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  1. #1
    Join Date
    Oct 2004
    Location
    Thailand
    Posts
    2,205

    Default Why is my shaft pitted here (see pic)?

    Removed the shaft the other day and upon inspection it has some pitting in one area. It's the part that fits through the stuffing box (now to be replaced with a dripless seal) and the pits are quite severe. I'm guessing it is electrolysis but wondered why only this part. The part outside the boat in the water, and the part at the flexible coupling, are fine. The shaft is 10 years old, has done many hundreds of engine hours, but the stuffing box was diligently greased regularly. Needless to say we're replacing the shaft but I wondered if anyone has any clues as to why it's happened only in the stuffing box.

    Latest video: http://bit.ly/ftb_106

  2. #2
    Join Date
    Jun 2008
    Location
    Northwest
    Posts
    1,978

    Default

    That is anaerobic pitting. For stainless to be stainless it has to have a constant supply of oxygen. When the shaft sits idle, the tiny sliver of water between the cutlass bearing material and the shaft quickly become denuded of oxygen and the shaft corrodes.

    Interestingly this is almost only seen in tropical (very warm) waters. I have seen a hole appear in the tubing of a boarding ladder in a matter of weeks where a barnacle has become attached. It drilled a clean hole straight through.

    With you in Thailand it's also a severe problem with the stainless steel expansion bolts that have been placed on the sea cliffs for belays for all the rock climbers. The anaerobic atmosphere in the 'drilled hole' quickly reduces the stainless steel threaded bolt and expansion cone to nothing.

    It's much less common in temperate waters but I have seen an extreme case on the backing plate in the structural box for the upper chainplates in a Vancouver 34, but she had spent her life cruising the Med.

    The solution with the propshaft is to make sure the engine/propshaft is run regularly.

  3. #3
    Join Date
    May 2005
    Location
    up on the moors.
    Posts
    33,290

    Default

    local corrosion and pitting probably initiated by particles in the stuffing box area. The corrosion may extend more deeply into the metal than the surface pits seen, weakening the shaft and making it vulnerable to shear failure. Some kind of insidious ion/anion interaction.

    What is the material of the stuffing box and liner ? Is there a linear weld in it ? (pits seem to be in a line)

    Solution - VicS and Vyv Cox, I suggest.

    And Tim Bennet
    I think, therefore I am. I am, therefore I sail.

  4. #4
    Join Date
    Oct 2004
    Location
    Thailand
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    2,205

    Default

    Quote Originally Posted by TimBennet View Post
    That is anaerobic pitting. For stainless to be stainless it has to have a constant supply of oxygen. When the shaft sits idle, the tiny sliver of water between the cutlass bearing material and the shaft quickly become denuded of oxygen and the shaft corrodes.

    Interestingly this is almost only seen in tropical (very warm) waters. I have seen a hole appear in the tubing of a boarding ladder in a matter of weeks where a barnacle has become attached. It drilled a clean hole straight through.

    With you in Thailand it's also a severe problem with the stainless steel expansion bolts that have been placed on the sea cliffs for belays for all the rock climbers. The anaerobic atmosphere in the 'drilled hole' quickly reduces the stainless steel threaded bolt and expansion cone to nothing.

    It's much less common in temperate waters but I have seen an extreme case on the backing plate in the structural box for the upper chainplates in a Vancouver 34, but she had spent her life cruising the Med.

    The solution with the propshaft is to make sure the engine/propshaft is run regularly.
    Great answer, thanks Tim. We sat idle for three years in India, only turning the engine occasionally and rarely putting it in gear (maybe once every three months for thirty seconds). Lesson learnt.
    Latest video: http://bit.ly/ftb_106

  5. #5
    Join Date
    Oct 2004
    Location
    Thailand
    Posts
    2,205

    Default

    Quote Originally Posted by sarabande View Post

    Solution - VicS and Vyv Cox, I suggest.

    And Tim Bennet
    Latest video: http://bit.ly/ftb_106

  6. #6
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
    Posts
    21,852

    Default

    Not quite. Stainless steels most definitely do not need a constant supply of oxygen, the alloys are used in all sorts of environments that contain no oxygen whatsoever, eg natural gas, a whole range of chemical manufacturing, etc. Pitting and crevice corrosion occur by the same mechanism, explained in detail on my website under 'metallurgy'.

    Briefly, stainless steels depend for corrosion resistance on a film of chromium oxide that forms very rapidly in air and is extremely stable. Thus the extraction of chromium from its naturally occurring oxide requires a huge input of energy. This film can be ruptured quite easily in the form of micro cracks and grain boundaries, which then allows corrosive media to attack the metal below. The galvanic potential of the 'active' metal below and the 'passive' film above differs, with the result that a corrosion cell develops, attacking the active metal. Thus pits just keep going once they have started. In the case of the shaft beneath propellers, cutless bearings, rope cutters, fouling, the cause is crevice corrosion but ultimately it is all pitting. Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
    Answers to some technical queries at new website http://coxeng.co.uk

  7. #7
    Join Date
    Oct 2004
    Location
    Thailand
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    2,205

    Default

    Quote Originally Posted by vyv_cox View Post
    Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
    I hadn't seen this page before, Vyv. Very informative and almost encouraging. Not that you need them but feel free to use any of my images for your records (PM me for the originals).
    Latest video: http://bit.ly/ftb_106

  8. #8
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
    Posts
    21,852

    Default

    Quote Originally Posted by demonboy View Post
    I hadn't seen this page before, Vyv. Very informative and almost encouraging. Not that you need them but feel free to use any of my images for your records (PM me for the originals).
    Thanks, I will do that when I return home, end October-ish.
    Answers to some technical queries at new website http://coxeng.co.uk

  9. #9
    Join Date
    Aug 2013
    Location
    SW Leicestershire
    Posts
    1,107

    Default

    Quote Originally Posted by vyv_cox View Post
    Not quite. Stainless steels most definitely do not need a constant supply of oxygen, the alloys are used in all sorts of environments that contain no oxygen whatsoever, eg natural gas, a whole range of chemical manufacturing, etc. Pitting and crevice corrosion occur by the same mechanism, explained in detail on my website under 'metallurgy'

    Briefly, stainless steels depend for corrosion resistance on a film of chromium oxide that forms very rapidly in air and is extremely stable. Thus the extraction of chromium from its naturally occurring oxide requires a huge input of energy. This film can be ruptured quite easily in the form of micro cracks and grain boundaries, which then allows corrosive media to attack the metal below. The galvanic potential of the 'active' metal below and the 'passive' film above differs, with the result that a corrosion cell develops, attacking the active metal. Thus pits just keep going once they have started. In the case of the shaft beneath propellers, cutless bearings, rope cutters, fouling, the cause is crevice corrosion but ultimately it is all pitting. Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
    Bit confused by this; referring to one of the photos on your website where possible cause of corrosion is "spash of antifouling"; wouldn't that be lack of oxygen? Spent a very unpleasant hour yesterday snorkelling in the River Itchen trying to scrape barnacle encrustation of the prop; unable to make much impact on the shaft which was also covered with them. If in same location next year will need to antifoul both prop and shaft - will that likely cause corrosion?

  10. #10
    Join Date
    May 2001
    Location
    North Wales, sailing Aegean Sea or Menai Strait
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    21,852

    Default

    The example on the website is a fairly unusual one in that the shaft is on a Jeanneau, who choose to use a 400 series stainless steel for their shafts. This lends it more susceptible to crevice corrosion. My 300 series shaft has had its fair share of barnacle encrustation but no sign of crevice corrosion.

    I recommend Velox antifouling for your shaft. This stuff is brilliant, not cheap but it really does do the job. We have lots of tube worm growth in the Med but since using Velox on my prop, P-bracket and parts of the shaft as a trial, none at all.
    Answers to some technical queries at new website http://coxeng.co.uk

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