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ted_yosem
Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
finishing.com -- The Home Page of the Finishing Industry

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Mixed metal pretreatment configuration

Q. Greetings! I have a question on how to reach a favorable compromise in a situation where you have to chemically treat two kinds of metals prior to powder coating: aluminum and galvanized steel.
My jobshop does window/door and ventilated aluminum glazing systems. Next year we are supposed to launch a powder coating line. The material to be powder coated is primarily aluminum linear extrusion (about 85% of the volume). The smaller part (about 15%) is galvanized steel from which metal sheets are made to later be installed in doors/windows (much like glass, but painted galvanized steel in this case).
It is worth mentioning that conversion coating is only needed for aluminum extrusion. The surface of galvanized steel does not need to be activated prior to coating, it just needs decreasing and rinsing, so that it can be coated right after this process. To cut it short, there is no point in high level pretreatment of galvanized steel in the context of our manufacturing.
That being said, we are planning to launch immersion baths for aluminum profile to be dipped it. That of course will include degreasing/etching and conversion coating with a rinsing stage after each function. The aluminum to be treated has little to no oil and grease on it. The galvanized steel has the standard amount of protection oil/grease in its surface (it is pretty greasy).
Right now it is unclear what kind of decreasing solution can be used to effectively clean and etch aluminum extrusion without attacking it too aggressively while also effectively degreasing galvanized steel (likewise, not eating up the zinc layer). As I understand, aluminum is usually cleaned and etched with acid solution instead of alkaline. And galvanized steel on the contrary is usually cleaned with alkaline solutions.
A separate series of baths for galvanized steel is not an option right now.
Can this configuration work in a typical layout that includes:
(1) Degreasing
(2) Rinsing with tap water
(3) Conversion coating
(4) Rinsing with tap water
(5) Rinsing with DI water

What kind of problems may appear if we are to exclude the stage (2) and rinse the material after degreasing in a stage (4)? Will be very grateful for an answer or an advice.

Regards,

Manbaa Al Anwar
Employee - Dubai
November 13, 2024


A. Hi Manbaa.

Although it is certainly best to give metals the most appropriate pretreatment for each individual type of metal, mixed metal treatment is fairly common. However, the usual mixed metal pretreatment is to do phosphatizing, even though the phosphate doesn't "take" on the aluminum. So in the case of aluminum and galvanizing, the usual mixed-metal approach would be zinc phosphatizing. Your situation is unusual though, because you intend to process far more aluminum than galvanized. I wonder if you could obtain aluminized steel or Aluzinc steel rather than galvanized and chromate it all.

Personally, unless you have strong experience to prove it, I do not share your confidence that you will be able to properly powder coat non-phosphated galvanized steel. It would come as a great surprise to me if galvanized steel "just needs degreasing and rinsing".

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




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