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topic 3518

Paint Adhesion Problems


(1999)

We are zinc plating and chromate passivating large 19" equipment racks. After painting these racks we are discovering paint adhesion problems. The paint is adhering to the chromate plating but the chromate is not adhering to the zinc plating. We have tried the following with no luck:

Wet Coat-
1) Zinc plate-Chromate-Yellow zinc chromate primer-Baking enamel at 350° F = no bondage
2) Zinc plate-Chromate-Vinyl wash primer-Yellow zinc chromate primer-Baking enamel at 350deg. F = no bondage

Powder Coat-
1) Zinc plate-Chromate-Polyester Powder @ 450° F=No bondage.
2) Zinc plate-Iron phosphate-Polyester Powder @ 450° F= Some bondage
3) Zinc plate-Acetone wipe down-Iron Phosphate-Polyester Powder @450° F= 100% bondage, OK on rub test, Failure at 20# impact test.

Can anyone offer any input on this issue?

Michael Fedoruk
Canada


(1999)

I suggest you will try another passivation solution , not all the passivator solutions after zinc plating , are compatible with paint.

If you can, omit the passivator stage ,it does not help under paint.

Another possibility is that the yellow passivation solution is not operate correctly , please check if you are working at the right parameters.

yehuda blau
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel


(1999)

Hi Michael ,
One of the things that people usually forget with painting after chromating is that the chromate must "rest" for a MINIMUM of 24 hours prior to painting to achieve good adhesion ( I prefer to wait 48 hours ) But first you need to assure yourself that the chromate is properly adhered to the Zinc , test it by rubbing your finger over the chromate if it comes off on your finger then its not adhering to the Zinc & that problem needs to be addressed , if your plater is plating Zinc from a cyanide solution then he needs to activate the Zinc before chromating with a Nitric Acid dip , If it is a Chloride Zinc solution , he needs to rinse in agitated rinses prior to activating the deposit in about 2% by volume Hydrochloric acid , if its a non - cyanide alkaline he needs to do the same as for chloride Zinc Solutions. That should fix the problem; if it doesn't, get back to us here at Finishing.com and I am sure someone else will have another solution.
regards

John Tenison-Woods
John Tenison - Woods
- Victoria Australia



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