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Zinc Anodes

 

Anodes are one of those items that make you cry. You pay good money to watch something disolve away. But it is an essential item and not having one will most likely result in another item of significantly more value, dissolving away in an even shorter period of time. Gord posted some valuable technical info he has gleaned from the Net. So I thought it would good value to all of us to lock that info in here in our study hall.

 

A voltaic cell is created whenever dissimilar metals, connected in some way, are immersed in a conductive fluid. The voltage created depends on the relative positions of the two metals in the galvanic series. Since the dissimilar metals create an electromotive force, a voltage, we can gauge the adequacy of the protective zinc by measuring the galvanic voltage.

 

The most common method is to use a "half-silver chloride cell." The resulting cell consists of a piece of pure silver, connected by a copper wire to a sensitive voltmeter and then with another copper wire to a probe that can be used to make electrical contact with various pieces of underwater metal. The Silver/Silver Chloride (or Ag/AgCl) reference electrode is the reference electrode of choice, because it’s easily and cheaply prepared. It is stable, and quite robust. It is sometimes referred to as "SSCE" (Silver/Silver Chloride Electrode) but that abbreviation can be confused with the Sodium Saturated Calomel Electrode.

 

Almost any digital voltmeter can be used to take the measurements. Analog voltmeters that can read voltages as low as 1/1000 of a volt (one millivolt, or mv) can also be used, except an analog meter will give you a very low reading (if any at all) in fresh water.

 

To use the voltmeter to check on the adequacy of the sacrificial zinc, one lead wire is connected to the silver electrode and immersed in the water in which the boat is floating.

The other wire from the voltmeter is connected to a piece of metal in the boat that is in contact with the seawater (the prop shaft, for example).

 

The amount of zinc required to protect other underwater metals varies with the type of metal involved. To make a metal last forever, simply lower its relative voltage 225-250 mV by means of a sacrificial metal (zinc).

 

Voltage Ranges vs Silver/Silver Chloride Reference Electrode

 

Bronze: 500 - 700 mV

700 mV Bz overprotected - Remove zinc

 

Steel: 750 - 950 mV

 

Aluminum: 800 - 1050 mV

 

Lover milli-Voltages indicate metal erosion - ADD Zinc.

Higher mill-Voltages indicate over-protection - Remove Zinc.

 

Voltage Ranges vs a Saturated Calomel Electrode:

http://www.ocean.udel.edu/mas/masnotes/corrosion.pdf

Attached Thumbnails

 

Galvanic Series of Metals in Sea Water from the least noble to the most noble and their potential voltage (note: the table is not complete).

 

Corrosion Potential in mV

Magnesium and Magnesium Alloys -1600 to -1630

Zinc: -980 to -1030

Aluminum: - 760 to -1000

Mild Steel: -600 to 710

Copper; -300 to -570

Brass: -300 to -400

Lead: -190 to -250

18-8 S/S Type 304: -50 to -100

18-8 S/S Type 316: 0 to -100

Graphite: +200 to +300

 

In order to have proper protection, each metal in the grounding circuit's should have a reading at least -200 mV below its stated potential range of corrosion.

 

Example: a brass through hull should give a reading of no more than -500 to -600mV (-300 + -200 = - 500 mV) to have proper protection in the grounding system.

Another way of looking at it is that the zinc should be absorbing the electrolysis given its corrosion potential at -980 to -1030 and hence will functions as the anodic agent in the current flow between all the equipment connected in that series of grounding. If the voltage at the brass through-hull is -300 to -400, that means that it is functioning as a potential "sacrificial" anode.

 

The voltage on all underwater hardware connected to the bonding system should be the same (IF NOT, there are problems in the wiring or connections)

 

See also:

 

Marine Metals Reference by Michael Kasten

http://www.kastenmarine.com/mbqMetRef.pdf

and

Corrosion, Zincs, & Bonding ~ by Michael Kasten

http://www.kastenmarine.com/mbqCref.pdf

 

Excerpted from “The Future for Sacrificial Anodes” ~ by Bob Crundwell

Goto:http://www.jcse.org/Volume4/Preprints/V4Preprint2.pdf

 

The calculation of the weight of anode alloy required to protect a structure is given by a simple calculation:

 

W = (A x C x L) ÷ Z

 

Where:

W = Weight in Kg

A = Area to be protected in M2

C = Polarisation Maintenance Current Density in Amperes / M2

L = System life in Hours

Z = Anode current capacity in Ampere Hours per Kg

The individual components of this equation, with the exception of design life, are known but only approximately*.

 

* It is a brave designer that claims to know the true surface area of the structure. Dimensional tolerances of rolled sections of the sizes from which offshore structures are made are known quite accurately but it is surprising what bits get left out of the calculation let alone any correction for surface irregularity (one authority quoted the difference to be a factor of x2).

* Maintenance current density is variously quoted at figures between 0.140 A/M2 and 0.040 A/M2 for the same location, a factor of almost 4 times.

* Anode current capacities for Al-Zn-In alloys are variously quoted between 2550 Ahrs/Kg & 2750 Ahrs/Kg In general the lower figure is on the basis of long term field tests and the higher figure is on the basis of short term lab tests.

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nigelphilp

PostPosted: Fri Jan 18, 2008 10:37 am Post subject: sacrificial anodes

 

all you clever ones out there, I am looking for advice about alternative methods of protecting all the underwater metallic bits from corrosion.

 

Townson yachts have a very short section of prop shaft available for siting of an anode, and I have the added difficulty of the standard anode that fits the shaft diameter being too thick (in a fore and aft sense) to fit into the exposed section of shaft AND the shaft has a step in it AND it then tapers as it goes into the prop boss.

 

I'm getting tired of hack sawing 30% of the new anode off not to mention having to replace the anode more often than i want to, so ... want to know of alternatives.

 

I am aware that the anode can be fixed on the hull, but if I can don't really want to drill a hole for the wire to go thru .

Some people I have talked to say that they connect an anode to a long bit of electrical wire, connect the wire to the motor? (and?) keel bolts and heave the anode over the side (when they are at anchor only hopefully).

 

The questions are; a) does this work adequately, and if yes how do I go about it B) if not - how else can i protect the boat from damage.

cheers.

 

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David Lackey

PostPosted: Fri Jan 18, 2008 3:36 pm

 

This is an area where lots of people think they know what they're talking about and a few do.

 

It is quite possible, for example, by the mis-placement of anodes, and poor bonding, to create exactly the opposite effect to what one might imagine is happening. This can have critical outcomes for stern gear on all boats, but also serious side effects for wooden boats.

 

I'm too far removed from the scene to recommend anyone, but I'm sure someone on the forum will be able to identify the real experts.

 

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pelagian

PostPosted: Sat Jan 19, 2008 10:04 am

 

I use the method of clamping a wire to the inside of the boat to an anode over the side, i have had no problem with this. I clamp it to the shaft inside the boat using a plastic clamp. In my mind on a race boat i dont want one of those half eaten anodes creating drag. you do have to remember to take it in before you leave the marina or it makes you like an idiot,

 

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Fusion

PostPosted: Sun Jan 20, 2008 6:03 pm

 

A combination of "pelagians" idea and adding a zink to the prop shaft nut end. pelagians zink on a wire will extend the life of you fixed under water zinks.

 

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L'escale

PostPosted: Tue Jan 22, 2008 11:38 am

 

If you are Auckland area, guy to talk to is Vern Linkhorne 0274 792 276.

 

One of a few 'experts' on the subject - worked with builders of new Navy patrol boats, superyacht builders etc - I heve used him on two yachts for advice incl. before & after meter readings (so you really know what's going on down there!)

 

The marginal cost is negligible compared to remedial work if all goes 'pear shaped' later

 

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quantum leap

PostPosted: Tue Jan 22, 2008 3:30 pm

 

The questions are;

a) does this work adequately, and if yes how do I go about it

B) if not - how else can i protect the boat from damage.

Cheers.

 

I have the same issues. but keep ,the whole issue in perspective.

 

1. Is your boat hull and steel work suffering from "paint blow off", Rust, corrosion, Electolysis...?

 

The zinc anode is supposed to sacrifice itself so the boat does'nt.

 

if there is no issue with the hull (and i presume you have had the boat a while) thyen the system may be working well.

 

The worst thing you can do is have the zinc anode not erodeing away.

 

If it is sacrificing itself to quickly then you may be underprotected.

 

The best bet is to get the expert (with his meter) on the boat and get a expert to work out the appropriate protection as there are many variables that contribute to correct protection.

 

Haveing said that their are only 3 ways known to man that stop rust on properly cleaned steel..

 

they are...

 

sacrificial coatings - Galv, zinc coatings, anodes...

Barrier coating technology. - Pure epoxies.

Inhibitive coatings - chromate / phosphate... primers.

 

either way you need the right protection from your anodes and a correct paint system, wiring system (SSB's in particular cause problems) and a qualified technician to balance the "whole system" equation.

 

one thing is for sure it is cheaper to get it right now that to keep repainting the boat.

 

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