Electrical corrosion on a wooden boat with no metal in the water?

Discussion in 'OnBoard Electronics & Controls' started by ianduncanqld, May 3, 2016.

  1. ianduncanqld
    Joined: May 2016
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    Location: Brisbane

    ianduncanqld New Member

    Hi, I have a wooden folkboat with nothing metal except the keel in the water (the motor is an outboard and is always kept out of the water). I bought an electronic anti-fouling system after reading up and thinking it would be ok but it appears that it is attacking the copper planking rivets. Is this possible? The electronic system is connected directly to the shore power and then to a copper anode hanging off the bow and a cathode off the stern, both suspended from non-conducting plastic line.

    I'm very confused as I can't see how the boat can be part of any circuit (there's no shore power connected to the boat, just an extension cord going to the anti-fouler which is sitting on a bench)

    The system is called Barnaclerid and I've been really happy with it, I just don't get whether it could be causing any corrosion. Any suggestions or feedback would be most gratefully received! Ian
     
  2. MikeJohns
    Joined: Aug 2004
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    Location: Australia

    MikeJohns Senior Member

    Barnaclerid

    Ian ?
    For the copper ions to drift from Anode to Cathode there has to be a significant and constant DC electric field in the seawater across the boat.
    In an impressed electric field in seawater, all metals exposed to the water and not bonded to the Cathode can become Anodic and subsequently corrode.

    The anodic metal surface loses electrons and produces ions in your case cu2+ which migrate in the electric field to the Cathode. In doing this it the anodic material corrodes.
    At the cathode, copper ions gain electrons from the external current source and any that actually arrive are deposited as copper. That will be where your rivets are trying to go. In a large application a lot of the ions are lost and never actually arrive at the cathode, rather other ions take their place.

    Early investigations with corrosion determine if there is a significant electric field present in the water, for this system to work there must be one ! The corrosion rates will depend on the intensity of the field.

    How long does an anode last you and how heavy are they ? You must chew through a few to produce enough cu2+ ions to toxify the water. There would be a fair amount of gas at the electrodes too, Hydrogen at the cathode and Chlorine at the anode.
     
  3. ianduncanqld
    Joined: May 2016
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    Location: Brisbane

    ianduncanqld New Member

    Thanks very much Mike, that all makes sense.
     

  4. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    In addition to the above - imo, the corrosion of copper rivets might also indicate a locally moist wood.
    Wooden hulls are never 100% dry. Wooden planks have a certain percentage of water in them (12-15% in a new and correctly built hull, rising after months and years in water) and hence they can act as conductors, although very bad ones. The more water the wood absorbes, the more conductive it becomes.
    The conductivity also increases with temperature, approximately doubling for each 10° increase. The hull area around the engine might be affected by the latter factor.
    In both cases, that's how the electrical circuit between anodes (rivets) and the cathode closes and permits the corrosion of rivets.
     
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