Cleaning Mercathode Unit

Discussion in 'Sterndrives' started by Willallison, Mar 8, 2010.

  1. Willallison
    Joined: Oct 2001
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    Willallison Senior Member

    The Mercathode unit, located beneath the gimbal housing, on my new Bravo 3 was inadvertently painted with antifoul by the vessels builder, rendering it ineffective. :mad:
    All the Merc literature say not to do this, of course and says that you should avoid pressure washing or scrubbing the area. What they don't tell you is how you should clean it....:confused:
    I'm inclined to simply replace the unit to be on the safe side, but of course it will need cleaning in the future....
    Any suggestions?
    And has anyone had success with the single mercathode system on the Bravo 3 drive (which has 2 s/s props...) or do they all really require the fitting of a 2nd mercathode system?
     
  2. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Scraping the paint off the electrode areas should be enough.
    I cannot think of a valid reason why you should not pressure wash the area.

    Check the current at the controller's +, if there is 20 mA or more (gimbal unit submerged of course), the unit works as designed.
     
  3. Willallison
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    Willallison Senior Member

    Thanks CDK. The reason that they don't like pressure washers (or scraping I'm sure) is that the reference electrode wire has a coating on it that is easily damaged. The merc proprietory testing unit has the same coating on its electrode. Measuring current with just any old electrode can give vastly different results as the conductivity is effected by this coating. Of course, merc being merc, they don't tell you what the coating is, so you've no way of really testing it without using their equipment.
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The Merc reference electrode is said to be silver chloride. That means it's just silver: the chloride comes natural as soon as the electrode is submerged.
    Because the chloride has no strong bond with the silver substrate, it can be washer off easily with a cleaning jet but forms again. Some silver is of course sacrificed in the process.

    To satisfy my curiosity I opened the electrodes of a defective old-style Mercathode unit, where the reference is a separate plastic dome screwed to the transom.
    There was a pitted 1.5" thin silver disc inside with a ragged hole in the center where the orange wire once had been soldered.

    The active electrode contained a .5x1" metal strip, unaffected by years of service. Judging from the dull gray color it is probably tungsten.
     

  5. Willallison
    Joined: Oct 2001
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    Location: Australia

    Willallison Senior Member

    Thanks CDK
     
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