thibshman

Topic: Best pre-treatment for powder coating aluminum

Can anyone recommend a pre-treatment system, conversion coating, process, etc. which yields excellent adhesion of powder coating and 2000+ hrs of salt spray corrosion resistance (ASTM B117 chamber)  for 6061 aluminum?  I'm not sure that powder coating over Type II anodizing is very good, and I've had mixed results when powder coating over a chromate finish.  Most parts I deal with are machined from 6061 billet.  How critical is surface finish as well?   Suggestions are much appreciated!

ChemSlinger

Re: Best pre-treatment for powder coating aluminum

thibshman,

EnviroServe Chemicals, Inc. has been providing the aluminum powder coating industry with high quality pretreatment chemicals and processes for many years.

I recommend a good alkaline cleaner to remove soils from the metal surface, followed by a clean rinse. The conversion coating that I would recommend is our Low Temp Conversion Coating. This product is environmentally safe, and requires less contact time. A rinse is also needed after this step. 

These chemicals are providing up to 3000+ salt spray hours with ratings of 8 and 9. Our article in this forum can be found in the Industrial Cleaning Section and will give you so much valuable information.

For more information please visit our website at www.enviroservechemicals.com

EnviroServe Chemicals, Inc.

(910) 892-1791

DustinGebhardt

Re: Best pre-treatment for powder coating aluminum

Contact your chemical vendor (Or EnviroServe) and ask for a phosophate conversion coating process.  These typically have much better adhesion than chromate conversion coatings and give good corrosion resistance (which varies based on the type of coating applied).

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

hstockman

Re: Best pre-treatment for powder coating aluminum

Dear thibshman,

There are a number of processes to consider, but based on your spec a Chrome Phosphate should work for converstion.

Contact your chemical supplier of choice to perform tests for verification, but a good start is as follows.

Stage 1 Alk Clean

Stage 2 Rinse

Stage 3 Rinse

Stage 4 Chrome Phos

Stage 5 Rinse

Stage 6 Seal

Stage 7 Recirc DI

Stage 8 Fresh DI Halo

Hope that helps.

Regards,

Herb

812-453-5280

thibshman wrote:

Can anyone recommend a pre-treatment system, conversion coating, process, etc. which yields excellent adhesion of powder coating and 2000+ hrs of salt spray corrosion resistance (ASTM B117 chamber)  for 6061 aluminum?  I'm not sure that powder coating over Type II anodizing is very good, and I've had mixed results when powder coating over a chromate finish.  Most parts I deal with are machined from 6061 billet.  How critical is surface finish as well?   Suggestions are much appreciated!

MilesChemicalSolutions

Re: Best pre-treatment for powder coating aluminum

The Three musts in pretreatment of aluminum for coating:

Clean

Etch  

Seal (Passivate) (Note: Followed by Dry-Off Oven is recommended).

Disclosure: Consult with a Metal Finishing Industry-Experienced and Successful Pretreatment Technical Service Representative for your specific options on pretreating your aluminum processing requirements.

1) Cleaning and removal of all surface contamination that may inhibit adhesion or application of pretreatment or conversation coating is critical and normally accomplished with surface active agents (surfactants) built into a complex cleaner.

2)  Aluminum, like many non-ferrous and plastic substrates, requires a mild microscopic surface etch prior to coating to promote improved adhesion. This can be accomplished simply and safely, with the right chemistry additives in your pretreatment chemistry.  The industry standard chemistries used to microscopically etch aluminum have been primarily acids such as Phosphoric, Sulfuric or Chromic acids, many times used in combination, somewhere in the pretreatment process with a fluoride containing additive to promote the etch. Hydrofluoric acid was used in the past to etch and brighten aluminum. Hydrofluoric acid should be or literally could be your last choice to utilize. This is due to the extremely hazardous and potentially deadly handling concerns and extremely low human threshold limits.  Consult with your Pretreatment Representative for the best etchant and or conversion chemistry available for your specific process requirements.

3) The Seal is many times the last process applied prior to a low conductivity,  Deionized, Distilled or Reverse Osmosis conditioned water rinse. The purpose of the Seal process is to completely passivate and seal any microscopic voids on the surface from the environment prior to coating. This helps to additionally improve adhesion and inhibit corrosion. The Seal process can be accomplished in many ways with today’s technological advancements. In some of the latest pretreatment technologies, the seal and passivation steps are accomplished in a single step during the conversion coating application process. The chemistry can be comprised of Hexavalent Chrome (which is a known carcinogen in the hexavalent state), Tri-valent chrome (which is thought to be a safe alternative to the hexavalent chrome), non-chromic seal such as molybdate and other organic, inorganic and nano-polymer products are commercially available.    

Note: Most Finishing Job Shops and Coaters have a curing oven in-house, if you do not have the resources to purchase a dry-off oven to immediately follow the pretreatment process, I recommend the use of your Powder Coating Oven to quickly (10 minutes or less) dry any moisture from the pretreatment process prior to coating, that may create potential coating defects and coating rework.

Writer: Samuel Miles, Owner of www.MilesChemicalSolutions.com  A company focused on providing the Finishing Industry with eco-friendly and affordable, concentrated Coating Removal Solutions to re-claim coating rework.

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dpatterson

Re: Best pre-treatment for powder coating aluminum

Thibshman:

You should not have any problems achieving the salt spray requirements with the proper pretreatment for 6061 alloy.  For years I have sold products designed for a 5 stage system and ran this system on one of the first powder coated extrusion lines in the market.

Stage 1 - Clean with a good alkaline cleaner to etch the metal as well as to remove oxides and any oils and cutting fluids.  

Stage 2 - Overflowing water rinse

Stage 3 - A hexavalent chrome phosphate solution including a fluoride component.  While I agree with the potential danger of fluorides they have been utilized in these coatings for over 50 years with few incidents.  The benefits they bring to the process outweigh the problems.  This coating will provide a "green" color to the metal.

Stage 4 - Overflowing water rinse

Stage 5 - Overflowing water rinse

If possible I prefer a DI halo, however few of my lines have this

The Chrome Phosphate solution was designed for use under paint.  I am certain most chemical suppliers including A Brite the company I work for, would be happy to process some for your parts in their laboratories for you to paint and test.  This process is currently being used to pass the AAMA certification process for architectural applications.  The alloys used are 6061 and 6063 and must pass 4000 hours of neutral salt spray.

Diane Patterson
A Brite Company

ChemSlinger

Re: Best pre-treatment for powder coating aluminum

A hexavalent chrome phosphate solution might do the trick getting 2000+ salt spray hours, however the health hazards of using chrome may become a bigger issue than normal. Just something to consider.

EnviroServe Chemicals, Inc.

(910) 892-1791

dpatterson

Re: Best pre-treatment for powder coating aluminum

For new lines, not processing Architectural Aluminum, I always recommend a non-chromate process.  This technology has evolved during the past 20 years and it is possible to achieve 2000+ hours (in many cases 4000+ hours) of Neutral Salt Spray if applied and controlled correctly. 

Your post listed that you have tried a chromate and since that is much easier to operate, and I do not the specific's of your operation, I stayed with the chromates.  I still recommend you have a couple of suppliers process some of your parts for testing.  Include both the non-chrome and chrome process to evaluate the different technologies for your specific application.

Also there are classes such as the Powder Coating Workshop put on by the Surface Finishing Academy which discuss the different pretreatment processes and when to use each.  I do know they welcome questions by users.  There 3 classes left for this year.

Diane Patterson
A Brite Company
214-291-0400


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Last edited by dpatterson (08/21/2009 - 07:58 PM)

andeehunt

Re: Best pre-treatment for powder coating aluminum

Aluminum surfaces develop an oxide layer when exposed to the
atmosphere. Abrading the surface removes the oxide layer and allows
paint to adhere. The oxide layer can be removed chemically as well.
Once the oxide layer is removed, the surface has to be painted
(powder coated/primed) or chemically treated to prevent the regrowth
of the oxide layer. Aluminum treated with a chemical treatment
(pretreatment) remain paintable with good paint adhesion than abraded
aluminum.