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Epoxy powder-coat adhesion to hexavalent and trivalent fasteners
I have a problem with chipping of black epoxy powder-coat on the head of a screw that has trivalent base plate. I never had this much chipping when the base plating was hexavalent.
Is there a difference in the adhesion between the tri and hex when used with powder-coat?
- Fastener Mfg/Dist - Wheeling, Illinois, USA
2006
It would be quite unusual to put powder coating onto chrome plating, James, so I'm going to infer that you are not speaking of chrome plating but of the chromates applied on top of zinc plating?
If that is the case, the situation is that all trivalent chromates are proprietary, and there are a number competitive products available. Some are well suited to powder coating (probably the thick film type); some are not well suited (probably the thin film + top coat type).
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006
James,
We are experiencing the same problem, or at least very similar, so I thought I would add a little to this thread and see if we could get anything else.
Here is the situation. We are having low carbon steel tubing plated with alkaline zinc and clear trivalent chromate, and are having severe adhesion problems when we powder coat (in-house). We previously did, and still do plate the same part with yellow hexavalent chromate with no powder coat adhesion problems. The parts are plated on the same line, only the chromate is different.
Ted, thanks for your advice to James, I will ask questions regarding the different trivalent chromates, and see if anything comes up.
Anything else anyone?
Thanks,
- Versailles, Kentucky, USA
2006
February 11, 2012
Q. Need your input about adhesion between powder coat over trivalent chrom[at]e.
1. The part is hardened steel followed with a machining operation (see no bake)
2. Part is then zinc-nickel plated (see no bake)
3. Followed with a trivalent chrome conversion coating
4. Powder coated and cure at 250° F
The problem is adhesion is lost between the trivalent chromate and the powder coating.
History about having problems with powder coating over trivalent chrome?
- Tucson, Arizona, USA
A. Hi, Armando.
I would suggest letting the chromate cure for 12 to 24 hours before powder coating, and looking for signs of discoloration or improper conversion as shown in the pictures in letter 42256. If that doesn't solve it, you will have to bring the supplier in, and possibly even have to change suppliers. Several different paths were taken towards making trivalent chromates with the corrosion resistance of hexavalent chromates, and each has disadvantages, and I think you will find that some just don't offer the adhesion that hexavalent chromates do.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 13, 2012
A. Hello Armando,
My experience is that it doesn't matter whatever trivalent passivation you have, you still need to phosphate the zinc plating before powder coating, in which case the trivalent passivation is going to dissolve into the phosphating bath anyway.
You can go from after plating neutralization direct to phosphating, unless there is too much shipping time in-between in which case a trivalent passivation won't harm, but you can't avoid phosphate again or else you will live with erratic peel off issues forever.
Khozem Vahaanwala
Saify Ind
Bengaluru, Karnataka, India
April 7, 2012
! Thanks for a terrific answer, KV! I'll try to remember it because this problem is always coming back since the invention of trivalent chromates.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 11, 2012
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