(I would firstly like to acknowledge the work of Gord for finding this info in the first place.)
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
< 500 mV Bz is eroding - Add zinc, > 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:
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)
Marine Metals Reference by Michael Kasten
Corrosion, Zincs, & Bonding ~ by Michael Kasten
Excerpted from “The Future for Sacrificial Anodes” ~ by Bob Crundwell
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
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.