Preventing rust inside steel pipe or tubing?

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

  1. bpw
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    bpw Senior Member

    Steel tanks then have been foamed in place (not uncommon on older yachts) are notorious for rusting out where the foam touches the tank.

    Seems the foam is never quite water tight enough to keep all the water out, but it does keep the moisture in against the tanks.
     
  2. parkland
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    parkland Senior Member


    This is also what I think.
    Oil prevents rust by coating it and not allowing oxygen to penetrate the metal right, so closed cell foam should also do that, although a different way.

    Adding chemicals or other products just introduces a way for the foam to fail.
     
  3. parkland
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    parkland Senior Member

    That sounds like open cell foam to me.
    I'm no expert by any stretch, but open cell foam could do that.
    Closed cell expanding foam should make a tight seal against where it's applied, and the surface underneath it should be protected unless the foam gets damaged somehow.
     
  4. Poida
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    Poida Senior Member

    I haven't seen it work in that application, but certainly as an anti rust on steel. I would imagine that any anti rust liquid would probably stop the foam from sticking to the inside of the pipe, but, I don't see why it would need too. But since my post it has been revealed that the pipe can't be sealed anyway so any anti rust solution would be washed over a short time.

    Maybe investigate a bitumen product like is used to anti rust cars and see if see how it goes against salt water.

    Poida
     
  5. waikikin
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    waikikin Senior Member

    I think fish is the best, when the seal is breached you got an indicator to repair:eek:

    Jeff.
     
  6. Poida
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    Poida Senior Member

    Well you'd know something's fishy.

    Poida
     
  7. SamSam
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    SamSam Senior Member

    Yes, but you also might go to use the vehicle and find it infested with cats and grizzly bears.
     
  8. Nick.K
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    Nick.K Senior Member

  9. brian eiland
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    brian eiland Senior Member

    I made reference to your contribution here, over in this subject thread discussion of steel tubes to support floating cottages / houseboats
    http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-13.html#post693034
     
  10. jehardiman
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    jehardiman Senior Member

    Sorry to get back to this so late, but I think there are some points that need made.

    To cause corrosion, you need an electrolyte, an electromotive force, and an electron source/sink. Generally, for marine applications, this means seawater and atmospheric oxygen for steel. For other materials, like stainless steel, it is seawater and the material itself, for NIALCu alloys it is ammonia, etc..

    Just like fire fighting (i.e. fuel-heat-oxygen makes a fire), brake the chain and you stop the corrosion. So we prevent corrosion by using noble materials (remove fuel/electron source), use anodes or self-oxidizing materials (remove the heat/electromotive force), or coatings (remove the oxygen/electrolyte).

    The noble materials are the ones with stable electron shells, the anodic/corrosive materials are those that freely give/take electrons. By using materials that are electron stable (i.e. hard to strip/give electrons like Gold or CuNi alloys), you stop corrosion. Other alloys such as most Stainless steels actually "corrode" freely but the "corrosion" oxide formed on the skin is stable (but can be worn off in the right conditions). And everybody understands the use of anodes which basicly move the corrosion to a consumable material.

    That leaves coatings. Coatings are all about two things surface prep and permability.

    In the case of steel, it comes to you two basic ways, hot rolled or cold worked. Hot rolled is where the red hot cast billet is formed into the final shape, either plate, thickwall pipe, or rolled shapes with deep draft/square edges (I-beams, square channel, rolled angle). In this case, the lubricant literaly burns and forms an oxide called mill scale. Mill scale is tightly adherant but will not take paint well and is permeable to water. Cold worked steel on the other hand is made from cold hot rolled plate and is formed by physically deforming the steel. Most light weight steel is made by cold rolling thicker plates into thinner ones, and some shapes without square corners, like thinwall pipes and square tube are made from this. For cold rolled products the mill scale generally falls off and the surface is covered in the rolling lubricant, which may or may not be water permeable.

    So factory direct steel generally does not have any coatings that will prevent corrosion (you can order it with a self-etching wash or other coatings but that is another topic). This leads to the first step of a good coating system, surface prep. You need to get the surface clean and provide a good tooth for the paint to hold. Generally this done by some form of blasting or self-etching primer. Once the surface is prepared correctly, then you have to lay down a coating system that will not be permeable to whatever the surface is going to be exposed to. For this, coal tar epoxies, some marine enamels, and possibly closed cell foams will work; open cell foams and water vapor permeable paints(like latex ones) will have less success, eventally forming rust scale which brakes the bond with the surface and gives a direct water path to the surface.

    So this leads to the issue of what to do with a closed section. In the example I cited, what happened was corrosion did occur, but stopped when all the oxygen in the space was consumed. The example cited by Nick.K, fish holds are gennerally accessable during construction and surface prep can be through because you know you are never going to be able to inspect it again for the life of the vessel. Similarly, when installing anechoic material you {REDACTED} to prevent it from coming off. In the case of a closed rolled section, such a square tubing, you may never get a proper surface prep to keep a good coating system on. In this case, it is just better to seal it totally than to rely on the closed cell foam sticking.
     
  11. Nick.K
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    Nick.K Senior Member

    Interesting post, thanks

    My own boat project has mild steel rub rails and a mild steel bulwark cap tube; the boat was built around the mid eighties and both sections had rusted out in places. I have replaced them completely with similar sections, about three long weeks of grinding and welding to do it all so obviously I have thought quite a lot about how to prevent them rusting out again!
    I haven't done anything yet but my intention was to use a two part foam pumped in at one end until it foams out at a hole made at the other. I am intending to do the insulation myself with this type of foam from something like a 'froth-pak' kit from DowCorning and thought to use the same for the tubes.
    What I observed from cutting off the old tubes was that even though the tubes had been leaking for a long time, most of the tube was uncorroded inside, the corrosion was mostly around places where water was sitting, flowing or dripping from condensate. I think the closed cell foam will prevent corrosion simply by filling the void and preventing air exchange which creates condensation, I don't believe the bond with the surface is that important in this case?
    My guess for the single pack expanding foam is that it may not expand or behave as expected in a tube. I used this type of foam a lot for installing windows, when you cut it, you see that the expansion is not uniform and often has large voids, I think in a tube that the foam may form large bubbles with slugs of denser foam between or it may simply fail to foam.
     
  12. SamSam
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    SamSam Senior Member

    You won't be able to squirt foam in a long section of tubing until it comes out the other end, it will have to be done in short sections.
     
  13. jehardiman
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    jehardiman Senior Member

    Don't forget that water has capillary action, which actually makes a very thin void between the steel and the foam worse than a small pool at the bottom. ANY void between the steel and the coating will wick up water and you will have corrosion there between the foam and the steel. The only limit to corrosion in this case is the availability of free oxygen. Oh, and by the way, the oxygen gradient in the water, caused by the distance from the source leak, actually drives the corrosion faster. Ever notice that steel rusts out at the waterline first? This is because the availability of oxygen nearer the surface actually set up an electromotive potential across the plate.

    FWIW, having worked with sealed steel voids in ships rudders and planes, pitch and pine produced the least corrosion. Because the pitch is flexible and adherent, you don't get wicking like the kind that happens when the syntactic cracks.
     
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  14. Nick.K
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    Nick.K Senior Member

    What would be your preferred treatment for the tube interior given that it is not possible to completely seal the tubes?

    The welding is likley to be porous in places, the tubes were stick welded with 6013, I welded the joints off the boat but the ends were done in place and some of the access was really difficult.
    My thought was that water ingress to the tubes would probably occur due to air exchange from temperature and pressure differences (either causing condensate or water being drawn in directly); filling the tubes would prevent this?

    I have read a bit about corrosion driven by oxygen differences, but would really appreciate learning more!
     

  15. bpw
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    bpw Senior Member


    Closed cell foam "should" be waterproof, but alas reality does not always agree with theory. Remember that even fiberglass hulls eventually water-log, and they are a lot less permeable than closed cell foam.

    Boats shouldn't leak either...so why do we have bilge pumps?
     
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