Preventing rust inside steel pipe or tubing?

Discussion in 'Boat Design' started by parkland, Jun 12, 2014.

  1. parkland
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    parkland Senior Member

    If something is made from 1" steel tubing, or 2" steel tubing, would it prevent rust by filling the pipe or tube with expanding closed cell foam?

    If it doesn't get oxygen, it can't rust, right?

    Closed cell foam should really seal it off, and prevent air or moisture from moving around at all.

    Just wondering, because how the heck else could you prevent rust inside a steel tube?
  2. daiquiri
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    daiquiri Engineering and Design

    That is pretty much correct.. But you can also seal-off the interior by welding two circular caps at both ends of the tube. Some air humidity will remain trapped inside and will condensate on cold days, creating a possibility for a small amount of corrosion anyways. But you can limit this by doing the welding job in a sufficiently dry environment.
  3. alan white
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    alan white Senior Member

    Pouring in epoxy (and draining what's left) would prevent corrosion if you can't cap the openings. First a solvent should be poured in to dissolve any oil in there that would prevent the epoxy adhering.
  4. jonr
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    jonr Senior Member

    For the sake of counter argument, maybe the foam will shrink very slightly or the tube will expand with heat and you will end up with a small crack between the metal and foam that holds in moisture and causes a rapid failure.

    I'd consider dragging a LPS3 soaked rag through it. It leaves open the option to inspect it.
  5. jehardiman
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    jehardiman Senior Member

    Yes, foaming it will just cause rust by providing a moisture path. Slavi is correct in that a sealed tank rusts a little, but it is minimal until all the oxygen is used up. I have opened void tanks of ships fabricated in WWII that were never painted. After 40 years (at the time) there was this slight fuzz of rust needles over the surface and an undetectable (i.e. below sensor level) amount oxygen. It normally took a day or two to ventilate and you could just watch the surface blush during that time. Also amazing was you could see all the chalk, grease pencil, and soapstone fabrication marks as well as the unchipped weld slag.
  6. rwatson
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    rwatson Senior Member

    I have a drain hole in a square section trailer. I fill the steel with Fish Oil anti rust, then drain it out and reseal it. You could do that with a rust converting paint i suppose.

    Extra weight and unavoidable voids that you can get with foam would be my first impression.
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    At the risk of sounding like a salesman for Penetrol, it will keep rust at bay very well, the foam option I would not even consider.
  8. parkland
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    parkland Senior Member

    Are voids bad if they are in closed cell foam?
    What can the metal do other than rust a tiny bit and run out of oxygen?
    Some of these foams cure with extreme outward force, it should remain air tight?!
  9. kapnD
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    kapnD Senior Member

    The foam will not prevent rusting, I have tried just about everything to prevent rust on tubing (trailer axles, mostly) and the best solution I have found is to put a threaded plug at the top and fill completely with waste motor oil. Then all you have to do is keep the outside in good repair, or you will be explaining the oil slick to the CG.
  10. portacruise
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    portacruise Senior Member

    There will be a certain amount of permeability and possible wicking of air or moisture for the closed foam permeability especially at the interface with the iron. There will be frequent cycles of higher pressure air trapped inside pushing out during heat and pulling in during cold which further contributes to this effect. If it is pipe which can be threaded and capped that is probably the best way. You can seal in some kind of non corrosive dessicant (silica gel?) packs to further capture moisture inside.


  11. SamSam
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    SamSam Senior Member

    I had need of some pipe and old automobile drive shafts were available cheap. All of them were heavily rusted and corroded and some were dented, but when the welded ends were cut off they were all in perfect condition on the inside, shiny bright steel with a light coat of oil. Whether they also had some kind of inert gas in there too..who knows, but it's doubtful.
  12. Yellowjacket
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    Yellowjacket Senior Member

    Aircraft tube fuselages are constructed from numerous small tubes that are welded to form a truss type structure. Obviously, if these structures were to rust from condensation inside the tubing it could be fatal, and since there isn't margin in the design to allow for rust, this problem has to be avoided.

    For many many years linseed oil and later boiled linseed oil has been used in aircraft tube airframes and structures like engine mounts to prevent rust from forming in the tubes. Typically the tube structure gets built up and after welding small holes would be drilled and tubes filled with linseed oil and then drained. The linseed oil formed a film on the inside surface and prevented rust from forming due to condensation inside the tubes.

    More recently LPS 3 has replaced linseed oil, but the concept is the same. Fill the tube structure and then drain and you will get good coverage and it won't rust.

    If you are going to weld up a tube structure also make sure that if you get condensation in the tubing it can drain out. If it can't drain then make sure there is a rust preventative coating inside the tubes.

    Some aircraft and some race cars that used tube frame structures have the tubes interconnected by small holes, and the tubing system is sealed and pressurized Usually with dry nitrogen). A pressure gauge is used to monitor the structure. If the pressure drops off you know the structure has cracked. This obviously means that every weld must be pressure tight, but if properly done that is case. What can be done in that case is to pump down the frame with a vacuum pump so that there is no air (and moisture) inside the tubing, and then it is pressurized with dry nitrogen and you are pretty much good for life.
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  13. parkland
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    parkland Senior Member

    Aren't aircraft aluminum lol?

    The tubing in question is part of a vehicle and welded at both ends, and not water tight either, so it can't be filled with oil.

    In fact it is a steel side by side vehicle I welded together. sort of a go-kart, but bigger, and using toyota 4x4 axles and parts. 48" wide, and about 10 ft long.
    Yanmar clone air cooled diesel for power, and the goal wasn't to have a racing machine that could jump banks and tear through mud holes, it was to have an economical toy to drive around in, cheap on fuel, and top out at 60 km/h or so.
    It's belt clutch driven to a jackshaft, reduced 5:8, then 5:7 with 60H chain to the 5 speed transmission. It should hit 46 km/h in 4th gear at 3800 RPM.

    You see a lot of steel boats with the 2" or so tubing used as the edge bumper along the top of the hull, I was kind of wondering how that is rust proofed.
    I looked on a boat, and it doesn't appear to have any cuts or anywhere that could be for paint or anything.
  14. SamSam
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    SamSam Senior Member

    Sounds like it's going to rust. "Not water tight" because of bad welds?

  15. parkland
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    parkland Senior Member

    I'd say bad welds, and also some bars are only welded on 3 sides because theres no room to weld the 4th, because of the angle.
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