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Joining CU and ALU busbars together

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LAN_403

Kizmo

Mon Oct 19 2009, 09:57AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Is there any way to prevent AL from creating oxide layer when joining cu and alu busbars together? Obviously both metals will be sanded before bolting them together as airtight as possible but im still worried about this connection.

Any ideas? One solution is to scrap al busbars and use copper but i prefer aluminium because its cheaper :)

Dr. Slack

Mon Oct 19 2009, 10:14AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Aluminium is immediately oxided or passivated by exposure to the air, so all alli is coated with a layer of alli-oxide. If you anodise, you can thicken it up, but there is no way to remove it, scratch it off and it reforms. A fun demonstration if you have some mercury to hand is to put a drop on some alli. Nothing interesting happens, until you scratch the alli-oxide under the mercury. Alli disolves, diffuses to the surface of the drop, and oxidises, generating a lot of heat and growing alli-oxide whiskers out of the drop (that's why they're not too keen on mercury being carried in aircraft).

However the layer you do get with exposure to dry room temperature air is very thin, with a low breakdown voltage. I don't think people generally have a volt-drop problem when bolting to alli. If you are really paranoid, you could solder a copper lug to it (search for alli solder, I've used the stuff, it works, vicious flux)

rp181

Mon Oct 19 2009, 12:58PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I have had success with kopr-shield, look for something similar. Sand the aluminum bus bars right before bolting the coppper to it, using the compound. The compound is a pertoleum based product, and has lots of copper suspended in it. It does not dry, so that is a plus too.
Try something like the cool-amp stuff, thats almost the same, but with silver.

Proud Mary

Mon Oct 19 2009, 09:03PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

doctor electrons

Mon Oct 19 2009, 11:22PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Kizmo!
Dr. Slack is 100% correct. Aluminum is oxidized immediately by the oxygen in the air. However your project can be done. I would first recommend NOT sanding the material. Sanding aluminum with abrasives of any kind will contaminate it. Machining is the way to go for that. Use no oils or cutting fluids or waxes. (very important) There are chemical treatments that will remove the oxides from aluminum and protect it from oxygen for an hour or two, long enough to join the materials. Joints must be airtight or oxidization is guaranteed! Regarding the sanding and why it is bad. If you imbed impurities whether it be abrasive dust, oils, and god help you ferrous material, you can cause massive oxidation and eventually corrosion. If you need to file it, use a stainless steel file thats brand new. If you happen to have a friend thats a welder it should not be a problem for him or her. If the parts are small enough, you can do your work in a sealed sand blasting booth purged with an inert gas (argon is best for al and copper). Good luck! Hope this helps!

rp181

Tue Oct 20 2009, 12:10AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Thats interesting, I did not know that. Kind of related, but I silver plated some copper a little while back (with a powder). I sanded the copper rather than using a wire brush like the instructions said (they said use a steel wool pad), and the silver coating got slightly yellow in some spots, even more when I tried baking it. Would this be due to the sand paper?

Bird-Dawg

Tue Oct 20 2009, 01:23AM

Registered Member #2360 Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14

I am not shure if you have a home depot in Finland (mabey you could get it online). I have had great succsses with this stuff. The only problem is it leaves kinda a waxy residue.

doctor electrons

Tue Oct 20 2009, 01:43AM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Bird-Dawg wrote ...

I am not shure if you have a home depot in Finland (mabey you could get it online). I have had great succsses with this stuff. The only problem is it leaves kinda a waxy residue.

Good call man!! Thats the stuff i was referring to! Its not permanent but it does work!
I could not remember what it was called off hand! Thanks!
Also, that waxy residue is what protects the aluminum from oxidizing

When thats gone, it will oxidize.

The answer to the question about the yellowing! Yes! Most likely that is due to the sanding. Most sandpapers use some form of oxide as grit. Check out some mfg specs for metal cutting abrasives. Steel wool would have probably had the same effect, possibly worse. That is why i recommend machining the metal #1. Try the ideal compound, he's right, you should have good results. Electricians use that stuff all the time!

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