Aluminum scrupper

Discussion in 'Materials' started by karthikna313, Feb 5, 2015.

  1. karthikna313
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    karthikna313 Junior Member

    Dear All,

    I am decided to use brass bolts in my Aluminum deck drain ( less corrosive) which is used in super structure....Can any of you please suggest me how and were to get details regarding Brass bolts ie, bolt grade, yield strength etc.,,,:?::?: awaiting for your kind replies.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Why would you use brass, let alone with aluminum? Brass is a lousy metal in the marine environment and coupled with aluminum which is easily 50% as noble, you're going to have huge corrosion issues. Engineeringtoolbox.com will provide the information you need, but without a clear understanding of what you're fooling with, no data in the world is going to help. Maybe you're confused with another copper alloy, but understanding the realities of the materials involved, would be a good start.
     
  3. gonzo
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    gonzo Senior Member

    That is a terrible combination. They make aluminum bolts and rivets.
     
  4. JSL
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    JSL Senior Member

    Any copper alloy on aluminum is not a good thing
     
  5. karthikna313
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    karthikna313 Junior Member

    Actually I will be providing Teflon to avoid contact with brass and aluminum....
    @ Gonao My requirement is more so I cant afford Aluminum bolts and rivets.....Thats why I am moving to brass bolts since it is less corrosive.
     
  6. PAR
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    PAR Yacht Designer/Builder

    If this is a client's boat, they should hire someone that knows what's going to happen with these combinations. If it's your boat, well the galvanic scale is easy enough to decipher, so have a look and pick a metal you can afford that's not going to get eaten up. Using a budget to justify a lousy material choice, can't be qualified.
     
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  7. gonzo
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    gonzo Senior Member

    It is surprising that brass bolts are cheaper than aluminum rivets. Can you post a photo of the drain? Can you weld the drains?
     
  8. karthikna313
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    karthikna313 Junior Member

    ass1.PNG

    ass2.PNG

    ass3.PNG


    These are snaps @ Gonzo. Pls suggest me brass bolt grades I was not able to find in engineeringtoolbox.com
     
  9. gonzo
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    gonzo Senior Member

    It probably doesn't show because it is not a recommended alloy. Silicon bronze would be the preferred copper alloy. However, stainless steel is cheaper and easier to get. Can you get aluminum rod? Making threads in aluminum is not too hard.
     
  10. michael pierzga
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    michael pierzga Senior Member

    Gee.. Combining .brass and aluminium is the start of a new cold war ! Dont do it !

    Use aluminum fasteners, plastic fasteners, steel fasteners or stainless steel fasteners.

    Isolation strategies are complex and soon fail when the paint system breaks down and seawater enters the joint.

    Do whatever you can to avoid using dissimilar metals .
     
  11. michael pierzga
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    michael pierzga Senior Member


    I can purchase a good selection of aluminium fasteners off the shelf at my local hardware store. It shouldn't be difficult to source the correct fasteners. Plastic is also an option. Plastic plus structural bonding caulk such as 5200 is a common method .
     
  12. michael pierzga
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    michael pierzga Senior Member

    I see teflon on your drawing. Be careful. Teflon swells with temp and moisture. Choose a plastic that is more stable so that it may be machined to tighter tolerances.

    When a material swells it breaks down any protective paint film and if used as a bearing it will sieze up.
     
  13. PAR
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    PAR Yacht Designer/Builder

    The spec's are in Engineerstoolbox, I use the site regularly. The question (again) is why in the hell would anyone want common brass on a boat, particularly against and/or in close proximity to a considerably less noble alloy.

    As to finding screws, brass or other wise, there are literally hundreds of places to buy various brass alloy screws (C-230, C-260, C-385, C-790 are most common), but prices can vary, but a 1" long #8 flathead wood screw is about 15 cents apiece in a box of 100. A 316 stainless screw of the same dimensions will be about 18 cents per box of 100. A 18-8 stainless screw will be less then a brass screw of the same size. A silicon bronze screw will be about 29 cents per box of 100, so I don't see the issues you're having.
     

  14. Kevin Morin
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    Kevin Morin Junior Member

    Galvanic Vs Crevice Corrosion?

    karthikna313, your section design only addresses metal-to-metal isolation so it will potentially reduce, but not avoid, galvanic corrosion between the two metals. As has been mentioned brass or bronze are extremely poor choices since a wetting of the isolation interface edge will bridge the two metals and galvanic corrosion will begin.

    More to the design's inherent flaws; the plastic (poorly chosen as Teflon is very malleable and will deform and loosed the flange in time) will help to reduce joint galvanic corrosion but in so doing it will create the very basis for crevice corrosion of the unsealed moisture retaining joints.

    When water is allowed to form a thin film and exposed on that film face to aluminum (even marine alloys) it will become acidic by de-aeration. The cycle begins as the water's entrained O2 is 'robbed' or taken up by the aluminum oxide reforming from being disturbed during fastener make up.

    As soon as the aluminum oxide is 'set' or formed, the water has become slightly acidic and there is no 'spare' or replacement oxygen from the atmosphere due to the thin film seal. So the now ph shift allows the water to begin to re-attract the oxygen and cycle of crevice corrosion begins; renewed by as little as ambient humidity or deck wash when afloat.

    It would be the least corrosive of all combinations to use bare aluminum and slightly over-bored holes in the flanges secured with hot dipped galvanized fasteners. The zinc coating is the least galvanic differential, has the least smooth surface to form and retain films and over bored holes will drain more than form fitting plastic sleeves.

    If the threads are not dipped after rolling on the bolts then they will rust, but plain old galvanized bolts are low cost, and have low corrosive potential. SS bolts, all alloys, must be passivated to use directly if the joint will be wetted and it seems that a scupper will be constantly wet?

    SS itself will begin crevice corrosion if sleeved tightly with plastic and subject to regular wetting to keep the water film acidic.

    Another point of view for your consideration.

    Cheers,
    Kevin Morin
    Kenai, AK
     
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