When to replace the Anodes ?

[fstbttms]

Originally Posted by Idiens

The more the better if they are connected properly.
You didn't mention if you are in salt or fresh water. In fresh water you need aluminium anodes instead of the zinc ones for salt water. Zinc will grow a white oxide in fresh water and stop working.

No offense, Idiens, but if I had a buck for every piece of bad info I read from you on various forums today, I'd have two dollars!

Absolutely you can over-zinc your boat. I'm not sure you need to go to the lengths that Alan suggests to figure out what you need, but fiberglass sailboats will generally have one or two anodes on the shaft and/or one on the prop (if applicable.) The strut may have one and some boats will have a plate zinc that mounts on studs that extend thru the hull somewhere aft of the mast (the Taiwanese seem prone to this).

[Dave-Fethiye]

OK - so its 2-1.

What is the problem created by having say 6 anodes instead of 2 ?
In electrical terms what goes wrong if you have too many anodes ?

Obvioustoo many ( like 100 ) would sink the boat ! - so I am taling about the electrics. They are called sacrificial anodes - so with more doesn't the sacrifice just get spread over several of them and happen more slowly ?


Dave.

[fstbttms]

Originally Posted by Dave-Fethiye

What is the problem created by having say 6 anodes instead of 2 ?
In electrical terms what goes wrong if you have too many anodes

I have to admit that I can't give you a definitive answer on this, regarding fiberglass boats. I suspect that you would have to try pretty hard to over-zinc one. Wooden and aluminum boats are a different story, where the actual integrity of the hull can be degraded.

But hey, it's your boat and your dime. You wanna put six zincs on it, knock yourself out. Just don't say we didn't warn ya.

[Idiens]

Originally Posted by fstbttms

No offense, Idiens, but if I had a buck for every piece of bad info I read from you on various forums today, I'd have two dollars!

Absolutely you can over-zinc your boat. I'm not sure you need to go to the lengths that Alan suggests to figure out what you need, but fiberglass sailboats will generally have one or two anodes on the shaft and/or one on the prop (if applicable.) The strut may have one and some boats will have a plate zinc that mounts on studs that extend thru the hull somewhere aft of the mast (the Taiwanese seem prone to this).

No offense taken as you are determined to disagree.
You don't mention the size of anodes you recommend. Do you recommend lots of little ones, a few big ones, lots of big ones or what?
You mention your other buck earned, do you really think anodes should erode completely in two months. Do you recommend adding more or larger anodes to extend this period, or look for reducing stray currents as well?
Without knowing anything about the boat, I still think more anodes is better advice than less.
However, I agree my blanket "more is better, if properly connected" has limits.

[ronbo1]

This site is a good source of information for zinc installation. Gives voltage measurements to determine if you're under or over-zinced.

It's also a good source of zincs.


BoatZincs.com (978-841-9978) - Discount Boat Zincs

[Idiens]

Originally Posted by ronbo1

This site is a good source of information for zinc installation. Gives voltage measurements to determine if you're under or over-zinced.
It's also a good source of zincs.
BoatZincs.com (978-841-9978) - Discount Boat Zincs

Thanks for the reference, now we have to determine how many and large our zincs should be to stay within the recommended voltage range.

[fstbttms]

Originally Posted by Idiens

Do you recommend lots of little ones, a few big ones, lots of big ones or what?

I recommend using the proper size for the part being protected. A 1.25" collar zinc for a 1.25" shaft or 70mm cone for a 70mm Max Prop hub, for example. Where on a sailboat do you have any leeway in choosing what size anode you use?

Originally Posted by Idiens

You mention your other buck earned, do you really think anodes should erode completely in two months. Do you recommend adding more or larger anodes to extend this period, or look for reducing stray currents as well?

Of course an anode should last longer than two months and if it doesn't, adding more is the quick and dirty solution. But you previously indicated that there is no upper limit to the amount of zincs you can safely use, and that is simply incorrect.

[senormechanico]

No one has mentioned the method of measuring this voltage.

You need a silver contact for one connection to the meter.
I used a silver earring soldered to an old meter lead, and sealed the connection to the earring with 3M5200. I just drop the lead into the water and measure my bonding system in several places to make sure all the connections are ok. Works like a charm.

Steve B.

[Idiens]

Originally Posted by fstbttms

.... Where on a sailboat do you have any leeway in choosing what size anode you use?

Well, I have two shaft anodes, one each side of the p-strut. They come in different sizes. I have a disc anode on the p-strut, probably bigger than necessary, there are smaller discs, so there is choice.
My apologies for my throw away line, the intention was not to sink the boat but to have enough anodes to last a season and not be surprised to find them all gone. As you said, it might be difficult to over-anode a fibreglass boat, whereas too little anode mass or area or position or connection might be easier to achieve. Since the goodness criteria seems not to be anode number or size but milli-Volts, the problem lies elsewhere.

[Dave-Fethiye]

Thanks Ronbo1

That link is JUST what I needed - it explains why we should not under or over zinc.

I read somewhere that to make the reading you just attach one of the multimeter clips to a COPPER tube and put it in the water near the boat and the other side of the is touched on the metal object you are teasting. There was no mention of SILVER being needed !
Does it matter if its silver or copper ?

Thanks
Dave

[Alan Wheeler]

Sorry, I was out last night and missed this.
There is a ruff rule of thumb that works very well for determining how much protection. Work out the surface area off all metal under the water. The surface area of the anode needs to be about 1% of the total metal surface. That gets you pretty close to spot on.
The accurate way of measuring is with the meter and silver prob. Why silver?? Well actually any metal can be used. But the voltage measured will change. There is a scale (I'll try and find it in my archives). Basicaly it is to do with where the metal fits on the Nobility chart. As two metals get further away from each other, the voltage rises. So for measuring, we need a metal with very good conductivity, excellent corrosion resisitance and far away from the metals we would want to measure. Gold is a little expensive. Several other metals around are a little radioactive. Silver tends to be the best choice of all, thus it is used. So now an industry standard has been reached and all voltage measurements are referenced to a common point of standard = silver. If copper is used, the voltage difference has to be factored in. The distance between copper and brass on the nobility scale is not far. So getting a meaningful and accurate measurement could be difficult.
Now if you have picked up on that we are measureing a voltage, hopefully it is clear that we have a current flow in the water. This part is complex and I could write pages on it. Infact there are already pages written on the subject. But to put it simply, there are many "things" in the marina that metals on your boat want to tiki tour off to via an electrical current. As the current flows form areas to the metal on your boat, electrons flow the otherway along the path back to the object. The electrons carry along the electrons that are the metal. However, they rarely make it to the other area because of current and so the metal just disapears into the water. The Zinc anode being low on the Nobility scale, "sacrifices" it'self first. If the current flow is kept within a "range" this happens in a controlled maner. If the surface area is too large, the electrical current becomes so strong, that paint can simply be blown off. Ususally the anti-foul, because it is so loaded with copper. A sign of too much zinc is often attributed to large blisters under the anti-foul. Even blisters in fibreglass can occur. Infact, sometimes Osmosis can actually be protection problems.
Aluminium can be used. But before you all rush out and buy aluminium anodes, aLuminium can also be bad. Ally is close to other metals on the scale and so one, the surface area needs to be much larger than zinc. The volatges are smaller, so accuracy is nmore important. Certainly voltage measurement is best to obtain the correct ally size. There are many grades of ally and each fit in very different places on teh scale. So you can't just put any ally down there. Ally is best used in very warm and high salinity waters.
Magnesium is at the other end of the prefered anode scale. It reacts very quickly and should NOT be used in salt water. It is common to be used in fresh waters. Once again, it is best to use a voltage measurement to determin what size is required. Magnesium can be expensive and you don't want to waste it.
I hope that helps.

[senormechanico]

Dave,
I'm searching for a link on the subject but don't have time right now.
The difference has to do with how much voltage the meter will read.
There's a difference, but not a lot. I just used a silver one because it's easy to make, and was recommended in a professional kit owned by a cruising friend.

If I can find the link, I'll post it.

Steve B.

[Dave-Fethiye]

Hi Alan,

If I look at the surface area of the metal in the water, it seems pretty low. The rudder already has anodes on it. The propeller, shaft and rudder attachments plus a water outlet and cables are run up to the bolts going into the boat frame. 1% of that surface are is going to be only ONE smallish anode. There were two big anodes on the boat. Maybe it has been damaging the paintwork and wood - too much zinc does that on a wooden boat, I beleive. I shall calculate the surface area much accurately - but the anode has two sides to work - so that surface area isn't tiny anyway.

[GordMay]

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.

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

[GordMay]

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