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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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Simultaneous etch-passivation solution for aluminium pretreatment
Q. Hello!
I am in powder coating business. We used to coat ferrous metals only in the past and the standard pretreatment method prior to powder coating was either sandblasting or mechanical roughening.
Right now we're planning to dedicate one coating line fully to powder coating of aluminium extrusions. Having consumed a lot of information on this forum and elsewhere, I am aware that chromate conversion coating is the best way to go in terms of aluminium pretreatment.
Unfortunately, due to certain laws and restrictions it is very difficult to built a dipping pretreatment line that has anything to do with Chrome VI. At least in my area.
Recently I came across a Henkel solution called Bonderite MN-T 2040 R2. it is a chrome-free process which contains hexafluorotitanic acid. It is marketed as a solution that offers one step etching and passivation. Standard example of its use is "Alkaline Cleaning -- Rinse -- Demi Rinse [ed. note: presumably '2nd rinse'] -- Etch-Passivation with 2040 R2 -- Rinse -- Demi Rinse.
An alternative example of its use proposed by Henkel is "Cleaning and etch-passivation with 2040 R2 with the addition of a Henkel surfactant Bonderite C-AD D-4 -- Conversion coating seal with pure 2040 R2 solution -- Rinse -- Demi Rinse". Official Henkel representative stated that indeed it is possible and tested, although the first bath needs to be heated to 30-40 °C and should ideally have ebullition [ed. note: presumably 'air agitation']. Other baths can be at room temperature and without ebullition.
Of course I am more intrigued about the second scheme, because it cuts cost, has less stages, water consumption etc. The aluminium that is going to be coated comes pretty clean (no oil or grease whatsoever), just oxidized aluminium. I want to ask you how viable and realistic does this sound to you?
Before this I have not heard about solutions that can offer one stage etching and conversion coating. Furthermore, with a surfactant this magic solution is marketed as a mild degreasing/etching/conversion coating 1-2 stage process without the rinse in-between.
Henkel is big name in industrial chemistry but.... Should we be wary?
Waclaw Kazarnowicz
Shop employee - Rzeszów
November 7, 2024
Aluminum How-To"Chromating - Anodizing - Hardcoating"
by Robert Probert
Also available in Spanish
You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)
A. Hi Waclaw.
We can't comment on sources & suppliers, including Henkel( huh? why?), but only about the generic approaches they represent.
I realize that you are in Poland and I'm in the USA, but I would suggest that only conversion coatings accepted by MIL-DTL-5541 and QPD-81706 be used. To my knowledge that includes trivalent chrome coatings, including at least one from Henkel. Other coatings, including those based on hexafluorotitanic acid might be as good but without broad independent testing & certification how can you know?
As a philosophical point, a 7-course dinner is never improved by reducing it to 6 courses via mixing together the soup & salad, or the desert & coffee. Separate cleaning, etching, desmutting, and conversion coating offers better quality and flexibility than combining steps although you may not need that level of quality and flexibility.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Thank you for your answer.
My apologies for mentioning products and suppliers, will take it into account in the future.
Can you recommend a good book or other kind of source that covers the topic of engineering and construction of bath pretreatment lines for aluminium extrusion prior to powder coating?
Waclaw Kazarnowicz
Shop employee - Rzeszów
November 9, 2024
A. Hi again Waclaw,
The usual sequence for pretreatment is: non-etch cleaning, etch, rinse, desmut, rinse, chromate, rinse.
When a non-chromate process is required, there will often be an extra sealing step, i.e., chromate-substitute followed by a proprietary sealer.
I have a vested interest in saying Robert Probert's "Aluminum How To" (linked above) is good, but on the other hand finishing.com has sold nearly a thousand copies without a single return request -- which means a lot of people agree :-).
When it comes to how to actually fabricate the tanks and other equipment, info is pretty sparse. The Electroplating Engineering Handbook ⇨
is good but it's getting dated in this age of plastics instead of steel in plating shops, and there probably isn't enough market for authoring any new such books today unless you read Chinese :-)
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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