WARNING: change when half worn.

[seafox]

Now that I have your attention. Why do Volvo say to replace the anodes when they get to half worn?

Is it so that they can sell twice as many anodes or is there a good reason for it? I would have thought that provided there was still some zinc in touch with the metal you want to preserve it will still be doing it's thing.

[GordMay]

An anode’s activity is based upon it’s chemistry and it’s mass/surface area. Anodes are designed to be effective in a diminishing range down to about 50% of their original size. Smaller, and they don’t provide adequate cathodic protection. It’s a bit of an arbitrary figure.

[seafox]

my prop anode gets down to half in a few months. It still keeps going till its at about 80 percent worn in 5 to 6 months. It must be still providing protection to get from the 50 percent to the 80 percent so quickly. No noticable electrolysis wear on the prop.

I asked the Auckland volvo parts man how long the prop anodes last when I bought some more in the weekend. He told me about 6 months. I thought mine was wearing pretty quick at 6 months but appears to be about right.

[Alan Wheeler]

There is a way to slow it down. But you have to be able to do two things first. Firstly, you need to be able to get a means of electricaly cnnecting to your prop in a easy way. If the Bronze prop is totaly electricaly isolated from the rest of the box, maybe via a rubber bush for instance, then the next point is of no use.
OK, if the prop is connected electrically, then you now need to test the 'protection" with a galvanic meter. You should be able to borrow one from a boat sparky. This will tell you where you level of protection is at. If it is a little low, your anode will eat away too quickly, along with poor protection being supplied to the prop and leg.
OK, if you are measuring that you need more protection(I suspect you will) then connect a decent weight cable to a point that is in electrical contact with the prop. Connect a "regular" anode to it and dangle the thing over the side and I suggest you dangle it to the side that that alloy boat is on. The anode will take over from your prop one and leave you prop one intacked. All you have to do is haul it in when you want to go sailing and drop it back in in the Marina for protection. I do this and it is a great way to monitor my anodes 6ft down below. I don't like divign down in this cold water here to check on them.

[seafox]

The leg is isolated from the motor. It has it's own big zinc between the leg and the prop. Lasts the normal year. the Prop is apparently isolated again from the leg and has its own three zincs. It is the one that chews away. so no way to attach another zinc to that.

Just wondered why they say change at half way. That would mean every 3months for me. Bugger that. I'll let it chew to 80 percent before I change it. Prop looks fine and the saildrive has its own protection.

[Sonny]

I put 2 zincs on my shaft at the beginning of the season. By the end of the season (early November), usually there is only one partially left on the shaft.

[seafox]

How long is the season?

[Pblais]

Just wondered why they say change at half way. That would mean every 3months for me. Bugger that. I'll let it chew to 80 percent before I change it. Prop looks fine and the saildrive has its own protection.

I think the problem is you assume it should last until it is totally gone. There is no rule of thumb for galvanic activity. If you are in a marina then it's double what the those that are not in one have. If the boat next to you is leaking a lot of voltage one season may not be enough for the zinc. I've seen it be 6 months or 2 years in the same slip with the same boat. My neighbor fixed his problem and suddenly 2 years is how long one shaft zinc lasts where it was 6 months. I don't haul except to re-paint.

[GordMay]

I would emphatically urge you to either:
Test the level of cathodic protection your anodes are providing (see below)
or
Change them out at the recommended intervals (annually or when 50% eroded).

The amount of protection and longevity a zinc anode provides, in a given cell*, depends on its surface area, and of its mass (weight). The larger the anode, in relation to the cathode, the less the corrosion.

* The rate at which galvanic corrosion takes place is basically governed by the chemical components which comprise the corrosion cell (the anode material, the protected metal (cathode) material, and the electrolyte (water).
Every metallic element or alloy possesses, or generates its own specific solution potential (voltage) when immersed in seawater (electrolyte) . That is, there is a difference in voltage between the metal and the water. This can be measured very precisely by attaching the negative lead of a milli-voltmeter to immersed metal, and the positive lead of the milli-voltmeter to a reference electrode placed in water.
A lower voltage metal will last forever if its normal freely eroding voltage is [u]raised[/u\] by 225 to 250 millivolts.

Such measurements in water commonly are made with a silver/silver chloride reference electrode.

Excerpted from ”Building your own corrosion meter” ~ by Chuck Husick
”... The amount of zinc required to protect other underwater metals varies with the type of metal involved. Underwater bronze, such as the propeller, is considered to be fully protected by the zinc on the prop shaft if the voltmeter reads between 500 and 700 mv. Voltages less than 500 mv indicate that the bronze is eroding, and more zinc is needed. Voltages above 700 mv indicate that the bronze is overprotected; there is an excess area of zinc available. (This condition won't necessarily harm a fiberglass boat but can cause trouble for wooden hulls.) The protected voltage range for steel is 750 to 950 mv and 800 to 1050 mv for aluminum ...”
(Ocean Navigator magazine, Jan/Feb 2002
Goto: Ocean Navigator Online - Building your own corrosion meter

Or, you could purchase the Portable Corrosion Test Meter & Workbook, from
Professional Mariner - Yacht Corrosion Consultants

Products :: Galvanic Isolation :: Corrosion Test Meter (Portable - Analog) - 20' Lead

Southeast Marine Services :: Other Equipment, Lighting, Corrosion Control/Isolation

Transformers, Battery Monitors Voltage and Amp Meters :: Corrosion Control

[fstbttms]

All zinc manufacturers recommend replacement at 50%. I don't believe that an anode that is 60% depleted is no longer providing protection, but it's cheap insurance to keep the zincs topped up, so to speak. Any zinc that lasts more than a year (in a marine environment) is not doing its job, probably because it is either improperly attached or has become passivated. On the flip side, a 3-month depletion is pretty rapid and may warrant investigation. In the case of the Volvo prop zincs, in my experience they frequently do not last very long and as someone pointed out in another thread, it may be due to cavitation corrosion They are not a particularly substantial piece of metal to begin with.

[delmarrey]

Zinks will also deplete faster if the vessel is used more often. Especially shaft zink, I believe due to the static that is created while the vessle is in motion...................................._/)

[Alan Wheeler]

Ummm, no that is not correct Del. While in motion, or for that matter, moored, yet a good tidal current flowing, the electrical action is greatly reduced. This is however, also in relation to the distance between the two items the current is flowing between.
Lets say you have just pulled into the birth and tied up. The electrical current it'self is established quite quickly. Much like the fact that if you switch a wall swtich on, you have current flow at the end of your extension cable instantly. However, the actual current flow of electrons inside the copper is very slow. I forget the actual speed, but it is extremely slow as in snail type speed. So back to the boat situation, the current "path" is established quickly, but the flow of electrons through the water is very slow. Now this is where that becomes important. In a crude sense, the electrons are what are carrying the ions of metal. The metal is flowign trhough the water. They disapate away from the anode and carry to the cathode and plate themselves to what ever the cathode is. But it takes a great deal of time to travel and any strong current will quickly stop this action. So in general, a boat under way can not set up a path that electrons and thus the metal can flow along. However, they metals are subject to Cavitaion corrosions and this is far more damaging at a greater pace than galvanic corrosion. It is easy to spot Cavitaion corrosion. The metals often pit out turning to something that resembles swiss cheese.

[rebel heart]

Originally Posted by Pblais

I think the problem is you assume it should last until it is totally gone. There is no rule of thumb for galvanic activity. If you are in a marina then it's double what the those that are not in one have. If the boat next to you is leaking a lot of voltage one season may not be enough for the zinc. I've seen it be 6 months or 2 years in the same slip with the same boat. My neighbor fixed his problem and suddenly 2 years is how long one shaft zinc lasts where it was 6 months. I don't haul except to re-paint.

If I remember Nigel Calder's recommendation, I can save myself some zinc life by unplugging my shore power, right? Since that's what the leaky stuff is grounding out to? Or completing the circuit to, I suppose.

[Alan Wheeler]

Yes and No. Problems between your boat and someone elses boat is via shore power. Namely the Earth. This can be isolated using a Transformer or a Galvanic isolator which are two very high current Diodes back to back, effectively breaking the earth system from galvanic currents.
Galvanic problems between different metals on your own boat can not be stoped this way.

[Moby Dick]

Gord

With no evidence other than my eyeballs, I would say a zinc at 50% erosion has more surface area than a new, smooth, one, as it is very pitted. My guess would be that protect tion would be similar (but will erode more quickly from the 50% gone mark)