protecting aluminum in outdoor environments

[Higgler]

We are making an all aluminum antenna to be used in the California desert. It will remain outside for 10-20 years. We would like aluminum that has good conductivity for low loss and we need to paint it white.
Questions:
1) I read alloy 1100 is the best conductor, do you suggest using this or another aluminum alloy (3000, 6061)?
2) What paint is suggested to protect from the environment. We also have to paint a plastic front cover (the plastic has not been chosen, Tefzel maybe).
Thanks,
kch

 

[CoryPad]

What are the mechanical property requirements? There are 6XXX series alloys with high strength and conductivity used for electrical transmission applications.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

What kind of restrictions do you face in California concerning types of paint?

 

[swall]

I forgot to ask--does the antenna have to be painted in situ after assembly, or can it be pre-painted in the shop?

 

[swall]

From AMS 2658--conductivity of 1100-O is 57-62%. Conductivity of 6061-T6 is 40-47%.

 

[Higgler]

The antenna can be painted in a shop. I'm not up on types of paint, but my wife is an environmental engineer - what are the best paints? We can't use lead based on an antenna.

We are making a 4 foot box shape with an open top.
We need only the outside surface painted. The inside must remain unpainted.

The type aluminum will be chosen by Mechanical consultants unless I specify a type, or some options. They are doing wind load analysis and may choose their own for maximum strength and lightest weight.
Regarding conductivity: what is the percentage referenced to? copper or some other standard?
Thanks for the 6xxx reference.

Side-question:
We need to make good electrical contact with the sheet aluminum and our RF connectors and cables. Is there a reference to search regarding, conductivity and aluminum coatings (i.e. is anodizing or iriditing or powder coating better for electrical contact between our connectors&cables and our chosen aluminum?)

Thanks,
sorry if I've strayed to another forum with the side-Q.
kch

 

[btrueblood]

If you need good electrical connections across the contact points between members, you might be better off using an unpainted structure. The so-called marine aluminums (50xx series) with Al and Mg, are pretty robust.

 

[swall]

Anodize is an insulator. Irridite is conductive, unless you use a heavy coating. For paint, you can mask off the areas you need for conductivity. As for paint types, consider powder coating with epoxy. The epoxy would have good abrasion resistance, a consideration if periodic dust storms would be expected. Many small shops are doing powder coating these days. Pre-treatment for powder coating could either be Irridite, Alodine or zinc phosphate.

 

[IRstuff]

Good electrical bonding precludes irridite, so wherever you need good electrical bonding, you'll need to mask that off or grind it off afterwards.

TTFN

 

[Higgler]

Ir stuff and swall, it seems anodize is out (unless we anodize only the external surface), irridite is bad if it's thick, so can we agree thin irridite is OK if it's thinly coated. I was told by shop workers, that irridite is typically used for parts internal to boxes. It protects, but scratches easily and the scratches look ugly externally. It's also a simpler brush on material. Sounds like a light coat of irridite inside our box is warranted, and maybe very light coat in the areas needing good conduction with other alumimun parts. I know that getting good contact with cast aluminum is a real pain in the butt (from my days in the EMI labs at Raytheon working on Navy hardware).
There will be a few good dust storms per year hitting our antenna, so powder coat with epoxy sounds recommended.

btrueblood "The so-called marine aluminums (50xx series) with Al and Mg, are pretty robust." I would think that the title "marine aluminums" would be pretty resistant to corrosion. Do you know if the conductivity at the 50xx series surface is good, and whether it oxidizes alot to protect itself in the marine environment (that would be bad, I can see our antenna going bad after a few years). I had heard that standard Aluminum at sea doesn't do too well.


Sounds like good progress,
Thanks for the inputs, addition inputs welcome,
kch

 

[btrueblood]

Alloy 5052 is the one I know works well for boat hulls, both marine and inland (fresh water). Yes, these develop fairly thick oxide layers, but fairly slowly relative to 6061, at least in Pacific NW (Seattle) marine environments. Electrical resistivity is about 5 micro-ohm-cm, compare to pure Al (alloy 1100) at about 3 microohm-cm.

The biggest "problem" with the 5xxx series is that they are not hardenable, so you are stuck with a yield strength of about 13 ksi (for 5052) to 23 ksi (alloy 5456), unless you get the product in a strain-hardened (work hardened) condition. But, I've routinely formed 90-deg. bends in 1/8" sheet 5052 at half-hard (30 ksi yield) condition. And it welds beautifully.

 

[CoryPad]

Epoxy-based materials are susceptible to ultraviolet radiation damage. If your antenna will see significant sunlight, I recommend a three layer paint: 1) primer (Irridite, Alodine, or zinc phosphate), 2) epoxy powdercoat, and 3) polyurethane or acrylic topcoat (powder, spray, liquid, whatever you want).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

Alodine and Irridite can be easily removed with a Scotchbrite pad or similar abrasive. So, a local area can be removed for electrical bonding with little trouble. CoryPad's suggestion of a topcoat over the epoxy powder is a good one. A single powder coat, utilizing one of the polyesters would also work. This is where the final selection will need to be closely coordinated with the paint vendor and with the input of the paint suppliers.