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Thread: electrolysis?

  1. #1
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    Default electrolysis?

    Last spring I pulled the shaft and prop on a clients boat to replace the cutless bearing.
    While the shaft was out I polished it and the prop up nice before re installation.
    I scuffed up the shaft good before I installed two zincs.

    Saturday I was at the yard and thenprop is ashy white, like cured concrete. The shaft not so much but not bronzey looking. The owner could not recall what was left if anything of the zincs when she was hauled, but they are not there now.

    Thoughts?

    Thanks.

  2. #2
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    Default Re: electrolysis?

    The white material could be calcium carbonate. If you put some acid on it CaCO3 would foam. (vinegar or Ospho, muriatic if you rinse it off right away). I have seen this on some parts, but I don't know what they had for anodes.

    The zincs should have left a mark if they were removed after it was taken out of the water.

    https://pubs.rsc.org/en/content/arti.../ra/c7ra03709k

    pdf of A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater (Chris, higher voltage generated hydrogen gas)
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  3. #3
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    Default Re: electrolysis?

    Quote Originally Posted by MN Dave View Post
    The white material could be calcium carbonate. If you put some acid on it CaCO3 would foam. (vinegar or Ospho, muriatic if you rinse it off right away). I have seen this on some parts, but I don't know what they had for anodes.

    The zincs should have left a mark if they were removed after it was taken out of the water.

    https://pubs.rsc.org/en/content/arti.../ra/c7ra03709k

    pdf of A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater (Chris, higher voltage generated hydrogen gas)
    Yes, I think that the deposit might be a sign of over protection. I am wondering whether there is a stray current involved raising the potential between the prop and some source of direct current?
    It really is quite difficult to build an ugly wooden boat.

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  4. #4
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    Default Re: electrolysis?

    I can see where the zincs were, but only because I Scotchbrited the shaft for good contact.
    The marina she lives in is full of powerboats that are always hooked up to shorepower when at the dock, so the possibility of stray current is pretty high.

    Over protection? Thats a new one for me. Guess its time for some homework.

    Ill splash some white vinegar on the prop and see what happens.

    Thanks.

  5. #5
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    Default Re: electrolysis?

    Quote Originally Posted by willin woodworks View Post
    I can see where the zincs were, but only because I Scotchbrited the shaft for good contact.
    The marina she lives in is full of powerboats that are always hooked up to shorepower when at the dock, so the possibility of stray current is pretty high.

    Over protection? Thats a new one for me. Guess its time for some homework.

    Ill splash some white vinegar on the prop and see what happens.

    Thanks.
    http://www.logixgroup.com.au/product...-Explained.pdf

    It is as if there is an unintentional impressed current system created in addition to the zincs.
    It really is quite difficult to build an ugly wooden boat.

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  6. #6
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    Default Re: electrolysis?

    Quote Originally Posted by Peerie Maa View Post
    http://www.logixgroup.com.au/product...-Explained.pdf

    It is as if there is an unintentional impressed current system created in addition to the zincs.
    That is a better link than the ones I used. It explains the consequences of over protection; paint failure, increased fouling and destruction of wood around metal parts.

    The carbonate film is usually there, but too thin to see when the anodes are sized properly. It is possible that if there is a stray current problem, it isn't always polarized the same way. If the polarity reverses periodically as different boats come and go, the anodes could have been dissolved earlier and the carbonate could have been built up just before the boat was hauled. Worst of both worlds, periods of stray current corrosion and periods of over protection. If you look at boats from the same place periodically, you might see different things going on at different times.
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  7. #7
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    Default Re: electrolysis?

    Chris McMullen of the McMullen Wing Co. in NZ wrote a very in depth post concerning this problem some six months ago. Both Cris and I are against the use of zinc plates to a hull that is neutral in its iconic potential. The usual result of over zincing is wood cooking in the area of the zinc attachment. In some places this has proven to be disastrous in that it has made replacement of major keel or rudder components necessary.
    Jay

  8. #8
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    Default Re: electrolysis?

    This a wood boat?

  9. #9

    Default Re: electrolysis?

    Quote Originally Posted by MN Dave View Post
    The white material could be calcium carbonate. If you put some acid on it CaCO3 would foam. (vinegar or Ospho, muriatic if you rinse it off right away). I have seen this on some parts, but I don't know what they had for anodes.

    The zincs should have left a mark if they were removed after it was taken out of the water.

    https://pubs.rsc.org/en/content/arti.../ra/c7ra03709k

    pdf of A Study of Calcareous Deposits on Cathodically Protected Mild Steel in Artificial Seawater (Chris, higher voltage generated hydrogen gas)
    Thanks for the very interesting references Dave. A huge amount of research has been done on the Cathodic Protection of steel. I wish the same resource was available to scientifically prove the reason, the by-product of Cathodic protected Copper Alloys destroy wood around the protected metal. I have some reading to do.
    Thank you.
    Chris

  10. #10
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    Default Re: electrolysis?

    Its not a wood boat, its glass. Didnt get out there yesterday but will head over later with some white vinegar.

    Thanks for the links.

    The waters in the marinas around here are hard on zincs. On my last boat, a glass boat, I hung an auxilliary zinc that weighed a few pounds that was clipped to the shrouds. It got eaten up pretty good but spared the shaft zincs. Most sailors that I know do the same.

    I have not seen this ashy white prop condition before.

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