Metal finishing Q&As since 1989

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Powdercoating brass

I work in the door hardware industry. We have a vendor that supplies brass handles that are powdercoated. We have received and rejected a number of handles where the powdercoated finish is peeling. Where the finish has peeled, the surface of the brass looks like copper, and is developing a green patina. I filed a groove into the metal and below the surface it has a uniform brass appearance. What could cause this finish failure? Is this indicative of a process problem or an imperfection in the substrate?

Ogden Guthrie
Quality assurance tech. - Poquoson, Virginia
June 18, 2008

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June 22, 2008

I believe that the color differences you are seeing are oxidation (tarnish) of the brass. Unfortunately, what you have here, until investigation is made into the manufacturing/finishing process, is a case of the chicken and the egg. To wit, did the oxidation cause the powder coat not to adhere or did the exposure to air from the break in the surface cause the oxide to form? The best way to go into this is to speak with your vendor about inspecting the parts immediately after finishing and also before shipping to see what if any defect is visible. A troubleshooting matrix needs to be developed, etc.

I am assuming that the part is a solid brass casting. If this is incorrect please let us know.

Gene Packman
process supplier - Great Neck, New York

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Thanks for the fast response. The part is indeed solid brass. We also offer these same handles in non-powdercoated finishes. The oxidization of these parts typically appears brown and spotty, whether in the lab or after years in the field. This is the first time I've seen the copper and green tarnish, so I wondered if it had something to do with the powdercoating process.

I think you are right about the oxidation causing the powdercoat not to adhere in the first place. To correct this, what is the best way to the prepare the brass before powdercoating?

Ogden Guthrie
- Poquoson, Virginia
June 25, 2008

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June 27, 2008

Ogden,

The best way would be after a knowledgeable person has inspected the manufacturing, storage and finishing processes for the handle. The problem could be a result of any of them or a combination.

To be more specific:
How is the handle manufactured?
What is the alloy of the handle?
What processes take place before the handle is powder coated?
How is the handle stored?
What is the powder coating procedure and operating parameters

From what I can surmise, based on my experience, I would say that you have a two day job for a consultant.

Gene Packman
process supplier - Great Neck, New York