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  1. #1
    Loose Gravel's Avatar
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    aluminum-aluminum threads seize

    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-
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  2. #2
    Dave Parker's Avatar
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    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

  3. #3
    Ole
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    If in contact with aluminium, the brass will not tarnish. But the aluminium will...
    -- Ole Tjugen, Luddite Elitist
    Norway

  4. #4
    Loose Gravel's Avatar
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    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?
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  5. #5
    Dave Parker's Avatar
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    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

  6. #6
    Loose Gravel's Avatar
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    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.
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    Ole
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    Quote Originally Posted by Loose Gravel View Post
    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!
    -- Ole Tjugen, Luddite Elitist
    Norway

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    Quote Originally Posted by Loose Gravel View Post
    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.

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    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.

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    Quote Originally Posted by Gerald Koch View Post
    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.

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