
I have never actually tried this color matching between solid brass and plated brass, but I am confident that you are on the right track in identifying the switch from heavy brass plating to nickel-brass plating as a big part of the problem. The self-leveling characteristics of nickel plating would make it smoother than the brass, so it won't look the same no matter what color tint it has. Apparently someone predicted that problem and tried to minimize it by going with a satin nickel. But things are getting complicated when you are satin nickel plating, not as a final finish, but to give the right topography to a substrate for subsequent brass plating.
I doubt that there is a chemical reaction that is accounting for the appearance change after clear coating and baking. It is probably that you had two uncoated surfaces where the texture of one was compensating for the difference in tint of the other, and when you topcoat them, and the hills and valleys refract differently, you are back to a mismatch.
As I say, I haven't tried it; but I think the nickel plating is a big part of the problem and should be eliminated entirely or at least switched to dull nickel plating. Also, a vacuum impregnation step may be a better direction than mush buffing.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004
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