corrosion, bolts etc

Discussion in 'Boatbuilding' started by bob the builder, Aug 18, 2009.

  1. bob the builder
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    bob the builder novice

    hi all.

    i'll be wanting to attach 15 ft of stainless steel traveller track to an aluminium truss beam.


    what would different people here do?



    thanks,
    mal.
     
  2. wardd
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    wardd Senior Member

    be careful of dissimilar metal contact
     
  3. bob the builder
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    bob the builder novice

    would you put a strip of wood / epoxy fiberglass between them?
     
  4. wardd
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    wardd Senior Member

    when i worked on aircraft we used a tape to isolate them and a zink chromate paint the fasteners were dipped in. otherwise corrosion would develop

    that was many years ago, there may be better ways now

    its something youll have to look into

    some of the boat builders here could be a better source of info on current methods
     
  5. bob the builder
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    bob the builder novice

    ?!

    from
    http://www.scorpiosail.com/web/mast_2.html



    [​IMG]


    i don't ever want to play that game


    can you install a sacrificial anode on the mast?

    what about using aluminium rivets?
    surely this is better than coupling more dissimilar electrolytic metals?


    plastic washers & sleeves

    plastic bolts
    Nylon Bolts

    keep it dry

    epoxy with ceramic powder ie fumed silica to decrease the osmotic diffusion rate to the lowest you can




    plastic bolt manufacturers
    http://www.craftechind.com/materials.php

    Yield strength of reinforced plastic is comparable to aluminum
     
  6. mark775

    mark775 Guest

    An aluminum rivet is of a different alloy and thus dissimilar.
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Make or have an intermediate part made from epoxy and glass or carbon fiber.
    Two half circles clamping around the mast would avoid all direct contact between dissimilar metals.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    woahhhh....looks like it could be cheaper (inferior) 304 SS...which doesnt help!
     
  9. Jimbo1490
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    Jimbo1490 Senior Member

    Sensible anti-corrosion measures:

    • Use Monel rivets rather than stainless rivets. Monel is closer to the Al alloys on the Galvanic Scale, so less conducive to dissimilar metal corrosion.

    • As Ward mentioned, an isolation tape or other coating can help. A good coating of a primer with hexavalent chromium will also work here. The common 'Mil-Spec' primer is such a coating, containing strontium chromate. You can brush this on; it does not have to be sprayed. The beam (I assume) will already be anodized. This complicates getting a primer to adhere properly, but it certainly can be done. Tapes, OTOH, always have their best adhesion to anodized aluminum, so no prep would be needed there.

      One caveat:

      Since tapes are not mated nearly as intimately with the substrate as a primer, they may harbor moisture in the adhesive layer, which could potentially defeat the purpose of the tape.

    • An isolation/anti-sieze coating such a Mastinox can be applied to the fasteners as well, delaying the onset of corrosion for years or decades. While pricey, in the grand scheme of things, this is a small fraction of the cost of your project.

    Jimbo
     
  10. alan white
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    alan white Senior Member

    Formica laminate will be cheap and effective as an isolation material. Low compression and easily cut, free from kitchen counter fab shop scrap pile.
     
  11. Jimbo1490
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    Jimbo1490 Senior Member

    Great Idea!
     
  12. Jimbo1490
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    Jimbo1490 Senior Member

    Just as with we sometimes do with aluminum, we lump a lot of different alloys together with the term "stainless steel", when in reality this is a diverse range of alloys with varying properties.

    Some have no place on a boat just as some are not at all suitable for food contact.

    Jimbo
     
  13. wardd
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    wardd Senior Member

    lay some fiberglass tape down on a flat surface and make your own isolator
     
  14. owene
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    owene Mr Owen Charles

    To be honest, I'd forget it. Get back to basics. The basic issue is that mixing SS & ally ain't a runner so why not purchase or fabricate the track in ally. End of problem.
     

  15. Astute Boats
    Joined: Oct 2009
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    Astute Boats Junior Member

    A very good product for this application is DURALAC. I use this for all fastenings an alloy tracks when fitting out and maintaining composite race yachts.
     
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