Revelations about zinc electrodes.

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

  1. rwatson
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    rwatson Senior Member

    Thanks for the mental exercise guys - its always fun to retrace basic concepts and refresh minds.

    The best explanation of the +ve, -ve concept that trips people up that I have come across, is the link
    http://www.allaboutcircuits.com/vol_1/chpt_1/7.html

    where Benjamin Franklin screws up current flow concept.

    So now we have "Conventional" flow notation, where the current is designated as travelling from the Positive to the Negative side. Its not true, but it still works for circuit design and calculation.

    In reality though, in "Electron flow notation", the electrons travel from Negative to Positive.

    Then, I got onto the defintion of a Cathode , at good old peer reviewed Wikipedia.
    http://en.wikipedia.org/wiki/Cathode

    The quote "A widespread misconception is that cathode polarity is always negative.. In fact cathode polarity depends on the device type, and can even vary according to the operating mode"

    If you look up the definition of Anode, of course it says "A widespread misconception is that Anode polarity is always positive (+). "

    So, how is Cathodic protection working ?
    http://en.wikipedia.org/wiki/Cathodic_protection

    "Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the Cathode of an electrochemical cell.

    [1] The simplest method to apply CP is by connecting the metal to be protected with another more easily corroded metal to act as the anode of the electrochemical cell."

    So - as far as I can make out, for marine hull protection, the steel hull is the Cathode - so the Zinc/Aluminium must be ..... an Anode.

    What fun :)
     
  2. MikeJohns
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    MikeJohns Senior Member

    It depends on your frame of reference.

    Look at the materials in the electrolyte; Which electrode material produces the Cations and which electrode provides the electrons to reduce the cations? Which electrode do Cations migrate to ? Why ?
    Once you answer those questions you might start to see why it's not quite as clear cut a definition as a hoover when looking at a galvanic cell ;)

    In an electrolytic cell with an impressed current the correct terminology is to label the electrode attached to the electron sink as the Anode (+ve) and the electron source (-ve) as the cathode.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Of course RWatson. The term is sacrificial anode... When Zinc is used with aluminum , zinc is the SACRIFICIAL ANODE. Material flows from the sacrificial zinc anode and is deposited on the Cathode...the hull, prop or whatever metal that is of a lower value on the index.

    Its difficult to understand why the original poster has a problem with this .

    .

    When determining what anode you must use and which material will be the cathode, Simply consult the Galvanic table and make sure the your anode..zinc in most marine seawater cases..will be the sacrificial anode and the metal you wish to protect will be the cathode '

    Zinc is at the bottom of the table...it is most Anodic at 1.25 on the index, Bronze is more cathodic at the top of the table at . 045

    Zinc is the anode, Bronze is the cathode.




    Metal Index (V)

    Most Cathodic


    Gold, solid and plated, Gold-platinum alloy 0.00

    Rhodium plated on silver-plated copper 0.05

    Silver, solid or plated; monel metal. High nickel-copper alloys 0.15

    Nickel, solid or plated, titanium an s alloys, Monel 0.30

    Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys 0.35

    Brass and bronzes 0.40

    High brasses and bronzes 0.45

    18% chromium type corrosion-resistant steels 0.50

    Chromium plated; tin plated; 12% chromium type corrosion-resistant steels 0.60

    Tin-plate; tin-lead solder 0.65

    Lead, solid or plated; high lead alloys 0.70 2000 series wrought aluminum 0.75

    Iron, wrought, gray or malleable, plain carbon and low alloy steels 0.85

    Aluminum, wrought alloys other than 2000 series aluminum, cast alloys of the silicon type 0.90

    Aluminum, cast alloys other than silicon type, cadmium, plated and chromate 0.95

    Hot-dip-zinc plate; galvanized steel 1.20

    Zinc, wrought; zinc-base die-casting alloys; zinc plated 1.25

    Magnesium & magnesium-base alloys, cast or wrought 1.75

    Beryllium 1.85
     
  4. MikeJohns
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    MikeJohns Senior Member


    Yes! People get very confused because they get hung up on a definition which they misapply to the incorrect frame of reference.
     
  5. CDK
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    CDK retired engineer

    Beautifully phrased Wynand.
    The real process is more complex because the electrolyte plays an active role. The acid is ionized and acts as a vehicle to transport the metal atoms, but the outcome is what you wrote.

    Since the development of semiconductors the scientists have determined that electricity is transported from - to +, not the other way around, so the term positive charge is misleading: that is where the holes are.

    But I do like your first sentence above, the negative electrode is the cathode.

    It would be much more understandable for everyone if we used emitter and collector like for transistors, but even then the polarity issue remains because there are both NPN and PNP device.

    So as pointed out in post #1 I will keep calling the zinc parts electrodes so everybody can be happy.
     
  6. CDK
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    CDK retired engineer

    I have a problem with ignorant fools pretending to be experts. Try to cut&paste some brains first.
     
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  7. BertKu
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    BertKu Senior Member

    Yes, it is absolute fun. But lets turn it around. What is actual happening in the zinc block. If I can understand what actual is happening to the molecules and electrons, maybe we can come to an agreement. If I look at a zinc block after a few months, half of it is eaten up. Molecules and electrons have disappeared. Correct?? Can anybody explain where the electrons and molecules have gone to.

    In the seawater?? Maybe, possible, could be.

    Added to the hull ?? I doubt it, otherwise the hull will show this up.

    So, who is going to explain what happened to those barium oxide layers on the cathode in an old fashion valve after a few years. .......A big you pardon Wikepidia, anode. My apology, we call it anode now.

    Am I confused. All the old fashoned EL84, EL34, RS732 valves etc, has now the anode burning the electrons away and the cathode is ...... I am now ready for a metal institution.
    bert
     
  8. BertKu
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    BertKu Senior Member

    CDK, we should not get personal, this website is a fun place and we should be allowed to express our opinion, without a personal attack, even if sometimes somebody deserve it, but we should refrain from it.
    Bert
     
  9. rwatson
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    rwatson Senior Member

    Thats it Bert.... a METAL institution is very appropriate. The details confuse me too.

    I think the important clue is "In fact cathode polarity depends on the device type, and can even vary according to the operating mode"

    The quote "But I do like your first sentence above, the negative electrode is the cathode." is therefore not necessarily accurate.

    So, inspired by the METAL Institution, I read on at
    http://en.wikipedia.org/wiki/Cathodic_protection

    "Production of hydrogen ions: A side effect of improperly applied cathodic protection is the production of hydrogen ions, leading to its absorption in the protected metal and subsequent hydrogen embrittlement of welds and materials with high hardness. Under normal conditions, the ionic hydrogen will combine at the metal surface to create hydrogen gas, which cannot penetrate the metal. Hydrogen ions, however, are small enough to pass through the crystalline steel structure, and lead in some cases to hydrogen embrittlement.
    "
    Interesting, but thats is for "improperly applied cathodic protection"

    So I dug further, and at
    www.mrowen.com/chem12/electrochemistry/electrochemistry80.pdf

    I found

    "www.mrowen.com/chem12/electrochemistry/electrochemistry80.pdf"

    I had to do a screen shot of the pertinent section, but essentially, Zinc oxide (not Zinc itself) gets placed on the steel - in the process.

    Thanks for the inspiration Bert - its fun being an armchair researcher.
     

    Attached Files:

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  10. BertKu
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    BertKu Senior Member

    Hi RWatson,
    I always enjoy it, when somebody has a sense of humor. It keeps us alive.
    Thanks for your research, much appreciated. That basically says it all.

    Bert
     
  11. gonzo
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    gonzo Senior Member

    The more clear way of looking at all this, is to think of chemical reactions and electrical circuitry simultaneously. Sea water, whenever there is electrolysis, will free corrosive substances like chlorine. That adds to problem. On wooden boats, too large of a zinc will deteriorate the wood around the, should I say anode.
     
  12. CDK
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    CDK retired engineer

    Gentlemen, please pay attention.

    In post #1 I made just one casual remark about why I talk about electrodes.
    We are now at post #72, mostly discussing this one sentence.

    My small experiment is much more important because it explains why zinc disappears sometimes faster than necessary and why electrodes with a steel strap sacrifice themselves by protecting the strap and bolts unless they are covered with paint or epoxy.

    The US style electrodes, without the strap, but with copper bushings around the holes are much better designed but still need a smear of paint over the bolt heads to do what you bought them for.
     
  13. viking north
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    viking north VINLAND

    Thank you Gonzo for reintroducing the wooden hull problem re zincs and internal bonding of the thru hulls as i mentioned back in post #25. It was an effort on my part to change the drift of the post from sematics back to the topic as i knew from past experience what was about to evolve, re electron flow, field flow, hole flow. Any how this has been a good and very important thread on zincs and hull protection and thank you CDK for it's intro. Thruout the thread reference sites were given on info for the protection of aluminium/steel hulls, fastening of zincs, location and numbers of zincs. I installed 6 zincs on my alum. hull in addition to one on the rudder and shaft. Back to wooden hulls:would anyone like to expand on proper or popular internal electrical bonding and zincs on wooden hulls as the breakdown of wood fibers around thru hulls is no less important than metal hulls being eaten by galvanic action. Geo.
     
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  14. gonzo
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    gonzo Senior Member

    The problem with wooden hulls, is that you have thousands of metal fasteners that can't be electrically bonded. From that perspective, trunell or tree nail fastened hulls are superior.
     

  15. michael pierzga
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    michael pierzga Senior Member

    CDK..?????????????????????

    "The US style electrodes, without the strap, but with copper bushings around the holes are much better designed but still need a smear of paint over the bolt heads to do what you bought them for."

    what are you talking about ???
    __________________


    What is a US style anode ? Post the technical description of a US style anode ? Are these anodes used outside of US waters ? and post all documents concerning your theory of brass fastening zinc ...or some kinda stainless fastened , "O " ring, smear of whatever compound, combination.

    Post your technical documents CDK.
     
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