Revelations about zinc electrodes.

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

  1. michael pierzga
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    michael pierzga Senior Member

    All yachts are made of "fools gold".... Praise the Lord ! ...gold platted yachts are how I make my living..
     
  2. viking north
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    viking north VINLAND

    Gonzo is correct,For a detail galvanic scale see post #217 under Reverse Engineering (conversions /modifications)
     
  3. viking north
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    viking north VINLAND

    And may it continue Michael, to maintain a democracy always question authority vigoursly, to maintain capitalism is is the duty of those that need take (earn it) from those that have , much like the galvanic scale. mucho, mucho where possible, Geo.
     
  4. MikeJohns
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    MikeJohns Senior Member

    Cornelius

    I think they are correctly called Anodes

    If you consider the cell as a box filled with electrolyte with two immersed electrodes, one being the Zinc and the other steel (a bare patch of protected steel). The Hull of the boat is then the external circuit. Within the electrolyte just consider which way the cations flow. That more properly defines the electrochemical cell.

    As for self corrosion, protective zincs are alloyed to produce a surface which sheds its oxidised material.
    Just how the zinc responds will depend on salinity and temperature. Self 'cleaning' anodes are considered mandatory.
    Pure zinc can become considerably less effective and I've seen blocks of pure zinc with a crust that insulated it very effectively, in fact i could get no connection anywhere on the surface of the zinc. An alloyed sacrificial zinc never does this.

    Of course Just how the zinc responds will depend on salinity and temperature.

    Self 'cleaning' anodes ( alloyed) are considered mandatory and are incorporated into standards. the Milspec anode composition standard is just one such standard.

    I'd rather see steel straps cast into the anode and steel studs on a steel hull.
    I always specify that anode straps, mounting hardware and even the backs of the anodes be epoxied or sealed and I like to see the sides of the anodes painted too so the only bare face is away from the hull. This solves a lot of paint blistering problems around the anodes.

    If the boat is left and the anode material completely erodes you don't want more noble material around like SS anode straps.

    And we paint bronze props very effectively with epoxy and hard antifouling. Unless the prop cavitates the paint is often immaculate, it certainly isolates a large amount of very noble material. We have fishing boats that have been doing this for 15 years with no shaft anode ( although they do have a grease filled shaft and the shaft is also epoxied up to the bearing).

    Zincs are often problematic and will sometimes damage paint even stripping it. Ironically overprotection can lead to extensive coating failure followed by very quick anode depletion and then severe corrosion. Often boats labelled 'problem boats' would actually be better with no anodes at all.

    I posted something on over protection here:

    http://www.boatdesign.net/forums/materials/blisters-around-zincs-steel-boat-23941.html#post225055
     
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  5. CDK
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    CDK retired engineer

    Mike, except for the anode bit I fully concur. You clearly have studied the topic well.

    Isolating the straps is a good remedy to keep these expensive electrodes from sacrificing themselves by protecting their own straps. But you are an exception, I didn't find one boat in the marina where someone took the trouble to paint the straps, shafts or props.

    The anode/cathode confusion may well originate from the times electricity was supposed to flow from + to -, but now we know better. Even Wikipedia has confusing texts about it.
    Such differences are inevitable. I watch a TV program about your border control where they always talk about revoking "a visa". But visa is the plural form of visum, a word that is immediately red underscored by the Windows spell checker.
     
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  6. Ad Hoc
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    Ad Hoc Naval Architect

    Interesting debate. As for the anode or not to anode…this is my take:

    Immersing 2 different metals into an electrolyte solution, such as sea water, a current flows from the anodic, or base metal, to the cathodic, or noble metal. The noble metal tends to be protected, and hence the anodic or base is the metal that is depleted.

    Thus looking at the galvanic chart Zinc is the lowest down at a nominal -1130mV, with aluminium around -760mV and carbon steels around -610mV.

    To protect the ally or steel, using zinc, this is an anodic reaction from the zinc to the noble metal or ally/steel, the zinc experiencing corrosion whereas the noble metal is protected. Hence using zinc is termed an “anode”. I’m with Mike on this.

    Mind you, also worth noting when discussing aluminium there are anodic and cathodic polarization reactions that can take place on the surface of aluminium. As Mike noted, owing to the differing composition of electrolyte solutions because the behaviour of the Al-ions is related to the chemistry of the aluminium in the electrolyte. Since it is possible that in the presence of a crack/pit, aluminium can repassivate under the right conditions; anodic film formation.
     
  7. Joakim
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    Joakim Senior Member

    What about saildrives and bronze folding propellers? I have a Yanmar SD20 with Volvo-Penta two blade bronze folding (likely the same as Radice and Allpa). Both the SD and the propeller have zinc anodes. The propeller is not painted. The propeller zincs wear quite rapidly lasting only 1-2 seasons despite the conditions around here (cold water, salinity ~0.5%, 5 kk season). The saildrive zincs seems to last 3-4 seasons and outboard zincs seem to last forever.

    Does a bronze propeller even need the zincs? Not all brands have them. Looking at the rare painted propellers paint doesn't seem to last well. Should I paint? What paint should be used?
     
  8. michael pierzga
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    michael pierzga Senior Member

    What is Gonzo correct on ?
     
  9. viking north
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    viking north VINLAND

    CDK, re electricity flowing from + to - this is known as field as in E(electric field flow) and flowing from - to + is known as electron as in I (current flow). Both are correct as both make up electrical power ExI=Watts. In modern electrical/electronic theory both are intermingled, however the tendancy today is to use electron flow to follow the operation of electrical/electronic circuits and field flow for the operation of capacator,transformer and antenna theory. Geo. CET.
     
  10. viking north
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    viking north VINLAND

    Michael, on the standard listing of the galvanic scale, see my post #32 reference, unless i'm missing a point you were trying to make by your order of listing, If so I stand corrected. Powerful statement "I stand corrected" It's a no lose situation, IE maintainds civility without eating crow,(smiley face ) Geo.
     
  11. CDK
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    CDK retired engineer

    So you're another adversary John, I can live with that:D.

    The zinc has the lowest potential, so the current flows from the zinc to the hull (where exposed). Because my background is electronics, the negative side, where the electrons are coming from, must be the cathode.
    That goes for the now extinct electron tubes, diodes, thyristors etc. A CRT used to be a cathode ray tube, the anode is the screen, being the receiving end
    Also the carbon pile in a primary cell is the anode, because it is positive, the zinc can is a cathode.

    But it remains difficult. There is a lot of anodized aluminum on this planet, not suitable for cathodes nor anodes....
     
  12. CDK
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    CDK retired engineer

    The zinc bolted to the prop nut has the harsh life of a midget protecting a giant. I have Allpa shafts with bronze props: in the Adriatic sea the zinc electrodes are completely gone within 3 months.
    I tried to paint the props with expensive Volvo stuff, but that was a waste of money. Will try epoxy this season.
     
  13. michael pierzga
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    michael pierzga Senior Member


    Well Viking North, if you or CDK or Gonzo would like to re write the galvanic table then you must supply a link to prove of your observations and educate us readers.

    This is the link I abide by

    http://www.mcnallyinstitute.com/04-html/4-1.html

    So far I have not seen any new knowledge, nor observations in this thread.

    CDK is still not sure why anodes on props and shafts wear faster ...the literature concerning cavitation erosion of anodes on props and shafts is widely available.

    Questions are being posed about the compatibility of SS and aluminium. Please supply the relevant links to prove your observations..



    CDK appears to be not aware that Zinc anodes are an alloy...the literature on zinc "passivisation" and the proper alloy composition of
    zinc anodes are widely available.

    CDK appears to believe that steel inserts moulded into zinc anodes promote erosion...supply the relevant literature that proves this theory.
     
  14. BertKu
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    BertKu Senior Member

    CDK, I am fully in agreement with you. Only electronic people will confirm that the zinc is the cathode. It is logical, as the electrons are taken away from the zinc to the more noble metal. for that reason the noble metal does not deteriorate. All non electronic people will still believe that the current flows from the plus to the minus.
    Bert
     

  15. michael pierzga
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    michael pierzga Senior Member

    Anode Cathode marine corrosion table


    GALVANIC SERIES OF METALS AND ALLOYS .....


    CORRODED END ( ANODIC OR LEAST NOBLE)

    MAGNESIUM
    MAGNESIUM ALLOYS
    ZINC
    ALUMINUM 5052, 3004, 3003, 1100, 6053
    CADMIUM
    ALUMINUM 2117, 2017, 2024
    MILD STEEL (1018), WROUGHT IRON
    CAST IRON, LOW ALLOY HIGH STRENGTH STEEL
    CHROME IRON (ACTIVE)
    STAINLESS STEEL, 430 SERIES (ACTIVE)
    302, 303, 304, 321, 347, 410,416, STAINLESS STEEL (ACTIVE)
    NI - RESIST
    316, 317, STAINLESS STEEL (ACTIVE)
    CARPENTER 20CB-3 STAINLESS (ACTIVE)
    ALUMINUM BRONZE (CA 687)
    HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE)
    LEAD-TIN SOLDERS
    LEAD
    TIN
    INCONEL 600 (ACTIVE)
    NICKEL (ACTIVE)
    60 NI-15 CR (ACTIVE)
    80 NI-20 CR (ACTIVE)
    HASTELLOY B (ACTIVE)
    BRASSES
    COPPER (CA102)
    MANGANESE BRONZE (CA 675), TIN BRONZE (CA903, 905)
    SILICONE BRONZE
    NICKEL SILVER
    COPPER - NICKEL ALLOY 90-10
    COPPER - NICKEL ALLOY 80-20
    430 STAINLESS STEEL
    NICKEL, ALUMINUM, BRONZE (CA 630, 632)
    MONEL 400, K500
    SILVER SOLDER
    NICKEL (PASSIVE)
    60 NI- 15 CR (PASSIVE)
    INCONEL 600 (PASSIVE)
    80 NI- 20 CR (PASSIVE)
    CHROME IRON (PASSIVE)
    302, 303, 304, 321, 347, STAINLESS STEEL (PASSIVE)
    316, 317, STAINLESS STEEL (PASSIVE)
    CARPENTER 20 CB-3 STAINLESS (PASSIVE), INCOLOY 825
    NICKEL - MOLYBDEUM - CHROMIUM - IRON ALLOY (PASSIVE)
    SILVER
    TITANIUM (PASS.) HASTELLOY C & C276 (PASSIVE), INCONEL 625(PASS.)
    GRAPHITE
    ZIRCONIUM
    GOLD
    PLATINUM



    PROTECTED END (CATHODIC OR MOST NOBLE)
     
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