Revelations about zinc electrodes.

Discussion in 'Boat Design' started by CDK, Jan 7, 2011.

  1. CDK
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    CDK retired engineer

    The "inspiration" for this post are questions from local boat owners about the unexplicable rate at which zinc electrodes sometimes dissolve.
    Although I believe my findings and explanations are accurate, the following is just my personal opinion. Please note that I avoid the commonly used 'zinc anode' because that is wrong. The zinc attached to the hull of a boat has a negative potential, so it should be called a cathode: steel structures on land are also protected by negative electrodes, a system called cathodic protection.

    1. It is a widespread believe that zinc electrodes must loose their mass over time and that an electrode behaving differently is not properly installed.
    This is not the case! A zinc electrode is only sacrificed if it is electrically connected to a metal hull that suffers from paint flaws or damages, or purposefully exposed metal areas. A metal hull with a good paint layer will provide no electrical path, so the zinc electrodes will be inactive.
    For a zinc electrode to become active, a closed circuit must be present.

    In the left hand picture you see two pieces of steel tube that I cleaned and hung in a bucket of seawater. One had an electrode attached to it, in this case a sliver of aluminum, the other one had none.
    Before the test I measured the current between the electrode and the steel and also the open voltage potential. The current was slightly more than 30 milli-amps, the open voltage approx. 600 milli-volts.
    Both objects remained immersed for 5 days.

    2. External influences.
    The piece with the electrode attached came out almost as clean as I put it in, so the electrode does protect exposed steel. But the other piece was covered in light brown iron oxide. The coating was much thicker than expected, as much as 10 times the rust that is formed if a piece of steel is submerged for 5 days.
    The reason for this fierce attack can only be that the electrolysis near the sacrificial electrode (it was actually foaming) ionized some H2O and NaCl, making it much more aggressive even for steel in the vicinity without any electrical connection but the water it is in.

    The lesson is twofold.
    A metal boat in the berth next to yours having damaged underwater paintwork may cause damage to your hull as well or at least activate your zinc electrodes causing them to dissolve quickly.
    And a shore power cable with the ground wire attached to your hull can play havoc to your electrodes if other hulls are grounded in a similar way or if the shore is lined with steel plates.

    3. Faulty electrode construction.
    The center picture show a very common zinc electrode at the bottom with a cast in galvanized steel strip.
    If attached with stainless or brass fasteners, the electrode will protect these fasteners. But the zinc around the steel strip is very thin, so will quickly disappear and then the electrode will protect its own part until all the zinc has gone. During the process there is a current loop between the zinc and the very nearby steel, so the electrode is no longer effective against a paint scratch further away.

    The electrode in the picture weighs 2 lbs and costs $20 to $40 depending on where you buy it.
    The market price (LME) for zinc is approx. $1 per lb....
    In the upper half you see my solution: a simple piece of zinc with a weight of almost 3 lbs, cut from a 36 lbs strip I bought for $27. The drawing shows how to attach this $2.25 electrode.

    It is of paramount importance to cover the nut and surrounding area with an insulating coating. I used 3M industrial sealant nr #800 but you can also use Sikaflex or a marine paint. Instead of an O-ring under the electrode a small piece of rubber hose could also be used. Even if it isn't 100% waterproof, the lack of motion within the seal will minimize chemical activity.

    4. Shaft electrodes.
    These will wear much less if the shaft and prop are painted. I've seen lots of shafts in our wintertime marina, 8 out of 10 were bare metal, causing the expensive shaft electrodes to work overtime.
    Some were almost like new, so probably not electrically connected to the shaft, in one case I could easily rotate the electrode with one hand....
     

    Attached Files:

  2. anthony goodson
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    anthony goodson Senior Member

    Thankyou for that CDK ,what material are the fastenings in your diagram ,and what are the parameters that lead you to that choice ?
     
  3. gonzo
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    gonzo Senior Member

    I find that stray current is the main culprit of excessive wear on the "anodes". Many boats have horrible electrical installations and older marinas are not any better.
     
  4. michael pierzga
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    michael pierzga Senior Member

    What alloy are your Zinc anodes ?
     
  5. gonzo
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    gonzo Senior Member

    Zinc anodes are not considered an alloy. They should be over 99% pure.
     
  6. michael pierzga
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    michael pierzga Senior Member

    Wrong...Marine zinc anodes are always a zinc alloy... MilSpecA-18001K. Pure zinc passivates. A passivated anode surface is no longer active...your zincs will last forever.

    Marine zinc anodes contain Aluminum and Cadmium to promote erosion of the surface and keep fresh zinc available. Do not use bulk zinc underwater and always use anodes with embedded steel plates to generate a perfect electrical bond between the zinc alloy and the bonding strap, plate, hull. Always fasten with fan disc washers to promote a secure electrical connection .

    When in doubt consult the technical documents.

    http://www.mgduff.co.uk/leisure-craft/fitting-instructions/aluminium.html
     
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  7. viking north
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    viking north VINLAND

    Gonzo, i think you hit the nail on the head, re stray currents.On my first conversion, an aluminium hull, I was given advise by an fellow alum. hull owner, when visiting marinas or where in a crowded anchorage to drop a big zinc over the side hanging off a #8 or so wire electrically connected to the hull. Seemed to work for him as his home port was a marina and his big zincs were being eaten.
     
  8. WickedGood

    WickedGood Guest

    What do you think of using conductive grease betwwen the Zinc and Drive Shaft?

    what about using the same grease on the Bronze/Nibil Prop to a stainless shaft?


    You reccomend painting the shaft & Prop?
    Would a copper based antifouling or Tin Base be better on a fiberglass boat that is bottom painted with copper.
     
  9. CDK
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    CDK retired engineer

    As long as you provide good electrical contact with the parts you want to protect and insulate the contact area against water, it doesn't matter what you use. I took stainless bolt and nuts because they can be removed without much effort even after years.

    On my own boat I use zinc to protect just the stainless objects like shafts, stern tubes and rudders. The questions put to me were from sailing yacht owners, where a grp hull is combined with an iron keel.
     
  10. CDK
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    CDK retired engineer

    I don't think there is a conductive grease for this task.
    But good quality shaft electrodes have two small pieces of copper protruding from the inner surface. Secured to a clean shaft they provide an immaculate electric contact. You could apply a smear of Molykote copper grease, not to provide contact but to fill the gap and keep the water out.

    If you paint the shaft and prop, you prolong the life of the zinc. But I know that paint on a prop won't stay there long.
     
  11. CDK
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    CDK retired engineer

    That's about right Gonzo. We are talking about industrial zinc here, used for protection of steel by hot dipping or galvanizing. It still is contaminated with other elements from the ore, but not enough to call it an alloy.

    For use in electrodes alloys are also used, but I would prefer pure metals because the properties are better known. For complex shaped electrodes on outboards or stern drives sometimes an alu/zinc alloy called Zamac is used because it flows better in the mold.
     
  12. gonzo
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    gonzo Senior Member

    I have never heard of zinc "passivating". In plating operations the zinc has less than .5% impurities and keeps on flowing to whatever is being plated.
     
  13. michael pierzga
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    michael pierzga Senior Member

    Reducing exposed metal reduces anode wear. Prop and Shaft anodes are subject to cavitation erosion and wear fast. Good shaft anodes have embedded copper conductors on the inside...shaft contact surface, for superior electrical contact. Normally all that is needed is to clean the shaft for good electrical contact and torque on. Be sure that your shaft is electrically bonded to the hull.

    They say tin based antifoul preserves anodes. I cant see the difference. On big yacts in the Med you moor stern too. The docks are made of steel covered in concrete. Aft facing rudder anodes erode fast. Not much you can do...that is why they are fitted. Not sure what zinc anode challenges you have in Maine.

    When anode dont erode...you have a problem.
     
  14. michael pierzga
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    michael pierzga Senior Member


    Err emmm, read. Youre a marine surveyor......The Internet is a good source of knowledge. Thousands of documents concerning passivisation .

    http://www.boatzincs.com/use_milspec_a18001k.html
     

  15. gonzo
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    gonzo Senior Member

    The link confirms what I said. Less than .5% impurities. MIL SPEC .005% iron
     
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