Installing Aluminum Gas tank

Discussion in 'Fiberglass and Composite Boat Building' started by Jack Jr., Mar 14, 2010.

  1. Jack Jr.
    Joined: Nov 2006
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    Location: Clveland OH / NYC

    Jack Jr. New Member

    Old topic, new thread.....

    I did search and found this thread but it would not let me repost since it was older..........

    I need to re-install my Aluminum fuel tank back in my boat. It is a 21' Apache scout. It was originally foamed in and fastened to the main stringers. Underneath the tank was 3/4" x 3" x 3" blocks in all four corners to keep it off the hull.... I currently have the tank out and getting ready to have it cleaned out. It has minimal pitting. It did not prevously leak. I am looking for the best way to install it. I am good with putting a good "coal tar epoxy" or other coating on it. I am questioning whether I should foam it back in. I understand the corrosion issues but I do like the support it gives.. FYI, the boat will be able to hit speeds +65mph. What are your thoughts. Here is an old & up to date picture..

    http://www.boatdesign.net/forums/fiberglass-composite-boat-building/fuel-tank-11403-3.html
     

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  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    A problem with a foamed in tank is that it is hard to remove. But you already did that once and probably won't make a habit of it.

    If you apply coal tar epoxy on the cleaned tank I see no reason why you shouldn't foam it back in.

    There is a 150 ft long iron propane line dug in my garden 1 to 3 ft deep. 15 years ago the guys welded it together from 18 ft lengths, wrapped glass fiber cloth soaked in coal tar epoxy around it and pushed it in the sand and mud while it was still warm and tacky.
    That is the approved standard procedure for underground pressurized fuel lines.
     
  3. gonzo
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    gonzo Senior Member

    The problem with coating aluminum tanks is that if you get a pinhole or scratch all the corrosion by electrolisis will be concentrated in that spot. It will soon eat through. The original design seems to have worked well, I would stick with it.
     
  4. SamSam
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    Location: Coastal Georgia

    SamSam Senior Member

  5. CDK
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    CDK retired engineer

    All kinds of studies have been done, but not always meaningful.
    The 95 gallon aluminum tank in my boat is 31 years old, painted brown and foamed in. There are several scratches, no corrosion.

    It was repaired approx 20 years ago when seawater made a tiny hole in the bottom FROM THE INSIDE, because the vent valve got stuck and the waves reached it. In gasoline is no free oxygen, so aluminum cannot repair itself by forming Al2O3.
     
  6. SamSam
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    SamSam Senior Member

    Well, I guess statements can also be made that aren't always meaningful. That yours has lasted 31 years doesn't disprove studies that show 10 years as an average lifespan. As you yourself stated, yours got a pinhole leak after 11 years, that seems to back up the study pretty good.

    How yours was made and from what thickness and type of aluminum is not stated, nor is Jack Jr's. Jack has a go-fast boat where weight is critical so the tank could be thin material, etc. I'm assuming his deck has rotted out, and maybe the stringers and transom too, so there might be quality of construction differences also.

    Mainly I was making a suggestion that odds are the tank will fail before the deck and if the tank is inaccessible, which is usually the case, he might have to rip up a perfectly good deck, maybe a week after the boats back in the water. If a hatch or semi permanent cover is not provided (which can create their own problems) at least make some measurements so cuts can be made in the deck with the least amount of damage to the stringers or frames, etc.
     
  7. CDK
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    CDK retired engineer

    SamSam, my point is that most information and advice from internet sources about AlMg alloys is incorrect.
    The newboatbuilders link shows an article that looks authoritative, yet states that moisture dissolves aluminum oxide. It also gives rules for painting Al like chemically removing the oxide. Those are fairy tales.

    Aluminum oxide (and Mg oxide as well) is a very strong, hard material that does not dissolve in anything. It can only be removed in an inert atmosphere by reduction, electrolysis or mechanical means. The only useful purpose to do that is for welding.

    The bond between the metal and its oxide is much stronger than with any other paint or coating, so why should you try to remove it?
    The factories that produce Coca Cola cans do not prepare the metal in any way, they just print and bake it in an infrared tunnel.
    Discarded cans turn up in fishing nets after years and hardly show any deterioration.

    The most important or in fact only rule is to avoid contact with other metals unless you can guarantee there will never be moisture near the contact area so there can be no electrolysis. And for that you need a good, somewhat flexible paint like coal tar epoxy or neoprene based coating. That dissimilar metals do not cause problems when properly protected is proven by car manufacturers like Porsche: the 928 has an all aluminum front and a galvanized steel bottom, roof and rear end, except the rear bumper, which is again aluminum under a plastic shell.

    Preventing corrosion from within seems very difficult to me. Should my tank develop a leak now that I switched to diesel engines, I will probably replace it with a glass fiber one.
     
  8. SamSam
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    SamSam Senior Member

    Yes, most info and advice about aluminum alloys on the internet could be incorrect. But I have some faith in Ike and his Newboatbuilders site...
    http://newboatbuilders.org/

    Here is another forum thread where the water dissolving oxide statement is made, along with the problems of ethanol.

    http://www.boatdesign.net/forums/powerboats/fuel-tank-life-construction-26316.html

    I'm a little puzzled where in your last post you say water won't dissolve the protective oxide but your other post says seawater made a hole in your tank.

    Anyways, I'm no metallurgicaloligistician but I do know that aluminum tanks can and sometimes do fail quickly, notwithstanding that it might possibly be possible for them to last forever. And I do think it's no fun cutting and grinding fiberglass or to have to rip apart what you just made because of a buried problem. I guess I'm sort of questioning the wisdom of re-installing a used tank that already has pitting, but I understand the reason why $$$.
     

  9. CDK
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    CDK retired engineer

    I'm sorry, I understand why you're puzzled.
    While composing post #5 I deleted a few lines to keep the post short; I shouldn't have done that.

    Both gasoline and diesel contain traces of organic compounds called thiols, in which sulfur is present in a non-ionized form, so chemically inactive.
    But when a few drops of seawater are present, a reaction takes place and a strong acid is formed, which breaks down the aluminum oxide skin and digests the -then bare- metal.

    It takes several weeks for the acid to form, but once it is there and the oxide barrier is eliminated, the process accelerates dramatically. I watched the droplets roll over the tank bottom through the inspection hole, leaving a snail-like trace .
     
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