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Will citric acid passivation of stainless steel and aluminum improve Sol-gel adhesion?

Does passivation help in the adhesion of subsequent coating? My coating is Silica-Organic composite formed by Sol-gel condensation. Thanks a lot for your attention!

Mei Ge
product designer - Singapore
November 9, 2009

Hello, Mei. I don't know much about sol-gels, but my understanding is that they do not adhere in the fashion of electroplated metals, but more in the fashion of paints, where mechanical "keying" is important to the adhesion. That being the case, I would think that anodizing the aluminum and leaving it unsealed would provide a great key for the nano particles. Bead blasting or shot peening the stainless might be a good approach. I guess I wouldn't expect passivation of the stainless steel to have much effect either way.

There is a special technique for producing a huge surface area black electroless nickel plating that, under high magnification, looks like a sponge. If a nano-sized sol gel were applied to this surface, it's hard to imagine "wild horses" being able to pull it off. But again, I don't know enough about sol-gels to know if they would form quite properly amoung all the nooks and crannies. Good luck,

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 9, 2009

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I get asked from time to time if passivation will improve the adhesion of this or that. I agree with Ted, it's far more likely that the surface finish/roughness affects the adhesion far more than passivation does. Passivation just removes iron to promote formation of a better chrome oxide layer. Unless you have an adhesive/coating that is specifically affected by the presence or lack of iron, I'd say passivation should be left to its usual purpose - corrosion protection.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

November 17, 2009

adv.