STEEL HULLS with Composite Superstructure / Topsides

Discussion in 'Boatbuilding' started by brian eiland, Jun 16, 2013.

  1. MikeJohns
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    MikeJohns Senior Member


    Unreliable sources in this case :rolleyes: Metals don't absorb water unless they are manufactured with a porous structure.
     
  2. brian eiland
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    brian eiland Senior Member

    That one got me also. I don't ever recall hearing that steel absorbed water.:rolleyes:
     
  3. pdwiley
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    pdwiley Senior Member

    Sure. It's called 'rust'.

    PDW
     
  4. MikeJohns
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    MikeJohns Senior Member

    The interesting thing about rust is that 2/3 of it comes from the atmosphere (one mole of iron and two moles of oxygen). So if the oxygen can get to it along with moisture it expands with a loose porous structure up to around 6 times the amount of actual corrosion. Chloride ions in sea water really accelerate the process.

    When polyurea for example is used as a coating it doesn't stop the water reaching the steel, it stops the oxygen. Damaged coatings on metals like steel and Al alloy peel because the resulting oxide layer pushes the coating off in the interface where oxygen can get at it. On alloy it can peel at an amazing rate, on steel it's slow and very visible.
     
  5. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    They are tested.

    Please see attachment.
     

    Attached Files:

  6. michaeljc
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    michaeljc Senior Member

    I am going to investigate this further. I am not so convinced that steel will not absorb water ( to a degree)
     
  7. Titirangi

    Titirangi Previous Member

    I studied historical metallurgy for a masters - production of puddling iron (wrought iron) in early European, Middle Eastern white cast steel (wootz or bulat for damascene) and Japan's Tamahagane HC steel plus contemporary steels and heat treatments at QUT and University of Southern Illinois.
    Steel can surface sweat in certain conditions by one side being heated or chilled one side drawing moisture from the atmosphere but none of the ferrous or non ferrous (royal metals) can absorb moisture into its mass.
     
  8. MikeJohns
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    MikeJohns Senior Member

    That chart says nothing about test conditions, has only 2 decimal places for water absorption weight gain at a max time of 7 days ! Full material properties of polyurea are available for long term saturation. Why didn't they put them in the table? You'll get the same result for most plastics. short term. The water absorption will be low at 7 days because the diffusion rates are low but long term they will saturate if they can't dry out again. And it certainly wasn't zero
     
  9. MikeJohns
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    MikeJohns Senior Member

    It's too dense and the atoms are packed tightly together. There's simply no room in the lattice structure for a water molecule, that's what makes metals such great materials.
     
  10. Ace Dragon
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    Ace Dragon Polyurea and Spray Foam

    It figures.

    Their customers are our military and heavy industrial customers, might be a little testing going on. Something tells me the military test some what - they just don't release all their data.
    A small company I know, conocohphillips, has hired a lab to test two different polyurea's for tank immersion linings now. Samples are ready and testing will start later this year. Its around four months to complete test. They will be either applying 1 or 2k psig trying to force liquid in to the coatings. I will at least know if the coatings pass or not.
    Forth and fifth decimal point may not be of interest to them - I may verify or may not.
    Past Projects can be found on web sites of Specialty-Products Inc. and VersaFlex.
     
  11. brian eiland
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    brian eiland Senior Member

  12. brian eiland
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    brian eiland Senior Member

  13. MikeJohns
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    MikeJohns Senior Member


    I'm not sure what you are saying here?

    But you need to understand that Polyurea is Hygroscopic.

    If a material data sheet says it doesn't absorb water its usually taken to mean its water absorption is <.01% by wt in 24 hrs. Or "effectively waterproof".
    But whether it is hygroscopic long term is a completely different issue. Water absorption response to polymers is a significant engineering material property that is well researched.
    People often intuitively feel that because a material is "waterproof" that it cannot absorb water ..... !

    To get a bit more detailed with some factual definable figures from a source you should be happy to accept:

    Polyurea water absorption is relative to ambient humidity the stable water content over time for all grades of Polyurea is as follows:

    in 50% humidity it absorbs 0.8% water by weight
    in 100% humidity ( immersed fresh water) 2.4% water by weight

    I have not searched for further data for sea water immersion, but it usually occurs at the same levels but at accelerated rates since metal salts speed up the process.

    If you have a void in a hygroscopic material coating with even a 1% water absorption then you will get water passing into the void, that can in turn lead to further disbondment of the coating since the water molecules can significantly weaken the coating-substrate bond. That's why it's very important to apply it very carefully. If it's well applied it's good; if it's poorly applied to steel it's not much better long term than putting the metal in a plastic bag with some water.


    The figures I used above are from these sources:
    a Naval Research Laboratory, Code 6120, Washington DC 20375-5342, United States
    b Naval Surface Warfare Center, Research and Technology Dept., Indian Head, MD 20640-5035, United States
     
  14. michaeljc
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    michaeljc Senior Member

    Re water absorption of steel. evidence both ways still inconclusive. Talked to a trade welder about this. He told me of a job where high quality welding of large 12 mm plate was required. They heated it before welding to drive out any water. Water was dripping of the plate as they heated. No - it had not been outdoors. Moral of the story?: heat steel sections and let cool before applying first coat. Disbelieve me if you must :)
     

  15. powerabout
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    powerabout Senior Member

    yes seen the same
    I assume it was just on the surface?
    the test would be to drop a piece of steel in a hot furnace.
     
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