plating, anodizing, & finishing Q&As since 1989
Epoxy Powder Coating Over a Chrome Substrate
We have some nickel-chrome plated products and, for some aesthetic applications, we apply a powder coating over the chrome plating. We send the chrome plated product through the same five stage pre-treatment (iron phosphate) spray system that we use for bare steel. Our powder coating is electrostatic (corona discharge) epoxy that we cure at approximately 300 °F for 20 minutes. We get satisfactory adhesion of the epoxy to the chrome in most instances. However, we have noticed that when the product is used in moist environments, the powder coating sometimes delaminates from the surface of the chrome. This occurs when the powder coating is damaged and water is allowed to creep between the powder and epoxy interface. We understand that this delamination may have something to do with the surface tension of chrome and/or the overall bond between epoxy and chrome. On steel products in the same environment, we see corrosion of the substrate when the epoxy is damaged, but delamination only occurs as a result of spalling of the steel.
We would like to know if others have found similar adhesion issues with respect to chrome and epoxy. Are there other pre-treatment methods that would work better than a classic five stage (iron phosphate) spray system? Also, is there a way to roughen the chrome surface with a treatment system (either mechanical or chemical), and would this improve the overall adhesion between the chrome and epoxy? Finally, are there better methods of applying an organic coating to chrome at thicknesses greater than 4 mils?Brett Drozic
I would blame your delamination on dirty rinse water but if you don't have any problem with the steel thats not it. Are you using a reactive polymer final rinse? Several companies including mine have this technology and it has fixed that problem in many cases. These products are a little selective about which paints they work with, so testing is required. As far as the "classic five-stage iron phosphate" don't forget that the iron in that coating comes not from the bath but from the metal surface, since your parts are plated you are not getting any iron phosphate. If you notice any water spots you may be better off without that stage, or with a sheeting agent (surfactant) in there. Talk to your chemical vendor, he should be able to help you.
- Pearland, Texas
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