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topic 15733

Aluminum pretreatment for powder coating

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A discussion started in 2002 but continuing through 2018

2002

Q. I'm doing research on aluminum pretreatments for parts that receive powder coating. My company usually specifies chromate conversion for aluminum. It is used on unfinished aluminum, as well as a substrate for liquid primer / paint. It is sometimes used as a substrate for powder coated parts (aluminum). I find literature that states the corrosion resistance benefits of chromate conversion coatings degrade above relatively low temperatures (150 °F?). The bake temperature of the powder coating we use is 375 °F. I am concerned that the conversion coating on parts going through the bake process is rendered useless. If this is the case, I want to eliminate the process from the manufacturing flow, as it is not adding value. Would Iron Phosphate be a better process for this application? Are there alternatives you can suggest?

Peter C. Maher
- Skokie, Illinois


Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby
from Abe Books

or

2002

A. Hi Peter,

The temperature sensitivity of chromated aluminum is well known because of changes in the structure of the conversion coating. Although I think is is difficult to quantify how strongly temperature affects the corrosion resistance of the coating, chromating is probably your best bet. Iron phosphating is an inferior pretreatment for aluminum. If you have corrosion specs that need to be met, I imagine that a somewhat degraded chromate conversion coating will beat the best iron phosphate coating every time.

George Gorecki
- Naperville, Illinois


2002

A. Peter,

Chromate, like other conversion layers, is only restricted temperature stable at the first initial drying (e.g. the drying done directly after the chromating process). Once dried completely the temp. resistance is much higher. Although I don't now the exact figures, chromate layers withstand the temperatures for curing powdercoating without any problem.

Nevertheless I could imagine you generally want to get rid of chromates e.g. chromium, due to the health hazards. Depending of the circumstances your parts will be in, be aware that actually other, less hazardous processes are available.

Regards,

Remmelt Bosklopper
Remmelt Bosklopper
- Enschede, The Netherlands


2002

A. Aluminum Chromate under heat cured powder paint is a proven and acceptable pretreatment, HOWEVER, the chromate gel coating must be thin. The moisture left in a heavy coating will "out-gas" (sort of explode) through the powder paint and leave corrosion opportunities. Again, put the chromate on very lightly for bonding powder paint.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como


2002

thumbs up signThanks to all who answered my question. I want to share information on an experiment I did using chromated aluminum test panels, which were then powder coated. 2024 test panels were processed through our standard 10 stage chromate line (Ultrasonic alkaline clean, running rinse, 2 ultrasonic DI water rinse, de-ox, rinse, chromate, rinse, rinse, DI final rinse). The panels were allowed to dry for at least 24 hours. They were then powder coated (1/2 of the plates were masked prior to coating) and cured at 375 °F for 15 minutes. The panels were unmasked, and put through ASTM ASTM B117 [link by ed. to spec at TechStreet] 168 hour salt fog test. The powder-free areas of the plates corroded badly, almost as badly as the unprocessed panels placed into the salt fog chamber. They did not come close to passing the requirements of B117. The powder coated surface held up perfectly. So, the cure temp of the powder coating renders the chromate coating useless. This must be kept in mind if uncoated chromated surfaces will be used in a corrosive environment.

Thanks again for the input.

Peter C. Maher
- Skokie, Illinois


2002

A. Hi Pete!

When any aluminum is cleaned using alkaline chemicals, the bath will etch the substrate. Etching gives the topcoat a surface in which to adhere to. This is particularly important when looking for salt spray performance. As you are aware, aluminum does not accept a phosphate coating. The etch really improves performance. Also, aluminum is not generally put into a salt fog chamber to be tested because properly pretreated panels will take forever to corrode (under the topcoat). You said the powder coated areas did not corrode, yet the powder free areas corroded. It would seem that they would corrode without a topcoat application. The quality of the aluminum will also make a difference upon your test outcome. I would retest using only the powder topcoat area and test panels using ones that were chromated against those that were simply alkaline etched. That will be the true test. As far as bare aluminum, I am not sure. And remember! Even if the bare aluminum corrodes in the salt fog chamber, does not necessarily mean it will corrode in the real day scenario.

bob utech
Bob Utech
Benson, Minnesota

A Guide to High Performance Powder Coating
by Bob Utech
[link is to Amazon]


2002

A. You repeated and reconfirmed what the specification writers had in mind when they wrote into the Mil specs not to dry bare chromate over 140 °F. Left bare it is common knowledge that it is destroyed (dehydrated) by heat, however, under any kind of paint it will hold up.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como


2002

A. Pete,

I performed a test very similar to yours, but with slightly different results. I masked some chromate-converted panels using standard high-temp tape and applied epoxy powder coat. Removed masking carefully from some panels prior to cure. Cured 15 min @375 °F; finished de-masking. Subjected panels to salt spray. The panels unmasked prior to baking corroded as though they had no chromate conversion coating on them. Those which were baked with masking in place, wherever the masking had been firmly adhered the panel was fairly resistant to corrosion, but not as good as required by Mil-C-5541 [link by ed. to spec at TechStreet]. But ... at gaps in the masking the corrosion was heavy.

The deteriorating effects of heat on chromate conversion coatings on aluminum is relatively easy to demonstrate. Bake chromated panels at different elevated temperatures (150 °F, 200, 250, 300, etc) for a standard time, say 15 minutes, or an hour. Then salt spray them. You'll see the progressive deterioration. The limit for continuous exposure is as stated in the spec.

John F. Brewer
- Akron, Ohio


(2004)

Q. Peter, et al, is it possible that I get a copy of a doc that says "chem-film degrades as temperature increases"? Our vendor is drying the pre-treated aluminum to 400 °F oven and it always is failing the salt mist test. He said he just wants to make sure all dries up and the cause of the failure is dents or scratches (which for the fact is correct), but if this temp really degrades the chem film, then I must recommend he stops his drying process with proper docs to back me up. Thanks, looking forward to your help.

Ronald Diocera
telecommunications - Philippines


February 16, 2012

Q. My question pertains to which solvent should be used for prep cleaning chromated aluminum before powder coating.
One painter we were using switched from MEK to alcohol.
After this switch in solvents we started to experience a significantly higher amount of paint chipping around screw holes. The paint adhesion, specifically around these holes (where screws are torqued in) appears to be worse after the solvent switch.
Could there be a connection with this or am I looking in the wrong area?
Thanks for your thoughts,

Eric

Eric Carlson
- Woodridge, Illinois, USA



May 13, 2013

Q. Hi my name is Joff and I am having some problems with powdercoated aluminium: there are marks left on the parts after the powder has cured, these marks look like another part has been resting on it whilst in the oven. My supplier says that the chromating process is the problem. Has anyone had this problem themselves? Thanks in advance.

Joff Evans
- Melbourne


May 20, 2013

A. Joff,
some pictures if you can do it, would be a big help in trying to answer this one.
Hard to see how chromate pretreatment would leave marks in the cured powder unless something was seriously wrong there. Chromate rarely leaves any significant dimensional residues.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




March 9, 2014

Q. I am a fabricator of equipment used on boats. My current powder coater chromates my bait boards before powder coating; I have a competitor of his vying for my business and offering to do the work for about half the price -- but he doesn't chromate before powder coating. How much does the chromate process affect/help protect my product, or do I really need it?

Rod maunder
bait boards - Brisbane Australia


March 2014

A. Hi Rod. I guess my question would be: if he doesn't chromate the aluminum before powder coating, what pretreatment does he do? And is he using the same brand and type of powder coating, complete with UV inhibitors, etc.?

It is relatively easy and inexpensive to have sample coupons salt spray tested to see if there is an obvious drop in quality. Cut a few small samples from your aluminum and send a couple to each of the proposed vendors, telling them to prepare and powder coat the samples exactly the way they are proposing to treat your bait boards. One salt spray test alone won't usually tell the whole story if there isn't much difference, but it will if there is a big difference. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey

simultaneous
March 11, 2014

thumbs up signHi Ted, thanks for the quick response. I just asked the other powder coater what his pre treatment is and he said he uses iron phosphate. Is there much of a difference between the two considering they both use the same brand of powder (PPG)? I look forward to hearing your opinion cheers

Rod maunder
Baitmate bait boards - Brisbane Australia


March 11, 2014

A. To be very brief: The Chromate gives Corrosion Protection.
The Powdercoat provides the aesthetic appearance.

No-one with any experience with the Powdercoating of Aluminium would ever consider Powdercoat in a marine environment over Aluminium without proper pre-treatment.
When Aluminium corrodes, the products of corrosion occupy a volume 140 times the volume of the metal consumed by the corrosion. Essentially these corrosion products explode the powder film off the Aluminium surface.

The ultimate approach would be to select a marine grade alloy, Properly clean the substrate, Chromate conversion coat the substrate, Properly rinse the Chromated substrate with DI as a final rinse and ensuring conductivity of run-off water  0  ^<10 micro-siemens, allow to air dry or warm air <80 °C force dry, select a warranty grade (TGIC based) polyester powder consumable, apply to achieve a cured film build of 80-100 microns, cure in strict accordance with powder manufacturers specifications and noting that the cure schedule recommended by the manufacturer refers to the actual metal temperature achieved not the air temperature of the oven.

The Australian Standard to be applied should be AS 3715 [link by ed. to spec at TechStreet].
There is a very informative book called: "Aluminium and the Sea".
I hope this helps,
Regards,
Bill

William Doherty
Trainer - Newcastle NSW Australia


March 2014

A. Hi Rod. Phosphating is sometimes done on aluminum so that the same pretreatment system can be employed on aluminum components as on steel, but phosphating of aluminum is not the equal of chromate conversion coating.

Still I caution you to proceed based more on actual results not just on theory, since anything can be done well and anything can be done poorly. William offers some sound advice which crossed in the mail with your follow-up question.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey


March 14, 2014

A. I would like to add that I made an error in my original reply.
The conductivity reading should be <10 micro siemens not 0.

I would also add that I have had experience with a very significant and costly failure of a coating system where the applicator was advised by his Iron Phosphate pretreatment Supplier to simply add some fluoride into the initial stages; the idea being that this would provide an etchant capability to remove Aluminium Oxides.
Unfortunately this advice was followed by the Powder Applicator.

The end result was a catastrophic failure of a balustrade system on a 30 story high rise.
This ultimately led to the Bankruptcy of the fabricator who was required by law to remove, repair & replace the whole kit and caboodle.

Sage advice is to avoid risk and cheap alternatives.
Do It Once, Do It Well, Build a Better Australia.

Regards,
Bill

William Doherty
Trainer - Newcastle NSW Australia

----
Ed. note: Actually, the error in William's posting was ours not his. In HTML (the language of web pages), the symbol "<" is a control character which can make the characters following it disappear; we need to be on the lookout for it when posting comments, and we weren't :-)


March 14, 2014

A. I can't see if it was specified what metal was being coated.
If aluminium, then surely chromate is the best pretreatment route, but there are phosphates that are adequate (but not as good, even if Chromate has health issues).
If steel, then chromate is not very good. Iron phosphate barely adequate and zinc phosphate far better.
If galvanized steel, chromate best, zinc phosphate Ok and some other phosphates also good (eg Mn phos).

In some countries TGIC powder is not used due to its health issues, but very good performance can be obtained from TGIC free powders.

Comparing two coaters, that they use the same brand of powder isn't enough. Within any brand there are varying grades of powder, varying polymers etc. Some powders are classed as "grade 1, Grade 2 " etc Ensure that the same powder is used, not just same manufacturer of powder.

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland


March 15, 2014

A. Hi Geoff,
Yes the TGIC issue has been raised repeatedly over the years here in OZ.
There was an extensive survey done here which concluded that the only real risk was with the raw chemical.
If you wish I can research my files and copy the report to you.
The problem with non-TGIC powders has always been that they exhibit permeability -- in other words they are porous.
Some actually create porosity during cure by the creation of water molecules.
Regards,
Bill

William Doherty
Trainer - Newcastle NSW Australia



Supplemental treatment after chem-film and before painting?

August 4, 2014

Q. Hello all,

Is there any post or passivation treatment on aluminium after chromate conversion coating (chromotizing) before painting or paint base ? If it is there what is the composition and parameters ?

What are the qualitative tests for chromate conversion coating or chromotizing on aluminium ?

Suggest me

Regards,

Surya Narayana
Process Engineer - Tumkur , Karnataka , INDIA


July 2014

A. Hi Surya. There are prepainting alternatives to chromate conversion coating (chem-filming) of aluminum, such as phosphatizing or phosphoric acid anodizing or proprietary passivation treatments, but chromate conversion coating is probably the most widely practiced. To my knowledge there is no supplemental chemical layer commonly added between the chromate and the priming paint.

As for the testing, I'd start by reviewing Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil]. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey



March 3, 2018

Q. Please tell me what are best chemicals for aluminium pretreatment? Degreasing, De-smutting and chromotising.

Umesh Madushan
- Colombo Sri Lanka


March 2018

A. Hi Umesh. The typical sequence for aluminum pre-treatment is non-etch cleaner, rinse, etch, rinse, de-smut, rinse, chromate conversion coat, rinse. One good and free guideline is Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil].

There are variations in the sequence and in the chemistry depending on your needs and the specific alloy. But the formulations been proprietary chemistry, rather than generic info, since at least the early 1950's. You buy these process chemistries from suppliers like Henkel, Macdermid, Atotech, Sur-Tec, or Chemetall [a finishing.com supporting advertiser] rather than formulating them yourself. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Food-safe pretreatment of aluminum before powder coating

July 3, 2018

Q. What is an appropriate surface prep for an aluminum part which will be coated with an FDA approved powder coat for a food contact surface? This is an indoor part, and far away from salt water. Is chromate conversion necessary or even permissible considering the surface will then be powder coated?

Justin Curtis
- Rochester, New Hampshire, USA


August 2018

A. Hi Justin. Regardless of FDA or any other regulatory codes, I would not put hex chromate beneath the powder coating on a food surface. But the major suppliers all offer TCP (trivalent chromate products) and, better yet, chromate-free conversion coatings.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"


August 27, 2018

Q. Hi, currently I'm doing aluminium powder coating and my process is as follows.
Degreasing, water rinse, De-smutting and etching, water rinse, chromating, water rinse.
Right now I don't have a pre heater and I'm doing it by sun light.
Please advice me this process is OK or not?
Also I would like to know after pre treatment giving some heat to dry the surface is giving good Quality to object?
If so what's the best suitable temperature to pre heat?

Thank you,

Umesh Madushan [returning]
- Negombo Colombo Sri Lanka


August 2018

A. Hi Umesh. I'm sure you realize that this is not ideal because of the chance of contamination after pretreatment and the difficulty in controlling the temperature, but necessity is the mother of invention. If you can keep the work clean, and at about 140 °F (60 °C) it sounds workable.

Etching normally precedes de-smutting. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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