Aluminum doubler question

Discussion in 'Metal Boat Building' started by milo12, Jul 12, 2015.

  1. milo12
    Joined: Nov 2013
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    milo12 Junior Member

    Does welding a doubler on a aluminum plate introduce the chance of crevice corrosion between the doubler and original plate?

    If yes, what is the correct method to follow when installing a doubler?
     
  2. gonzo
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    gonzo Senior Member

    Only if the weld is not well done all around the plate.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Only if the cutting edge and welding of the doubler is poor quality. In general, it should be ok, as Gonzo noted, it must be all around the circumference.
     
  4. Kevin Morin
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    Kevin Morin Junior Member

    Aluminum Doublers

    milo,
    There are a few conditions to add to gonzo's reply.

    First the size of the doubler and second the weld process? If the size is large (larger than an 8"x 8" plate/panel/doubler) then it may be necessary to air test the weld? In this circumstance you'd fit a small threaded fitting to the doubler in order to add positive air pressure and 'soap the seams'.

    Second the weld method can make a difference because many aluminum MIG process welds can be (semi) porous at a pressure/vacuum rating which allow leaks when there are temperature changes on the parent metal.

    So, if you're asking about 3"x4" TIG welded doubler? Chances are the weld was 'closed' or finished by welding the end puddle onto the beginning puddle (?) This tiny interstitial volume will probably not vacuum water into the space between the plates to begin a crevice corrosion cell.

    However, if the plate is 12" x 20" and the process is MIG welding then it would be prudent to pressure test the weld to insure no vacuum leaks that could pull moisture into the space between plates to allow the crevice corrosion to form.

    When closing a TIG weld on a plate like this it is customary to place a wetted rag (or acetone wetted rag) on the plate/padeye base/doubler so the temp is lowered radically which will 'pull' the weld, at closure, into the final 'vent' which has begun to pull air in to compensate for the cooling of the parent metal. If this is not done most weld spaces with air will continue to heat and expand and the weld will 'blow out'.

    Hope this helps see that there is no (completely) fixed answer to your question but the need to pay attention to sealing doublers is very important?

    Cheers,
    Kevin Morin
    Kenai, AK
     
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    There are many factors that effect porosity.

    The principal ones (apart from the obvious, such as poor cleaning/degreasing/de-oxidising) are:

    1) Different base metals. Below is a weld, one with 5083 plate welded to a rider bar of 5033 on the left and with 6082 rider on the right.

    Welds with different base metal.jpg

    The only change is the base parent metal. As you can see, the composition of the base metal can have a serious effect on the amount of porosity.

    2) Torch angel/position relative to baseline

    Porosity v angle of torch.jpg

    Here you can see the path for the escaping bubbles of moisture/hydrogen. The angle and travel direction can promote porosity too.
     

  6. milo12
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    milo12 Junior Member

    Thank you for the excellent explanations.

    A similar scenario is how to handle deck surfacing. Wood, plastic or rubber could be used on the deck to provide a nice walking surface.

    I have seen aluminum boats such as the FPB64 that have non-skid applied to the aluminum deck. How do they prevent water and corrosion from eventually getting under the non-skid.

    Edit - Here is a link to the FPB64 deck. http://www.setsail.com/fpb-64-deck-details/

    It looks like a rubber material that is bonded to the aluminum deck. Would a nick in the rubber or a corner lifting cause crevice corrosion under the rubber.

    This is the material used on the FPB64 http://www.treadmaster.co.uk/
     
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