corrosion of aluminium keel by ballast-advice

Discussion in 'Boat Design' started by sharkbite, Apr 14, 2014.

  1. sharkbite
    Joined: Apr 2014
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    sharkbite New Member

    I have purchased a 40' aluminium Finot "Reve de Seychelles" 1980. She has been poorly maintained and now that I have finally removed decades of oil and gunge from the bilge I found pitting of the plate inside the keel where steel ballast has come loose from the polyester resin used as ballast sealer and sat against the aluminium.

    I plan to do thickness testing of the keel and then plate over from outside.
    I will then dry the ballast by heating the keel with gas torches and seal with bitumen.

    In the interim I have flooded the keel sump with fresh water and dropped an anode in that is bolted to the hull. Has anyone experience or knowledge of using anodes internally?

    There are also random pits in the bilges where previous owners have stupidly bolted wood against the hull, luckily the hull is 8mm thick and the deepest pits are about 4mm, rather than cut out and weld plates to fit I was planning to simply weld patches(inside) over the worst pitted areas.

    These "remedies" are simply to get another ten years or so of safe cruising out of what is otherwise a well built hull, I do not want to try and make her "new' again.

    Any good advice out there?
     
  2. gonzo
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    gonzo Senior Member

    In aluminum hulls internal anodes, particularly in salt water use, are advisable. Bitumen is not a good way of sealing. One of the bad things about it is that is is soluble in fuel and oils. Eventually, you will end up with a sticky black mess all over the bilge. Epoxy or chlorinated latex (as in pools) are better choices.
     
  3. sharkbite
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    sharkbite New Member

    Thanks, I can't see that I will achieve a decent bond with the sides with epoxy, not knowledgeable about chlorinated latex but will investigate. May have to live with an anode in the sump forever. Not the worst outcome especially given metal fittings natural gravitation to the bottom of the sump.

    Any misgivings about me welding a doubling plate over the corroded area of the keel from outside? I know that it makes keeping the hull clean a little more of a issue but seems like a simple solution. Cutting out a piece and welding new with the ballast immediately behind does not seem feasible.
     
  4. gonzo
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    gonzo Senior Member

    The only area of concern would be the keel/bottom joint. That is where all the stress is concentrated. Probably it is also where most of the corrosion is. It may need some kind of connection between the second plating and the inside framing. It is possible to make holes and "button weld".
     
  5. sharkbite
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    sharkbite New Member

    Hi Gonzo. I should have been more elaborate, the line of pits is horizontal at mid keel depth where the ballast stops. Above there is smooth and pit free. I will have to do thickness testing below that line to determine whether the ballast has been interacting with the plate. So worst case scenario would be doubling the lower section of keel. I would place the doubling plate's top line a few inches above the pit line. I was thinking of plunge cutting a few slots to "button weld" the new plate to the old as I am mostly concerned about the strength of the keel and would like to knit the two plates as much as possible.
     
  6. gonzo
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    gonzo Senior Member

    I think you shouldn't have a problem then. Good luck
     
  7. sharkbite
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    sharkbite New Member

    I have spoken to some professional metal boat builders who advise against overplating/doubling the keel as they believe it will sweat in-between the two plates. This can cause stresses if the boat goes into cold regions and force the welds apart. I am not planning on going polar but would welcome any input about pros/cons of overplating below the water line. I would buttonhole stitch the two plates.

    Cutting out and replacing the pitted affected area is only an issue because I would only be able to weld new plates from the outside as it is too far down the keel to weld internally.

    About sealing the ballast. I have decided to weld cover plates over and pressure test. Then fill with vegetable oil.
     
  8. sdowney717
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    sdowney717 Senior Member

    Will the oil go rancid and rot and stink?
    There will be water and natural oil mixing and it will grow bacterias, fungus.
    Will it rot and make organic acids that will corrode aluminum?

    This says vegetable oils go rancid quickly.
    http://www.jctonic.com/include/healingcrisis/20rancid_oils.htm

    Consider that diesel fuel can grow nasty stuff that eats metal, why not vegetable oil even more likely to do that.
    http://www.finol.ie/which-oil/blog/2013/04/19/actoil-a550/

    Your basically got a large tank of oil with what your thinking of doing.


    Why not cut out the pitted aluminum in sections and weld in new plate from the outside?
     
  9. Barry
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    Barry Senior Member

    Sharkbite
    I cannot put my hands on the article that I had read about doubling aluminum plates together in a salt water environment but this is what I remember about the article

    If you end up having two aluminum surfaces together, ( or aluminum and another impervious surface) and salt water gets trapped between them, the aluminum cannot form a corrosion resistance surface due to the lack of oxygen. The water between the plates become acidic and corrodes the aluminum. Better to cut out the plate, if it is localized and weld in plates
     

  10. Kevin Morin
    Joined: May 2013
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    Kevin Morin Junior Member

    hull doubler in aluminum

    Sharkbite, I think Barry has it correct, doubling in aluminum creates the invitation to crevice corrosion by allowing any moisture, not even liquid, between aluminum plates to become de-aerated and subsequently acidic and thus the basis for a corrosion cell.

    To overcome this air testing the weld of a hull doubler is the traditional method to insure a dry interface. Tap the outer plate to the thread for an air fitting and when the exterior/perimeter welds are complete; pressure up the void/interstitial space with dry air or nitrogen and leak test with bubbling compounds along all the welds.

    To seal the thread opening; put wet, cold cloth on the plate to chill the entire perimeter welds of the doubler plate and then TIG up the threaded opening, while the rest of the entire plate is being cooled (chilled) and the weld will indicate sealing by pulling inward when the puddle is sealed.

    This method will insure both pressure and vacuum leaks are excluded from the welds and the space between the two plates will remain dry and corrosion free. It is most common for a void like this to leak in vacuum when the boat is floated and the hull is cooled by the water, pulling liquid droplets into the porosity in MIG welds, or cold laps in all welds.

    IF possible, cut out the 'band' of corroded keel plate and replace with a strip of material, beveling outside for a single pass outside weld allowing the weld to 'droop' or penetrate the inside of the beveled edges. Carrying a keyhole in this type of weld is sort critical to the full fusion required.

    cheers,
    Kevin Morin
    Kenai, AK
     
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