Reducing poultice corrosion

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Apr 8, 2022.

  1. fallguy
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    fallguy Senior Member

    B7F4CF64-0A3F-471B-A9BE-A9FF89A07009.jpeg I was going to put this boat in freshwater, but she is probably headed to salt.

    I did not anodize or paint these 6x10 aluminum beams. There is an obvious point of saltwater ingress, well four or six actually since 6 beams.

    Any comments are welcome.

    like would it be better to try to come up with a way to keep these dry or better to flush them or definitely coat them, etc.

    I am even wondering about sealing with wax or some silliness..
     
  2. fallguy
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    fallguy Senior Member

    Here is a picture of the forward beam.

    These are catarmaran beams in a demountable cat. image.jpg
     
  3. bajansailor
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    bajansailor Marine Surveyor

    Re how your cat is demountable, will you be taking it to pieces every winter to take her home, or will she remain in one piece once you have trucked the sections to the water (which might now be salt)?
    Are the beams made from marine grade aluminium?
     
  4. SolGato
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    SolGato Senior Member

    I would consider having them hard anodized.

    Second to that, a metal etching primer and a high quality topside paint has worked well for me in the tropics on old extrusions that were once well protected from anodizing.

    If the clamps aren’t aluminum, a hard durable plastic lining material might be considered to create a barrier against electrolysis.
     
  5. fallguy
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    fallguy Senior Member

    I have neoprene between the straps and the beam.

    The beam is marine aluminum, like 6061-t3 maybe. I forget. It is sparcraft s830 from 5 years ago....

    I had no plans to take the cat apart prior to launch. I could probably demount the thing next winter.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Poultice corrosion only occurs when aluminium is in direct contact with a wick-like material..i.e it is either: wet, constantly wet, or absorbs moisture.

    So long as you insulate the bear aluminium against the contact with this "material" - you're ok.
    Such as paint, gaskets etc..
     
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  7. fallguy
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    fallguy Senior Member

    The gaskets are installed on the boat side. So the likelihood of seawater stalling here is high.

    I probably have to tear the boat apart in the fall and treat the beams.
     
  8. SolGato
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    SolGato Senior Member

    Granted this is not directly related to boats and cross beams, but the basic idea still holds true in that not only does one have to worry about electrolysis due to dissimilar metals, but also water immersion, coatings, materials used for isolation, and any gaps or intrusion points where water can get in and create a “wet” environment.

    Pipe Supports: A Nagging Corrosion Problem Solved https://stoprust.com/technical-library-items/28-pipe-support-corrosion-solved/

    I suggested hard anodizing because I’ve seen 40 year old aluminum HobiCat extrusions that have stood up well in our harsh tropical environment having lived on the beach and been put away salt water “wet” their whole life. Sure, the rivets corrode and anywhere fixtures have loosened and rubbed against each other there is corrosion, but it’s amazing overall how well hard anodizing holds up.

    It’s certainly better than any kind of top coating, and the last time I checked, it still wasn’t too expensive of a process.
     
    Last edited: Apr 10, 2022
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Aluminium has its own protective oxide layer... why paint/anodize?
    Just as work and unnecessary expense.
     
  10. SolGato
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    SolGato Senior Member

    That only works if the layer is undisturbed and can heal itself, and if the Ph of anything it comes in contact with doesn’t dissolve the layer.

    Also if the aluminum is of high quality without additional metals added to change its characteristics and properties.

    So you get some salt water and some sand into a crevice that has some movement in a nice damp space, and suddenly you have a party with lots of “white powder” being passed around.

    Why would you take the risk with something as structurally important as beams, or a mast, etc.. on a boat?
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    If the protective oxide layer of aluminium if removed, from a knock, scratch etc...it repairs itself within milliseconds.
    The oxide is an incredibly fast reacting oxidation process.
    So long as water can run off...you're fine.

    Only if the joint design has such places. A good design does not, for very obvious reasons.

    There is no risk if one understand the mechanisms at play and mitigation.

    There are hundreds if not thousands of boats running all over the world, with bare unpainted aluminium.
    Risk is only risk, if you don't understand what you're doing.
     
  12. SolGato
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    SolGato Senior Member

    Well, you made a general statement about not protecting aluminum because you don’t need to, and then followed it up with a bunch of “ifs”.

    My statements are specific to FallGuys situation.

    He has unprotected aluminum with neoprene gaskets in an area where salt water might collect.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh.. you mean like this:

    So a 'might' trumps an 'if' ??
    Most odd.

    Your example is also incorrect, but decided at the time not to query it.
    But now I will.

    The clue is underlined in bold.
    That is not poultice corrosion..it is crevice corrosion - totally different mechanism.

    And "if's" are there to educate those making or attempting to make an informed decision....that they can select one way or another...and provide information to the decisions that lay ahead.
    If you make a wing out of steel..the plane wont fly...if you make the hull out of chocolate it will melt... etc.

    There is no contradiction - except those wishing to play semantics for cheap points.

    Aluminium - unprotected - does very well on its own.
    As noted there are examples the world over.. if one wishes to look beyond their sphere.
     
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  14. fallguy
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    fallguy Senior Member

    As much as I like the idea that doing nothing is good, I have to quantify and understand a few things better.

    poultice/crevice were the same for me, I assumed a spongy, salty blob of stuff in a crack or crevice---apparently, this is incorrect

    oxidation layer thickness

    from my understanding, aluminum oxide forms within seconds when raw aluminum is exposed to oxygen which is present in air; the oxide layer is some thickness X

    forcing the oxidation layer to be thicker is what happens when anodizing is done, this makes the layer thicker

    this thicker layer is only helpful within reason

    a hard, waterproof paint finish provides more protection against saltwater

    the paint I used for topcoat on my boat blistered under full time immersion...another builder warned me it would; the immersion was an accident, but proved him right

    the ends of the beams are open, so fluid can easily get inside and sit there while the water evaporates and the salt concentration increases---I could probably foam the ends closed, but there are hardware locations where stuff can get in; sealing them perfectly is likely impossible...perhaps foam in the ends and provide weep holes

    Anyhow, keep the contributions coming my way if possible.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Ok..some simple basis of ally corrosion.

    For corrosion to occur, it needs an electrolyte, without which corrosion does not occur, and finally it needs a current a flow of electrons, via the electrolyte.
    Aluminium has a very well protected oxide layer and makes the metal 'passive'. This layer is around 5-10nm thick. Doesn't sound much, but it is plenty.
    The oxide layer almost spontaneously occurs, since bare aluminium once exposed to the atmosphere forms an oxide layer within milliseconds.
    Thus if left as bare ally with its oxide layer - it protects itself.

    Anodizing is ostensibly for looks/style/aesthetics, as all it is doing is just adding a light thickness to the oxide - nowt else.
    So to anodize or not, is a subjective choice for the owner, but it adds little in the whole scheme of things to the parent metal.

    Ok..so painting...well, if that paint is scratched, and there is the presence of dissimilar metals, then you actually make the situation worse, as you can get very localised deep corrosion, owing to the ratio of areas being exposed.
    Thus, painting is not a panacea that many seem to think, for aluminium. Hence leaving it alone, is just as good. Since once scratched/scored, the oxide layer forms almost immediately - thus self protecting again.

    The killer for aluminium to break the passive layer is the acidity or alkalinity of the electrolyte.
    If you look at the classic Pourbaix diagram for aluminium you get this:

    upload_2022-4-11_12-47-1.png

    At very low potentials the immunity regions of Al can be seen, meaning that aluminium will not react.
    At low pH values aluminium is not stable in aqueous solution and the formation of Al-ions will take place, also at high pH values aluminate anions are formed.
    However, between pH 4 and 8,5 the oxide layer is stable.

    So back to the point.

    If you have a joint, of whatever means, where stagnant water may collect, this leads to a situation known as crevice corrosion, below is a classic example of this:
    upload_2022-4-11_12-50-13.png

    Any small region that has volume where sea water can collect is idea for this type of corrosion. Simply that stagnant water collects and the H2O slowly evaporates leaving a very high alkaline solution as the salts become more concentrated in the remaining solution. This high Ph alkaline solution, as noted in the Pourbaix diagram is not good for ally.

    Poultice corrosion, however, only occurs when bare ally is in direct contact with a wet/moist material, like wood, rope etc.
    The presence of the constant moisture sets up the conditions for Poultice corrosion to occur, nothing to do with a crevice.
    An example is shown below:
    upload_2022-4-11_12-55-3.png

    A foam fender was glued in direct contact with the bare aluminium ship side, over a period of time the results became clear, the fender slowly peeled away from the side. The foam, over time, slowly
    absorbed the surround sea water which had no protection between the fender and the aluminium. So what occurred was that the hydroxide by-product slowly forced, or separated, the fender from the ships side owing to its
    volume, leaving the mess shown above.

    So, like most boats, IF (there is that word again) if you apply good maintenance practice, like washing down your regions of where there are joints that 'could' collect sea water, with fresh water after each use, you'll be fine.
    If you don't do this, then anywhere on your boat that can collect water, may well be subjected to crevice corrosion of time.

    Other than the joints, which need attention and general inspection/maintenance, leaving the ally as it is with its own oxide layer is the best protection it can have!
    If it aint broke... don't fix it!
     
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