Propeller Nut - What happened to this Nut

Discussion in 'Materials' started by alby joy, May 26, 2020.

  1. alby joy
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    alby joy Junior Member

    MicrosoftTeams-image(1).jpg This is an Image of M30 Nut which is used as propeller Nut - Material as per the OEM is SS316L. The vessel was in the water and recently when we tried to check the propeller- This observation was made. Anyone have a comment - How this can happen?
     
  2. gonzo
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    gonzo Senior Member

    That is corrosion caused by electrolysis. What kind of sacrificial anodes do you have?
     
  3. alby joy
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    alby joy Junior Member

    Actually it is one of the sister vessel in the 2 nos of vessel - We have observed this condition in one side of the catamaran in a single boat. We had anodes in one of the vessel which is placed in the A-brackets. As the material is SS316 L - Do it have chance to corrode like this as they are top in electrochemical series.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Matters little if there is or are other metals in close proximity. It is the electrical potential between that bolt and something nearby that has set up the simple circuit for the bolt to corrode.
    And whether the SS is active or passive too...
    Thus sending a picture of the arrangement and listing the material each is made from ... shall assist.
     
  5. gonzo
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    gonzo Senior Member

    The electrical potential can also be caused by a leak on the electrical system on the boat or the shore power.
     
  6. alby joy
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    alby joy Junior Member


    Hi,
    Please find the material list and sectional view of the arrangement
     

    Attached Files:

  7. alby joy
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    alby joy Junior Member

    Hi
    Presently the boat have no electrical installations in it. The grounding plate in the boat also has no connection to the shore power
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    The sea water provides the source of the "potential" as the seawater is the electrolyte that carries the current.

    You may have some suspect stainless steel.

    upload_2020-5-26_16-45-3.png

    If the 316, is not 316.. but more 'common', something like 304, then the potential current set up suggests that it, being the anode, that will corrode first.
    Naval Brass is roughly in the typical range for most Manganese Bronzes.

    The potential of roughly 200 mV difference between the 2 metals is more than enough to initial corrosion.

    If the bolts are 316 and with certs to prove it - something else is going on....
     
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  9. baeckmo
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    baeckmo Hydrodynamics

    ...and the cap around the arrangement is causing a closed volume, which will create an oxygen-depleted environment. This means that you will have SS parts with different states of passivity due to a varying reduction of the protective layers. You may even have differing potentials between two ends of the same component. Check qualities as advised by Ad Hoc, check possible leak currents from shore systems, and take off the caps! Generally speaking, austenitic steels work fine as long as there is a free circulation of sea-water around it, but may see trouble in a closed volume.
     
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  10. alby joy
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    alby joy Junior Member

    The propeller cone nut was provided to protect the Propeller nut which keep the propellers to the shaft. Now it is suggested not to use Propeller cap & just put it open. As it is a boat build under a classification society they suggest keeping the cap for the protection. Additional to this I will do the chemical analysis of the nut and will find out the grade of the nut.
     
  11. bajansailor
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    bajansailor Marine Surveyor

    Instead of having the 'cap' over the nut to 'protect' it, would it be feasible to drill a hole through the nut and shaft such that a split pin can be used to ensure that the nut does not come off the shaft accidentally?
     
  12. Barry
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    Barry Senior Member

    A quick magnetic check will tell you if the nut is 304.
    GENERALLY 304 is magnetic and 316 is not.

    The ( magnetic attraction) strength is not as strong as a carbon steel nut though, so you may have to balance the nut vertically on a non-magnetic surface, to see if your magnet will influence the nut
     
    Last edited: May 27, 2020
  13. jehardiman
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    jehardiman Senior Member

    That type of corrosion, where the material is consumed from the inside out (Intergranular corrosion) is common across all stainless steels and like alloys (i.e. austenitic, ferritic, and martensitic Fe-Ni alloys) usually in an oxygen depleted location. Though the corrosion may start as crevice corrosion, the corrosion is self perpetuating once started. It is caused by the material becoming sensitized due to improper heat treating, that is the material at grain boundaries become anodic to the grain core due to alloy solubility not forming the proper interstitial. Material that is in contact with another cathode will stop corroding when the anodic material is consumed, which is why it hollows out.
    If the nut actually is 316l, it was either not heat treated after machining (which is what makes it non-magnetic), or has been re-heated at some later date. FWIW, it really doesn't take long for that type of damage to occur, only a week or so.
     
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  14. jehardiman
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    jehardiman Senior Member

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  15. David Jones
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    David Jones Junior Member

    This is not correct. Both 304 and 316 are austenitic stainless steel alloys and both can be magnetic or not. In the annealed condition, neither typically exhibit much magnetism, both in the cold worked condition will. This is a common misconception that seems to propagate extensively... It is wrong.

    dj
     
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