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topic 54432

Powder paint adherence problems




February 24, 2010

I have a problem with my powder paint on Al and Fe plates, which will be used outdoors in variable situations.

Our pretreatments for Al plates are: degreasing(10 min), water rinse, washing with acids (10 min),water rinse, painting in conveyor oven.
Pretreatments for Fe plates: degreasing, water rinse, galvanization(ZnCN), water rinse, drying with dry air, paint.
Pretreatments for Fe, zinc plating: degreasing, water rinse, washing with acids, water rinse,galvanization(ZnCN), water rinse, chromitization, water rinse, drying with dry air.

But after 2-3 months being outdoors, my paint is taken off in 1 piece, I mean my paint adherence to itself is good but it doesn't stick to the plates and is taken off easily after a while. We can see the corrosion effects under the paint in great amounts too.

what do you think about the causes? Do you think the gap between the last step and the oven place (about 2-3 hours) is the problem or the gap between the entrance of the oven and its main part for reaching the ideal temp (200 C) is the problem (2 meters)? or my steps may accomplished incorrectly?

Elnaz Shirazi
engineer - Tehran, Iran


February 25, 2010

Hi, Elnaz. I think the main problem is pretreatment. Aluminum requires chromate conversion coating or equivalent; steel requires phosphatization; galvanizing (it's not really clear if you are galvanizing or zinc plating) requires phosphatization. Zinc plating requires chromate conversion coating.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


February 27, 2010

Hi,Ted. Thanks a lot for your advice. sorry for not clarifying, I meant zinc plating by using galvanizing. so I understood from your response that I've to apply chromate conversion on Al and zinc plates, if not the adhesion of the powder paint to the plates will be weak.

Elnaz Shirazi
- Tehran, Iran


March 1, 2010

Hi, Elnaz. Hot dip galvanizing is different than zinc electroplating. It must be phosphatized for good paint adhesion.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


March 2, 2010

Hi Elnaz and Ted.
We had the same problem,but we changed into a different Al Pretreatment with a really good salt fog chamber performance, since then we don't have adhesion problems.
And sorry Elnaz, but I didn't understand you Fe pretreatment process.

Regards from Mexico!

Victor Kuznetsov Mendez
- H. Matamoros



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