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Alodine necessary with powder coating?
(to hopefully help readers more easily follow the Q&A's)
Aluminum is often 'chromate conversion coated' either as a corrosion resistant final finish or as a pretreatment for painting or powder coating. As with most metal finishing, 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 specification for the process.
Alodine & Iridite are trade names of major suppliers of the process, so people sometimes refer to it as "Alodining" or "Iriditing". "Chem film" is another term for the process, used mostly in the USA.
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 coatings 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
Production Manager for Plating Co. - AST, Arvada, CO
Junior Sales Representative - Resintech, Camden, NJ
Nadcap Chemical Processing Auditor - PRI, traveling
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