Galvanic corrosion protection

Discussion in 'Metal Boat Building' started by Ufling, Jan 18, 2010.

  1. Ufling
    Joined: Jan 2010
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    Ufling Junior Member

    Hello there
    Is there any basic rules on the galvanic corrosion protection of an aluminium hull. I'm thinking sizes/quantities of zinz bars vs. hull size. The material is EN AW-5083
    Thanks in advance!
    Uffe "Ufling"
     
    Last edited: Jan 18, 2010
  2. CDK
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    CDK retired engineer

    In the galvanic series, zinc and aluminium are too close to provide protection. Only magnesium electrodes would be marginally effective.

    An aluminium hull alone will show very little galvanic corrosion unless there are stainless steel parts present.
     
  3. Frosty

    Frosty Previous Member

    Outboard motors use zincs!!

    Whats in the boat --if its just a rowing boat then youlle have no trouble.
     
  4. Ufling
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    Ufling Junior Member

    Hi
    It's 6,6meter, to be fitted with a outboard, see pictures here
    http://picasaweb.google.com/TeamUfling/AluAngler6605#
    I'll use A4 stainless bolts in the boat where rivets can't be used. The outboard is mounted with Stainless bolts.
    I have some grease specially made for Aluminium/Stainless connections.

    @ CDK, if I use magnesium won't that protect the zinc on the outboard?

    Thanks
    Uffe "Ufling"
     
  5. gonzo
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    gonzo Senior Member

    Outboard motors use magnesium anodes in fresh water, so do outdrives. The term "zinc" is a generic name only. Some manufacturers use aluminum anodes too. Also, it depends on the aluminum alloy your hull is made of.
     
  6. CDK
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    CDK retired engineer

    Very good Frosty! If they wouldn't have a steel shaft they wouldn't need the electrodes we call zinc.
    But in that case they wouldn't be outboard motors....
     
  7. Frosty

    Frosty Previous Member

    Ahh,-- have you seen one with an alluminium shaft?


    They made wooden outboards in the war, didnt know that did you?
     
  8. gonzo
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    gonzo Senior Member

    Trojan war?
    Anodes need to be appropiate for the metals they are protecting. The location is also important.
     
  9. Frosty

    Frosty Previous Member

    Nope 2nd world war wooden engines, for limited use to escape the vicinity of a sinking aircraft.
     
  10. Ufling
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    Ufling Junior Member

    Interesting discussion (CDK and Frosty) but it dosen't answer my question..:?:

    @ gonzo, if you have seen the pictures of the boat, would it make reason to place only one magnesiun bar below the waterling on the transom?

    Thanks
    Uffe "Ufling"
     
  11. jonr
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    jonr Senior Member

    Normally zinc in salt water, magnesium in fresh.
     
  12. Zed
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    Zed Senior Member

    Can you get Monel Bolts?

    Is that an option people?
     
  13. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Ufling, if you want longevity using stainless parts in wet areas must be avoided at all cost. There are aluminium bolts and nuts.
    Above waterline, isolate stainless parts by using nylon washers.

    If you feel the need for some extra precaution, attach two 4" magnesium electrodes about midships, one on each side and check how they survived at the end of the season. If they have dissolved more than 50%, use larger ones the next time.

    As Gonzo wrote, what you call zinc on the outboard may be something else, so the magnesium electrodes may not provide protection there.

    There is also an electrical solution to prevent corrosion. Google for "Mercathode".
     
  14. jonr
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    jonr Senior Member

    Impressed current cathodic protection can provide better performance, longer anode life and anti-fouling protection.
     

  15. Ufling
    Joined: Jan 2010
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    Ufling Junior Member

    Thanks! I appreciate you guys taking the time to help. The boat will only be in the water when i fish, otherwice on the trailer. Mostly I fish in salt water, but somtimes also in fresh water. I think I'll look in to the impressed current cathodic protection.
     
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