aluminium fittings on carbon structures - Problems

Discussion in 'Materials' started by lausl, Oct 27, 2010.

  1. lausl
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    lausl carpe diem yacht design

    I have heard about problems fastening aluminium fittings on carbon structures when not paying attention to correct insulation of the fasteners.
    Say al. spreaders on carbon masts (in the past) and clamps on decks etc.
    Have you got experience and/or photographs of such problems and an indication as how fast these problems may show?

    Appreciate any hint
    Regards,
    Klaus
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    You need to look at the galvanic series to really appreciate the problem. With an electrical potential greater than -100mV aluminium will be in danger. Carbon, or graphite, has nearly a 700mV potential between it and aluminium. Thus without paying very serious attention to isolation, you’re in for big problems.
     

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  3. CDK
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    CDK retired engineer

    Good point Ad Hoc.

    In fact carbon sacrifices any metal electrically connected to it, even stainless steel, although the chemical reactivity determines the speed at which it happens.

    But the good news is that what we call carbon is a resin containing carbon fibers as reinforcement or at least something that looks like carbon fibers. And I doubt that it is conductive.
     
  4. AndrewK
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    AndrewK Senior Member

    A laminate that I tested was conductive.
     
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  5. daiquiri
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    daiquiri Engineering and Design

    True. But the problem is, when an aluminum fastener goes through the graphite/resin laminate it can get in direct contact with graphite fibers, so it has to be insulated - through the use of PTFE or nylon sleeves for example, or by some other mean. Avoiding the use of aluminum and carbon together is the best one...
    Aircraft industry uses either chemical bonding for joining graphite-fiber parts, or insulation with corrosion-inhibiting sealants, or titanium fastenings (the cost of this solution is probably prohibitive for your use). However, aircrafts usually don't get splashed by or submerged into the salty water (when it happens, corrosion issues are probably the very last ones to think about. :D ), so I don't know how do these solutions behave in a marine environment.
     
  6. PAR
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    PAR Yacht Designer/Builder

    I would think naval aircraft would need to address this issue. Living aboard an aircraft carrier can't be a pleasant experience for many of these material combination.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Corrosion-inhibiting sealants do work well for navy aircafts too, and are required where galvanically dissimilar metals or non-metals have to be joined. But they are not perfect solutions, and I guess (not sure though...) that exposure to damp and salty air is different from continuous wetting/drying cycles in a sea water.

    By the way, did you know this:
    http://www.f-16.net/news_article2579.html
    Two-thirds is a big number, though they don't say how severe those corrosion issues actually are on single aircrafts.

    We usually look at the aircraft industry as a source of ultimate wisdom in many technological areas, but materials for aerospace use are mostly oriented towards mechanical performance and weight saving. So it might be much more prudent to keep sticking to experiences from marine industry when it comes to corrosion protection and corrosion-aware design.
     
  8. CDK
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    CDK retired engineer

    Very interesting.
    Can you or any other member put 2 test pins on a carbon laminate and tell me the surface resistance and the distance between the pins?

    I once needed a conductive rubber profile for an electronics project. You would think that it would be easy because carbon is used as a filler, but nearly all samples had a high, sometimes infinite resistance or contained so much carbon they were hard as stone.
     
  9. lausl
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    lausl carpe diem yacht design

    Rivets fastening carbon laminate sheets to aluminium profiles

    I was too curious to know other examples of this corrosion problem in the marine context. In my case a box section had been built from four 90deg aluminium corner profiles and 4 flat single skin carbon fibre plates
    The through holes have not been fitted with sleeves or bushes but have been
    painted with a yellow primer (apparently in use in the aircraft industry).

    One can see on the close ups, that the primer had not penetrated all the holes, in addition one can see on the carbon fible surfaces that the laminate is quite dry, in places exposed to the environment.
    This section failed in service
     

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  10. daiquiri
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    daiquiri Engineering and Design

    What type of loads was this assembly designed for (torsion, axial, shear, bending)? I'm asking because corners are the areas of the beam section where high concentrated stresses (even up to 3 times the stress value in the flat sides) can occur in load cases involving torsion. Since aluminum is a less resistant between the two materials, a strategy of making corners with aluminum doesn't sound like a good one - unless it's thickness has been sufficiently beefed up in the stressed areas.
    That's in addition to all previous remarks about galvanic incompatibility of the two materials.
     
  11. baeckmo
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    baeckmo Hydrodynamics

    This discussion reminds me of observations we made on rubber hose connections in sea-water systems. Some rubber qualities seem to have extra high graphite contents (enough to create a continuous electrical connection), causing massive corrosion on unprotected alu stubs. The process is accelerated where water is stagnant, causing oxygen reduction. As you can see in Ad Hoc's tabula, this occurs also with the so called "stainless steels" in their active status. Even copper will see trouble with galvanic corrosion to graphite in fluid systems with oxy-depleted salt water.

    For those of you using I/O drives, this is a good reason to give the stubs for the bellows a good layer of paint protection. That will save you a set of cardan shaft bearings and ultimately save the boat from sinking.
     
  12. apex1

    apex1 Guest

    Not really surprising Bodo.

    Rubber, as we know, has a high "soot" (carbon) content in general. A inadequate spec. of these aggregates (there are some 30 - 40 different on the market) leads even to so named "Super Conductive" rubber.

    Where possible, use simple Teflon tape (for plumbing) to isolate.

    Regards
    Richard
     
  13. CDK
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    CDK retired engineer

    There is another mechanism at work there against which no paint layer can offer protection. Mercruiser sells something they call "liquid neoprene", being just a rubber cement that also doesn't work.

    The raw water cooled gas engines operate at 70 C. and produce staggering amounts of soot. A layer of carbon particles clings to the exhaust walls from manifold to prop and because of the (sea) water present, it conducts almost as good as any metal. Even with plastic hoses there is galvanic corrosion and bearing damage.
     
  14. Landlubber
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    Landlubber Senior Member

    ....for the very same reason not to use a lead pencil on alloy masts when marking out for cutting slots etc, use a Texta....it is my understanding from way back that mast makers are aware of this, and there was a failure that determined the carbon tracking of the lead pencil mark caused the failure.....
     

  15. lausl
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    lausl carpe diem yacht design

    Landlubber

    have you got more details on that failure (pencil on alu mast), or even other
    cases whre this problem has arisen in the marine field?
    Klaus
     
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