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Old 05-06-2015, 13:52   #16
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Re: What Anodes for my Corten Steel Yacht ???

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Originally Posted by sailorchic34 View Post
AH, in the galvanic series,

Aluminum is is -1.0v to -.077v depending on alloy mix.
That is true for "ordinary" aluminium alloys. These are designed for a mixture of strength, corrosion resistance, cost etc. Aluminium has very different properties depending on the alloy mix.

Aluminium anode alloys are designed for a very different purpose. Their sole aim is to sacrifice themselves to other metals. In many ways this is the exact opposite of the properties required for "ordinary" or structural aluminium alloys.

This table shows the galvanic series. It includes "ordinary" aluminium alloys and aluminium anodes. (As well as zinc anodes) As you would expect the type of aluminium used in anodes is considerably more active (more -) than ordinary aluminium and slightly more active than zinc anodes.
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Old 05-06-2015, 13:59   #17
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Re: What Anodes for my Corten Steel Yacht ???

Still the same reasons: tradition and lack of experience with aluminum anode alloys.

The only remaining reason, absent presentation of data, is the difficulty in matching anodes throughout the boat. For example, it is easy to get aluminum shaft anodes, but some heat exchanger anodes may be scarce. An while is it clear (I have seen) zinc and aluminum shaft - prop anodes fight with each other, does the potential from a HE pencil extend far enough that a mis-match to the shaft anode matters? I'm asking you to consider that the potential varies around the boat and even changes with the boat is moving vs. motionless. If this were not so, one big anode would be all we needed. I wonder if a HE anode would actually fight with a hull anode. I doubt it. I wonder at what spacing anodes are not in practical potential contact with each other (they are connected on the boat side, but not on the water side)?
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Old 05-06-2015, 14:01   #18
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Re: What Anodes for my Corten Steel Yacht ???

Almost as fun as an anchor thread!
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Old 05-06-2015, 14:22   #19
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Re: What Anodes for my Corten Steel Yacht ???

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Originally Posted by noelex 77 View Post
That is true for "ordinary" aluminium alloys. These are designed for a mixture of strength, corrosion resistance, cost etc. Aluminium has very different properties depending on the alloy mix.

Aluminium anode alloys are designed for a very different purpose. Their sole aim is to sacrifice themselves to other metals. In many ways this is the exact opposite of the properties required for "ordinary" or structural aluminium alloys.

This table shows the galvanic series. It includes "ordinary" aluminium alloys and aluminium anodes. (As well as zinc anodes) As you would expect the type of aluminium used in anodes is considerably more active (more -) than ordinary aluminium and slightly more active than zinc anodes.
Can you post where that chart came from. All the galvanic tables I looked at (DOD Milspec and others) all list aluminum above Zinc. generally less the -1.0V.

I have seen Aluminum at -1.6V but that was with a Standard Hydrogen Electrode reference. For galvanic series in seawater, I can't find where aluminum is a lower negative voltage then zinc. Not saying your wrong, I just seek enlightenment...
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Old 05-06-2015, 14:37   #20
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Re: What Anodes for my Corten Steel Yacht ???

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I'm asking you to consider that the potential varies around the boat and even changes with the boat is moving vs. motionless. If this were not so, one big anode would be all we needed. I wonder if a HE anode would actually fight with a hull anode. I doubt it. I wonder at what spacing anodes are not in practical potential contact with each other (they are connected on the boat side, but not on the water side)?
The electrical potential would not really change based on the boat moving or standing still. The only effect would be to travel throw less salty water, where the electron flow would be reduced.

In theory, one Big anode (not too big with wood boats) would be adequate for protection of all metal within the same circuit (connected/bonded together. Zinc and aluminum would not really fight, the more negative metal will be the anode first till it's mostly gone.

I have even pondered adding active cathodic protection, by placing a conductive metal plate on the outside of the hull connect to +12V DC with a mV meter and rheostat and getting away from mucking with anodes. It's quite simple to do with a fiberglass boat. Slightly harder to do with a metal boat as you wound need to insulate the plate from the metal hull. one could even add a silver cell reference for not much money. Figure I can make one for $40 (with out silver cell).
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Old 05-06-2015, 14:56   #21
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Re: What Anodes for my Corten Steel Yacht ???

Thank you for all your posts and interest. My yacht lives in salt water permanently.
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Old 05-06-2015, 15:24   #22
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Re: What Anodes for my Corten Steel Yacht ???

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Can you post where that chart came from. All the galvanic tables I looked at (DOD Milspec and others) all list aluminum above Zinc. generally less the -1.0V.
The table was from here:

Marine Cathodic Protection

For some reason the properties of different aluminium alloys are often regarded as very similar especially the general public. Yet everyone understands that steel and stainless steel have very different attributes. Maybe it is because steels are know by different names (mild, stainless, hi tensile etc) where alumium alloys are known by numbers (1200, 5083, 6082 etc)

The reference potentials I quoted:
"Zinc is -1.05v
Aluminium -1.1v
Magnesium -1.6v"
were from here:
Anode FAQs
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Old 05-06-2015, 15:59   #23
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Re: What Anodes for my Corten Steel Yacht ???

Let's see if I can add to the confusion. As a retired chemist, I actually haven't thought about the details for years but... First of all, the numbers aren't that important. They are given relative to the reference cell and there may be different references in different publications: silver/silver chloride, calomel, hydrogen are the usual ones. It's the position in the series. There is one thing that bothers me about the table. Pure aluminum is shown as a bar with a pretty large range. In fact, it should be a vanishingly thin line. All pure aluminum is like all other pure aluminum. Why is there a range? I think aluminum and zinc (pure) are pretty close with Zinc being slightly more active, less noble, more negative. I've never quite understood alloys. Are they simple solid solutions or are they like eutectics. I know we can draw phase diagrams and show the mixing I still don't quite get it. Especially because I've experienced "dezincification" or something like that in my marine bronze thruhulls and valves. If that happens in a thruhull why doesn't it happen in a "zinc" with the least noble metal being leached out first, leaving behind the more noble ones? And if that were the case then the electromotive potential of the "zinc" would be the potential of the least noble metal, wouldn't it?

Now to get to some nitty-gritty! What are we protecting with our "zincs". (I don't care what metal they are made of for this part.) Once I saw an engine that had been run without zincs and it was scary!! I've heard of propellers falling off and rudders falling off. So are we protecting things like the bolts on the rudder, or the prop shaft itself? Or are we protecting thruhulls as well? My thruhulls are not "bonded" but I can't swear that there aren't galvanic paths. And I agree with Sailorchic that in principle a single anode should work but I'd argue for having them close to the "stuff" we are protecting just to minimize questions of resistance and the random gremlin attack. Dammit, I was going to replace my prop shaft zinc next week, now I've got too much to think about!

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Old 05-06-2015, 17:57   #24
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Re: What Anodes for my Corten Steel Yacht ???

Well I did a simple test. I took a bucket of saltwater (IE what das boot is sitting in at anchor), grabbed my multi-meter, a spare zinc, a bit of aluminum tubing and a stainless steel hose clamp.

I then measured the mV's between the stainless steel hose clamp and the zinc then to the aluminum. The Aluminum reading was -120mv and the zinc about -90mv. So I can at least see that aluminum can have a more negitive potential. Oh its not scientific, as I'm sure my Multimeter is not 100% accurate in the mv range and I did not have a silver reference cell, but at least it shows the potential difference between a zinc anode and a aluminum tubing.

BTW a impressed current galvanic system is stupidly easy on a fiberglass boat with 12V batteries and solar. I may play around with that later this year.
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Old 05-06-2015, 18:06   #25
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Re: What Anodes for my Corten Steel Yacht ???

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And I agree with Sailorchic that in principle a single anode should work but I'd argue for having them close to the "stuff" we are protecting just to minimize questions of resistance and the random gremlin attack. Dammit, I was going to replace my prop shaft zinc next week, now I've got too much to think about!

Bill
In protecting underground steel piping, anodes are placed within 50 feet of the pipe, connected by wire to the pipe. Each anode will protect 200-500 feet of pipe line depending on soil conditions. For the metals in the typical <=70 foot fiberglass boat, location of the zinc is not that critical. It does need to be connected/bonded to the metal being protected, but location is not super critical.

Now if you have a aluminum sail drive, then yes you'll want a anode on the saildrive.
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Old 05-06-2015, 20:56   #26
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Re: What Anodes for my Corten Steel Yacht ???

See below in RED.

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The electrical potential would not really change based on the boat moving or standing still. The only effect would be to travel throw less salty water, where the electron flow would be reduced.

Yes, it does change, though not a great deal (US Navy). But I thought it was interesting. Also notice that the potential is not uniform.



In theory, one Big anode (not too big with wood boats) would be adequate for protection of all metal within the same circuit (connected/bonded together. Zinc and aluminum would not really fight, the more negative metal will be the anode first till it's mostly gone.

I think I will need to test the effect of distance on voltage. I suspect relative surface areas will matter.

I have even pondered adding active cathodic protection, by placing a conductive metal plate on the outside of the hull connect to +12V DC with a mV meter and rheostat and getting away from mucking with anodes. It's quite simple to do with a fiberglass boat. Slightly harder to do with a metal boat as you wound need to insulate the plate from the metal hull. one could even add a silver cell reference for not much money. Figure I can make one for $40 (with out silver cell).
Yup, I've installed very large impressed current systems on 10,000,000 gallon oil tanks. Whether it is robust enough for recreational us, with power surges and all. Running off solar will help, but will it work when the ownrer is gone for 3 months? I'm sure that it would work for you, since you would actually monitor it. A very flexible solution. I'm not sure what you will use as a durable seawater anode, and you will need to provide conductivity to the prop shaft, around the engine coupler. All-in, it will be much more than $40, I think.
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Old 05-06-2015, 21:54   #27
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Re: What Anodes for my Corten Steel Yacht ???

Don't you love it when anecdotal hearsay is as credible as well established science? Without even a pretense of scientific explanation? God gave you a brain . . .
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Old 06-06-2015, 00:41   #28
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Re: What Anodes for my Corten Steel Yacht ???

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Well I did a simple test. I took a bucket of saltwater (IE what das boot is sitting in at anchor), grabbed my multi-meter, a spare zinc, a bit of aluminum tubing and a stainless steel hose clamp.
I love it. A real engineers solution. Forget the internet, do the measurments youself .

However, your results are not what I would expect. Almost any aluminium, apart from an alloy specifically produced to be an anode, should have been less negative than the zinc. Were the multimeter probes or wire touching the seawater? The copper in the wire or the nickel plated copper of the probes would also produce a voltage difference compared to the hose clamp. The other possibility is that unless the zinc is sanded down to fresh metal it can be surprisingly hard to get a good electrical connection.

It is always a good idea to check for a low resistance connection after you have installed a zinc and it can often be difficult to to get a good connection between the multimeter probe and the zinc even on dry land. Zinc gets affected with a high resistance coating especially if exposed to fresh water.

Give those zincs a sand before you launch, especially on critical areas like the sail drive.
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