How to tell bronze from brass screws

Discussion in 'Wooden Boat Building and Restoration' started by sdowney717, Oct 24, 2017.

  1. sdowney717
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    sdowney717 Senior Member

    The screws are #12 50 years old. Is there a definitive test. They were used to secure teak decking. Most came out clean. I don't trust them. I have broken old screws off before. I can post pics later. I bought #10 1.25" ss instead which will go into new holes. I can keep them for plank repairs if bronze.
     
  2. rasorinc
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    rasorinc Senior Member

    are you using SS 316 for the new screws?
     
  3. gonzo
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    gonzo Senior Member

    Brasses and bronzes are families of alloys that overlap. Short of doing a material analysis, it would be hard to tell.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Both brass and bronze are copper alloys. Brass has zinc added, while bronze uses tin. Both can have other things added too, though bronze is much more augmented in this regard. Brass tends to be a dull gold color, while brass looks more brownish. Corrosion is the usual "tell", with brass just not up to the marine environment for very long. Brass is pretty soft, while bronze can be more brittle. I'd be very surprised if you have brass, it's just not acceptable except as a coffee cup hook. If the screws have some red on them, they might be too corroded to trust any more.
     
  5. sdowney717
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    sdowney717 Senior Member

    I rescrewed all the bottom planks with bronze screws back in 2006. I could tell those original screws were really quite strong, did not break off in the wood. I predrilled regular holes standard drill for the new screws in new frames.
    The screws in the upper deck area all seem softer, they tended to break when reused, so I quit reusing them. Mainly the toe rail and teak decking seem to not be the same quality as the planking screws. even looking close at them and not noticing anything, they tend to twist off at the thread. And their heads can be a mess of corrosion. Nothing worse than breaking off a screw in the wood when it is holding on a piece of teak..

    For all my upper hull above waterline repairs, I use the typical SS screw you will find at a hardware store. Been using for many years and have not trouble with any. The few I used under the water, yes terrible, became full of pitting and channels and holes.

    I bought 400 #10 SS by 1.25" oval head sheet metal screws to re-screw all the teak.
    They were .09 cents a screw at York Bolt.
    2.5" squared drive deck screw is .17,
    3" is .25,
    2" is .16 cents each.
    Above the water, all these kinds of screws have been fine, I know since I have pulled some out. My boat is in the lower Chesapeake Bay near Seaford, Va.
     
  6. gonzo
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    gonzo Senior Member

    Some brass alloys have tin and other metals in them. For example naval and tonval brasses.
     
  7. PAR
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    PAR Yacht Designer/Builder

    All the brasses have zinc, no tin, while the bronzes have tin and often manganese, silicon, etc. Navel brass is fairly tough, but nothing compaired to the usual bronze alloys. When bronze sees electrolyze, it gets particularly brittle. This is chloride corrosion and typically seen as a green residue that can be scratched off. If it's long term, it'll suck out much of the copper, leaving a red stain in the hole and dramatically weakening the fastener.
     
  8. hoytedow
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    hoytedow Wood Butcher

    Are you sure about that mix? I always thought bronze was copper and zinc and brass was copper and tin. I am open to correction. Now I know I had it backwards. I stand corrected.
     
    Last edited: Oct 25, 2017
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Brass is copper and zinc in varying proportions. "Honey Brass" presumably has a high proportion of copper. If you want something to break, use brass ! :eek:
     
  10. gonzo
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    gonzo Senior Member

    Naval brass has zinc and tin. Tonval brass has zinc and lead. There are many alloys.
     
  11. Blueknarr
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    Blueknarr Senior Member


    Why keep if you don't trust?

    Is there enough of a difference between the specific gravitys of the various alloys for use in identification?
     
  12. gonzo
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    gonzo Senior Member

    If the alloys had only two components, yes. However, they can have several elements in different ratios. A spectrometer can easily determine the alloy.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Yeah, there's lots of formulations for both bronze and brass, of course then there's the most common ones, which is what you see in screws, fasteners and marine use. Phosphorus, manganese, aluminum and silicon are the use additions to tin employed in bronze. Generally, tin is used in higher concentrations if the material needs more rigidity. The same is true of phosphorus. Dezincification is the primary reason you don't use brass in the marine environment. It leaves a very soft, porous material behind that easily breaks.
     
  14. gonzo
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    gonzo Senior Member

    Adding lead helps dezincification, but it is basically banned now.
     

  15. ned L
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    ned L Junior Member

    I would not consider this to be at all definitive. However, I recently heard a recommendation that if you drop the screws on a concrete floor a bronze screw will make a clearer "ringing" sound and a brass screw will make bit of a duller "ring".
    I have both tried this with screws that I know to be brass and bronze, and to sort some that were mixed. It does seem to work.

    This would seem to make sense considering the major component difference (Tin / zinc)
     
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