Conversion coat and powder coat aluminum sand casting
I am in need for expert opinions on the correct process and material for coating an aluminum sand casting (Alum. alloy 360 or 380) with chromate conversion and powder coating. The casting will be exposed to the outdoors, specifically mounted on a tower in all parts of the world. The external (exposed) casting surface is powder coated and the internal surface is coated with chromate,(unpainted),which needs to be electrically conductive.
Presently we have some housings which have been powder coated first and then chromated. The basis for this was that the chromate would break down at the powder cure temperatures had it been done first. I have not been able to obtain a definite answer as to which is the correct process for applying these coatings.
Also, with respect to powder coatings, which type of powder would best suit my application. Information I have tells me that a TGIC polyester with outgas tolerent characteristics is the way to go. Is there a powder which will produce a paint like finish with respect to covering imperfections in the casting? Please inform me on the above.
Thank you in advance,Scott Lincoln
Millitech Corporation - South Deerfield, Massachusetts
Mr. Lincoln: If you are in need of expert opinion, you need to hire a consultant and pay for his/her investigation of your situation, not rely on comments posted here. I know that this is nitpicking, but it is important that everyone understand that people are freely volunteering comments and experiences off the top of their heads -- NOT offering expert opinion.
But I think things were done in the right order.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
There is no good answer for your problem that is not expensive. The high temp will basically destroy the corrosion resistance of the unpainted chromate.
Take a look at a zinc phosphate that is made for use on aluminum. It can stand the temp and give some corrosion protection and is a good paint base.
Visit with the paint folks on their scheduled hour in the chat room on Wednesday nights. They like to see visitors with paint prep problems.James Watts
- Navarre, Florida
You will need to apply a conversion coating to the aluminium before you apply the powder if you wish to achieve outdoor resistance for any length of time. If you are using chromate it should be possible to rechromate after powder coating to obtain good corrosion resistance on the surfaces that are not powder coated.
With respect to your question about powder chemistry you do not give sufficient information for a definitive response. For example if colour retention is not important then you might consider epoxy chemistry. Where colour retention is important then polyester chemistry can be used.
You may contact me for more information.Mario Pennisi
- Powder Coater Painter - Brisbane, Queensland, Australia
You are proceeding reverse sequence.
We do treat 65000 and over parts of Permanent, SandCast and Hi-Pressure aluminum castings each year.
I've been working on the outgasing problem.
First you should consider visits you dealer or talk with them about your problems. Sometime the way that the operator, at the foundry, are casting the parts, especially in the case of the sandcast,the speed the aluminum path into the mold can create turbulence that could lead to a formation of bubbles beneath the surface. And when you go into the dry-off or cure oven then you have what we call outgassing problems. Sometimes minors changes in the molds at the places were aluminum flow is slow can make a big difference.
Second: After your cleaning, Rinse if you do use a Chromate you can wait 24 hours or do as we're doing dry them at 385DegF if the parts are painted right after the dry-off oven there should be no dehydration of the chromate once the Polyester TGIC, we're using Protech formula, is applied on. We obtained up to 2000 hours salt spray using those methods. It is sure that we're still having some problems with certain models of casting, we do have thousands of them ...,
Some are saying that an oven can be use to painting in order to remove all gas. The trick is to heat the parts a higher temperature than what you would cure them. That way when you paint them the polyester TGIC is already melting at the bottom its layer and fills the surface holes. That way you have no more air to worry about.
- Lumec Inc. - Boisbriand, Quebec, Canada
I don't know from your letter whether your parts are accessible to paint on the inside or not but we have a conductive powder coating that could be interesting to you along with our low cure powders which are available in TGIC POLYESTER and assist the out gassing problems greatly.John Underwood
- Mississauga. Ontario. CANADA
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