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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Is Alodine Necessary with Powder Coating?
Quickstart:
Aluminum is often 'chromate conversion coated' as a corrosion resistant final finish or as a pretreatment for painting or powder coating. The process actually involves many steps, usually including alkaline cleaning, etching, deox/desmut, the chromate conversion step per se, and water rinses between each step.
MIL-C-5541 / MIL PRF-5541 / MIL-DTL-5541
[on DLA] is the most commonly cited spec. "Alodine" and "Iridite" are trade names. "Chem film" is a slang term for the process.
Q. Hi,
I work designing communications equipment for the Navy and commercial marine markets. Almost all of our equipment is manufactured from aluminum. Every interior part protected from the environment gets Alodined only. This appears to be standard practice in the industry. In the past, we would Alodine and paint our external surfaces of enclosures or any item that would be exposed to the environment. We have recently changed to powder coating all of our equipment that got painted. Do I need to Alodine under the powdercoat or is this not necessary because of powder coating's known durability?
Thanks,
Bill
product designer - Hampton, New Hampshire, USA
October 26, 2010
A. Hi, Bill. Powder coating is a coating application method rather than a type of coating. It tends to result in thicker coatings than you get from wet paints, which is where your "known durability" idea comes from. But pretreatment by chromate conversion coating (Alodine), or a non-chrome substitute, remains equally important for adhesion and corrosion protection. Powder coating is as likely or more likely to have pinholes or missed areas as wet paints. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
A. Just be sure to put the chromate on very thin ahead of powder coat. It is a "gel" and the high curing temperature for powder paint will cause it to dehydrate and explode right through the powder causing loss of adhesion. The same is true of all paints but to a lesser degree.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
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