aluminum-aluminum threads seize

Loose Gravel · Oct 13, 2006

I'm designing a little electrical photo widget, but the battery door sometimes seizes to the case. It is one of these little threaded things that you take out with a coin. It has Al to Al threads and they can seize up tight and have to be drilled out. Not very handy. I'm thinking about using unplated brass, but I'm worried that the green may grow and seize it up, too.

Will brass - aluminum threads seize?
Should the brass be plated? With what?
One part will always be aluminum. The other, the male thread, may change. It must be electrically conductive and easy to machine.

Thanks for you help.

-LG-

Dave Parker · Oct 13, 2006

All you have to do, is put a very sparingly amount of vaseline on the screw before you put it in and it will stop this from happening, just dip the screw into the vaseline, then wipe off good and it will prevent this from happening.

Dave

Ole · Oct 13, 2006

If in contact with aluminium, the brass will not tarnish. But the aluminium will...

Loose Gravel · Oct 13, 2006

Ole

as I understand aluminum, it oxidizes in 10 milliseconds to aluminum oxide which is the same as sapphire. This is the problem with Al-Al threads is that a little dirt scrapes a little Al and whammie, they are one. The aluminum oxide is, however, so thin that it is still conductive.

So, will the aluminum 'tarnish' be aluminum oxide or some cancerous corrosive mess?

Dave Parker · Oct 13, 2006

Aluminum will corrode to a mess....we have had aluminum wires corrode in housing projects to the point they will no conduct electricity through the powder..

Dave

Loose Gravel · Oct 13, 2006

Dave, I'm a little worried about the vaseline or other anti-seize compounds because they can wear off or go away in time. Is there an easy to machine stainless steel that might work.

I remember now about aluminum wires in houses catching fire. The brass and copper connections must have eroded it.

Ole · Oct 13, 2006

Loose Gravel said:
Ole

as I understand aluminum, it oxidizes in 10 milliseconds to aluminum oxide which is the same as sapphire. This is the problem with Al-Al threads is that a little dirt scrapes a little Al and whammie, they are one. The aluminum oxide is, however, so thin that it is still conductive.

So, will the aluminum 'tarnish' be aluminum oxide or some cancerous corrosive mess?

With two metals in contact, you get the electronegativity playing a role, which every boat owner knows intimately. Under dry conditions, nothing happens. But in wet (and especially if salty) conditions you get a buildup of a mixture of Al oxide, hydroxide, carbonate, sulfate, chloride, and god knows what. Anyway it's white, hygroscopic, and porous.

Mating aluminium and brass is very common, and does not present a problem except in marine conditions. But as a boat-owner, I couldn't help mentioning it!

Gerald Koch · Oct 16, 2006

Loose Gravel said:
I remember now about aluminum wires in houses catching fire. The brass and copper connections must have eroded it.

The reason for the problem with aluminum house wiring is that aluminum oxide is an insulator. This prevents a resistence free connection from being made. Even if you should try to scrape the coating off it will immediately reform. Resistence at the connection points at outlets and switches caused heating which lead to fires. In contrast copper oxide is a conductor and does not have this problem.

Gerald Koch · Oct 16, 2006

For a frictionless sliding surface you need two dissimilar metals. A very good choice is aluminum and brass. You will find this combination used extensively in the focusing mounts of lenses. BTW, brass is considered to be self lubricating.

kmv2 · Nov 3, 2006

Gerald Koch said:
For a frictionless sliding surface you need two dissimilar metals. A very good choice is aluminum and brass. You will find this combination used extensively in the focusing mounts of lenses. BTW, brass is considered to be self lubricating.

But two dissimilar metals can cause a galvanic series, they should be carefully chosen if they are dissimilar metals.
For example, if you put gold (heavily cathodic) in contact with zinc (heavily anodic), you're going to have huge corrosion issues and the zinc is going to waste away to nothing.

The issue with aluminum in household wiring is that it is very succeptable to fatigue and this can be caused by successive repeated electrical loads, and even just the physical sagging of the wire.

I believe aluminum is now illegal to use in household wiring. In overhead lines, it has a steel inner core to provide structural stability and prevent the fatigue failure. This is ideal, as electrons flow on the outer ring of the metal, so the steel doesn't have to do any conductive work, just structural, and also the steel/aluminum pairing isn't too bad from the galvanic series point of view.

kmv2 · Nov 3, 2006

In answer to the question, IMO I would not use brass or any other metal screws. I would lubricate the aluminum screws to prevent oxidation (but this is extremely hard to do with aluminum, although the aluminum oxide will protect from further oxidation). In most applications you would want to aluminum to oxidize. Aluminum oxide is one of the hardest substances known to man, and is used as an abrasive for industrial applications.

Gerald Koch · Nov 3, 2006

In order for corrosion to occur you not only need two dissimilar metals you also need some sort of electrolyte. Unless you leave your lenses sitting in salt water when not in use you are not going to see a problem. As I said, brass and aluminum have been used extensively in the past for focusing mounts. If there were a problem then another combination would have been used.

barryjyoung · Nov 3, 2006

Loose Gravel said:
Dave, I'm a little worried about the vaseline or other anti-seize compounds because they can wear off or go away in time. Is there an easy to machine stainless steel that might work.

I remember now about aluminum wires in houses catching fire. The brass and copper connections must have eroded it.

303 stainless steel is relatively free machining. Stay away from 304 if you are concerned about machinability. 15-5 preciptation hardening stainless is fairly easy but is tough so requires rather heavy cuts with lots of horsepower. Have you considered Monel or Invar? If you do end up going to a high nickel content alloy be warned that it machines (and taps) quite easily but will work harden if you look at it sideways so do not let the parts touch each other or drop on the floor before they are finished. Let me know if you need more information.

Raphael · Nov 3, 2006

Why not anodizing ?

I think too that brass and aluminium will be a correct choice. We have here, on a telescope focuser, such assembly, in a rather harsh environment (basically outside, under a rolling roof, not so far from the sea coast) and there is no disabling corrosion issue.

Why not using aluminium on aluminium, but with anodizing of the two part ? It is not difficult to do, with little care, sulfuric acid (beware !) and a lead battery charger. As far I know (I tried), two anodized aluminium parts will not "glue" any more.

OK, this makes problems with then necessity of electrical contact, but if I understanded well your needs, you can do a little threaded hole in one of the parts to set a contact screw.

Best regards,

Raphael

aluminum-aluminum threads seize

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aluminum-aluminum threads seize

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