Stantion base deck corrosion

Discussion in 'Metal Boat Building' started by Steelboat, Feb 11, 2022.

  1. Steelboat
    Joined: Feb 2022
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    Steelboat Junior Member

    My first post here on the forum.

    I am trying to cure a long term maintenance issue, the deck under my stantion bases keep developing rust patches despite the best coating process. I think the coating is failing from stresses on the stantions flexing the deck slightly. Rust bleeds develop about two years every time :(

    I am looking at cutting the stantions off with an abrasive wheel, and adding a stainless plate underneath. I am not sure about how to weld the plate down other than TIG the perimeter. Perhaps some plug welds into the deck with 309l rod?
     
  2. Steelboat
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    Steelboat Junior Member


    Here is my proposed solution... any thoughts on plug welds to make the plate more rigid?
     
  3. gonzo
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    gonzo Senior Member

    What is your coating process? If the rust is localized to the weld, it may be possible to solve the issue by adding a stiffener.
     
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  4. fallguy
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    fallguy Senior Member

    I have a feeling the rust is caused by the original welding process and not stanchion stresses. If it were stresses; those would be localized to a few only.

    I am almost certain that when you weld the base down; there is a change in the deck steel and that this will not change by spending a bunch of money on a new thing to weld on which has the same effect on the deck below.

    I apologize for casting doubt on your plan without a great recipe.

    I recommend asking over on finishing.com or waiting to hear from some steel guys on the forum.

    My gut hunch is you need to modify your post weld coating process to include some type of means to remove free iron from the weld zone like pickling or acids treatments, but I am not an expert.

    You may want to share the 'best coating process' used here or over on finishing.com. The forum there is a bit clunky, but the responses are gold.
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum Steelboat.
    Thanks for posting your CAD drawings.
    Re the first one, is this showing the current arrangement, or is the plate with the hole in it an addition?
    That plate would probably work more effectively if it was aligned at 90 degrees (approximately) to the hull centreline rather than fore and aft, as it would then offer more resistance to bending when somebody grabs it when (eg) climbing up from a dinghy.
    Can you post some photos of your current set up please?
    Re cutting the stanchion off and adding a S/S plate underneath - this can only help to make the whole arrangement a bit stiffer, which is good.
    You would have to continuously weld the S/S base plate to the deck, all around the perimeter of the base plate, otherwise it will rust underneath.
    Re how you have a curved edge on the S/S plate, unless you have the equipment to do this, and it is relatively easy to do, it would be easier and almost as effective to just have a simple flat plate base.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Agreed.
    The bkt needs to be in-plane with the applied load:

    upload_2022-2-12_9-40-28.png

    You can make this by using a sectional piece of the channel and simply cut at 45 degrees.

    If you do as you're suggesting, then you have an even worse situation awaiting you.
    Water will seep through the fillet weld, it'll be stagnant and be exposed to the bare metals or any small scratch in coating/paint and work very quickly to corrode under, hidden away from view:

    upload_2022-2-12_9-42-53.png

    So when you grab the stanchion.. it could be a whole lot worse - it may just simply break free.

    Add a simple bkt..that'll do.
     
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  7. Steelboat
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    Steelboat Junior Member

    Yes the plates with holes are original construction (1993). They are mostly used to lash things down. I considered an athwartships brace, but it would be a terrible toe ripper (x 12). Any design ideas?

    I should note that the system is already very stiff by any yacht standards. Welded rails go bow to stern, top rail is 32mm 316 ss, lower rails 19mm. It forms an entirely welded structure.
     

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  8. Ad Hoc
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    Ad Hoc Naval Architect

    Then you should really be using 316L stainless.
     
  9. Steelboat
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    Steelboat Junior Member

    I should have noted earlier the decks and hull are all mild steel. Type 316 and 304 stainless items are welded to the deck- bollards, cleats, hatch coamings, windlass doubler plate, chain stopper doubler, etc. All used 309l filler rod and either TIG or MMA. I have not seen this corrosion problem in those places, so it seems unlikely that the welding process resulted in coating failure on the stantions.
     
  10. Steelboat
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    Steelboat Junior Member

    Thanks Ad Hoc for the drawings and notes. Did you mean 316 filler rod for the welds? We have been using 309l for all the mild steel to stainless joins.

    Is this what you are thinking, or brace both sides with a channel?
     

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  11. Ad Hoc
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    Ad Hoc Naval Architect

    Prior to your pother post, I was referring to the S/S grade... that being 316L.

    The L for the lower carbon content to prevent carbide precipitation.
    As you weld that region the temp of the metal will go through the range of 550-850c, this is where chromium carbide is produced at the grain boundaries.
    In a nut shell the chromium is drawn away from the weld HAZ and leaves regions lacking in chromium which leaves region of low corrosion resistance.

    Yes, something like that. But if you can, align the webs of the stanchion and bkt. for better load transfer.
     
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  12. Steelboat
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    Steelboat Junior Member

    Oh yes ok. When we buy ss plate stock and tubes it is almost always marked "type 316/316l" Makes me wonder what it exactly is! I guess the best would be to buy from a shop that carries traceable mill certifications. The real world is that most all stainless is now made in China, and we work with what we get.

    I have been treating the welds and HAZ with phosphoric acid paste to help that issue. Now I have a specialty paste with hydroflouric acid in it, leaves everything very silver and pretty.
     
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  13. Steelboat
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    Steelboat Junior Member

    latest revision. I did an ultrasound of the corroded area, and have lost about 40% of the base metal in places. Rather than an insert plate, I think will go with a doubler plate, using no slot welds because of the extra thick doubler. I plan to cover the corroded areas plus 8mm (5/16"). I'm sure I will hate the tearing of my toes every night, but no other idea to solve this!
     

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  14. bajansailor
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    bajansailor Marine Surveyor

    Re bashing your toes, would it be less painful if you welded on a section of tube to act as a gusset, rather than a piece of plate?

    How many of the stanchions are severely (as in 40% loss of base thickness) corroded underneath?
    Are you planning on cutting all of them off and re-welding them?
    If so, would it be possible to move them inboards a bit, and have the gusset stiffener on the outboard side instead?
    And then you would not have to worry so much about bashing your toes on them?
    I note that they are welded rails all around the boat, hence the rails at the bow and stern would need to be modified a bit if all of the stanchions were moved inboard a bit.
     
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  15. Steelboat
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    Steelboat Junior Member

    Cannot move inboard, would be an epic job. I did not ultrasound everything, but yeah corrosion.

    I did think about the stantions with a wall thickness of 3mm and base plate thickness 6mm. A steeper brace angle make more sense? I revised, a little less trip hazard.
     

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