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  1. #1
    Moderator RX951's Avatar
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    Understanding Marine Corrosion

    The Whats, whys, and Hows of MARINE CORROSION
    &
    CORROSION AND ELECTROLYSIS Definitions

    Below is a good explanation for the term galvanic corrosion and electrolysis. both of which we see as a boat owner.

    What is electrolysis : electrolysis (noun), pronounced elec.trol.y.sis. 1 a : the producing of chemical changes by passage of an electric current through an electrolyte.

    Galvanic Corrosion
    Galvanic corrosion is frequently referred to as dissimilar metal corrosion. Galvanic corrosion can occur when two dissimilar materials are coupled in a corrosive electrolyte. An illustration of galvanic corrosion would be joining two dissimilar metals in electrical contact in seawater.

    In a galvanic couple, one of the metals in the couple becomes the anode and the other metal becomes the cathode. The less noble material becomes the anode. The anodic metal corrodes faster than it would all by itself. The cathodic metal corrodes slower than it would all by itself.

    Many boaters use this knowledge to their benefit. Sacrificial zinc anodes are commonly used to protect metal components on boats. The zinc anode corrodes preferentially there by protecting the boat component. The zinc anodes are maintained and replaced as required to insure continued protection. Other alloys are also used as sacrificial anodes. Aluminum or magnesium sacrificial anodes provide better protection in some cases.


    In our situation with boats in general, the electrolyte solution is the "salt water"

    Good link here

    http://www.boatus.com/boattech/MarineCorrosion.htm

    For corrosion to occur, four elements must be in place. These are:

    Two dissimilar metals.

    A connection between the two metals.

    An electrolyte, such as water.

    A potential difference between the two metals.


    Note: It is imperative to always unplug your trailer lights prior to backing down an submerging your boat at the boat ramp. this will prevent the electrolysis corrosion process from taking place as you will eliminate the "variable" of the salt water; electrolyte, from the equation.

    The reason we have ZINC ANODES in our GPR cylinders is to help prevent the process from taking place in the cooling system of our motors. The "sacrificial" zinc anodes will allow the corrosion process readily take place on itself before it attacks the motor.

    Once 50% of the anode is let, replace it.

    Never paint or cover a zinc anode, doing so will reduce or eliminate its protective potential.




    Zinc Anodes: can be used in both salt and brackish water.
    Aluminum Anodes: can be used in both salt water and brackish water.
    Magnesium Anodes: can be used in fresh water only



    CORROSION AND ELECTROLYSIS

    EQUAL CONFUSION TO MOST

    WEAPONS FOR BOTH

    (As applied to fiberglass vessels)

    It has been said by many electrical (gurus) that the term electrolysis is the most confusing and misused term around the boatyard. Electrolysis is a term that is loosely applied to the corrosion processes. The term refers to "solution phenomena" and not to corrosion.

    There is a type of corrosion called electrolytic corrosion but it is not electrolysis. Electrolysis refers to the degradation of an electrolyte that occurs as a result of passing electrical current through it. Electrolytic corrosion is caused by a current from an external source, often the boat's battery or shore supply. The current that causes electrolytic action is called "stray current" and usually emanates from a poorly installed electrical circuit or a bad earthing (ground) arrangement, like a poorly installed electrical circuit or bad ground arrangement on a radio or power tool or a current leak due to damp connections.

    The confusion is usually between the terms galvanic corrosion and electrolytic corrosion. Galvanic corrosion is caused by an electric current generated by two different metals in a conducting medium such as seawater. When we talk about galvanic corrosion we're talking about an electric exchange. The results of each type of corrosion can be similar and can occur at an alarming rate. The bottom line however is that you can have a properly wired boat moored at a properly wired dock and still be at risk of corrosion.

    Where is the safest place to store my boat to repel these enemies of corrosion. It helps if you keep your vessel out of the conducting medium, like seawater, or well away from other vessel's feasting on shore supplied alternating current that may be leaking to or from God knows where. When your vessel is anchored out half the problem is eliminated (this makes a good case for cruising). Now some of you can dry store your vessel and that prevents a host of problems, but many of us are stuck in the wet stuff.


    References:


    http://www.boatsurveyor.com/corrosion.htm

    http://www.practical-sailor.com/news...6corrosion.pdf

    http://www.screamandfly.com/home/eng..._corrosion.htm

    http://schools.matter.org.uk/Content...ainApplet.html



    [/b]


  2. #2
    Happily Self-Employed WFO's Avatar
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    Good one Billy

  3. #3
    Pistonwash's Avatar
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    Excellant Billy..keep up the good work..these post are very informative to the board.

  4. #4
    ABBOTT's Avatar
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    great post!

  5. #5

    Galvanic corrosion in fuel tank?

    I have an RXP and have had 2 fuel pumps fail due to the positive crimp inside the tank in gasoline dissolving/eroding/corroding?

    Any ideas? The ski doesn't have any ethanol based fuel but the positive crimp has dissolved on both 2 pumps..

    Galvanic corrosion/electrolysis dissimilar metal erosion> Please help..

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Helpful material, but there are some errors and contextual omissions in it (as relates to PWC). Be sure to educate yourselves sufficiently that you can recognize incorrect or incomplete advice.

    Here are some things I noticed...
    Quote Originally Posted by RX951 View Post
    ... Many boaters use this knowledge to their benefit. Sacrificial zinc anodes are commonly used to protect metal components on boats. The zinc anode corrodes preferentially there by protecting the boat component. The zinc anodes are maintained and replaced as required to insure continued protection. Other alloys are also used as sacrificial anodes. Aluminum or magnesium sacrificial anodes provide better protection in some cases.

    Aluminum anode cannot protect aluminum, the anode must always be the least less noble metal present that is in contact with the water.


    ...It is imperative to always unplug your trailer lights prior to backing down an submerging your boat at the boat ramp. this will prevent the electrolysis corrosion process from taking place as you will eliminate the "variable" of the salt water; electrolyte, from the equation.

    Unplugging trailer lights can be helpful, but not because it eliminates the water. Hot glass bulbs can fracture on contact with cold water, rapid cooling can draw water inside the lamp enclosures.
    I suppose if you left the trailer parked in the water for a long time, you would get additional corrosion from the 12 volt power being present. Short term, corrosion isn't the primary reason to unplug.


    The reason we have ZINC ANODES in our GPR cylinders is to help prevent the process from taking place in the cooling system of our motors. The "sacrificial" zinc anodes will allow the corrosion process readily take place on itself before it attacks the motor.

    Once 50% of the anode is let, replace it.

    Never paint or cover a zinc anode, doing so will reduce or eliminate its protective potential.




    Zinc Anodes: can be used in both salt and brackish water.
    Aluminum Anodes: can be used in both salt water and brackish water.
    Magnesium Anodes: can be used in fresh water only

    Only Zinc and Magnesium will protect aluminum, aluminum anodes are not useful for PWC.
    ...

  7. #7
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by StrandedAtSeadooAgain View Post
    I have an RXP and have had 2 fuel pumps fail due to the positive crimp inside the tank in gasoline dissolving/eroding/corroding?

    Any ideas? The ski doesn't have any ethanol based fuel but the positive crimp has dissolved on both 2 pumps..

    Galvanic corrosion/electrolysis dissimilar metal erosion?
    Please help..
    Photos of the problem area?

  8. #8
    How about wire brushing the corroded areas, then spray with 100% zinc paint then over coat with a good enamel

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