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Old 05-06-2015, 00:44   #1
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What Anodes for my Corten Steel Yacht ???

My Corten Steel yacht is currently fitted with zinc anodes. I have been given advice that I should in fact be using Alumunium Anodes. The alumunium being a softer sacrificial metal than the copper, nickel, chromium and steel that makes up high quality corten steel? Any thoughts? Thank you
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Old 05-06-2015, 06:24   #2
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Re: What Anodes for my Corten Steel Yacht ???

Anodes work based on their position on the Galvanic Scale relative to the metal being protected. "Softer" has nothing to do with it.

That said, your steel boat will be well protected by either zinc or aluminum anodes.
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Old 05-06-2015, 06:43   #3
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Re: What Anodes for my Corten Steel Yacht ???

Zinc is best for salt water, aluminium is just OK
Aluminium for brackish water
Magnesium for fresh water

This catalogue has some good info:
http://www.mgduff.co.uk/pdfs/mgduff-...logue-2013.pdf
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Old 05-06-2015, 06:58   #4
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Re: What Anodes for my Corten Steel Yacht ???

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Old 05-06-2015, 07:18   #5
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Re: What Anodes for my Corten Steel Yacht ???

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... aluminium is just OK
Please explain. Though it is not traditional, I've yet to read concrete argument against aluminum anodes.

Another thing to consider is just how many people sail in fresh, but the marina is actually brackish at least a part of the year.
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Old 05-06-2015, 08:18   #6
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Re: What Anodes for my Corten Steel Yacht ???

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Please explain. Though it is not traditional, I've yet to read concrete argument against aluminum anodes.
Aluminum anodes are designed to work in saltwater. They are every bit as effective as zinc anodes. Even the article that noelex 77 linked to indicates that aluminum is fine for saltwater.
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Old 05-06-2015, 08:31   #7
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Re: What Anodes for my Corten Steel Yacht ???

Based on 35 years experience engineering steel offshore structures, the offshore industry uses aluminium alloy anodes. From memory they are not pure aluminium, nor I suspect are the comercially available yacht anodes.
If you did some google research I'm sure you would find that both Al and Zn are far enough down from iron in the electrochemical table to work effectively.
Another bit of amateur knowledge which hasn't been mentioned is that aluminium hulls use zinc anodes, so zinc must be further down the table.
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Old 05-06-2015, 08:50   #8
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Re: What Anodes for my Corten Steel Yacht ???

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Another bit of amateur knowledge which hasn't been mentioned is that aluminium hulls use zinc anodes, so zinc must be further down the table.
Actually, they use aluminum anodes (the anode is a different alloy than the hull or outboard). Use zincs and the warrantee is gone.

The only advantages I can see for zinc are...
* they are traditional. Matters to many people.
* they used to be cheaper, but not any more.
* they are more available, but that is changing rapidly.

... and none of them are actually advantages.
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Old 05-06-2015, 10:18   #9
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Re: What Anodes for my Corten Steel Yacht ???

Have steel boat too. Have used zincs for 15 years no problem. The list is correct Zinc is what you use! Russ
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Old 05-06-2015, 11:23   #10
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Re: What Anodes for my Corten Steel Yacht ???

Aluminium anodes have a higher negative potential (they are more active) than zinc. Magnesium is considerably more active again. Steel boats are designed (or should be) for the correct amount of anodic protection. A switch to aluminium or magnesium in salt water is not necessarily good.

Changing the anode material needs to considered carefully on a metal boat, although aluminium is quite close to zinc so it is generally OK.

Zinc is -1.05v
Aluminium -1.1v
Magnesium -1.6v

The lower conductivity of fresh water requires a higher negative potential for protection, so magnesium anodes are used. In addition zinc develops a protective coating which stops it working when exposed to fresh water. This coating can remain when the boat returns to salt water.

Brackish water in many ways in the most difficult. In practice boats often switch between salt and fresh water. Here the best option is aluminium, although aluminium can also develop a protective coating in fresh water so try and give the anodes a periodic sand.

If you do change to aluminium anodes make sure you change all the anodes. Some (especially prop and bow thruster) anodes can be difficult to get in aluminium. Make sure you have spares especially if cruising out of the way places. Alumininium anodes need to be a very precise metal (not "ordinary" aluminium which is an alloy) so only buy them from reputable companies.
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Old 05-06-2015, 12:02   #11
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Re: What Anodes for my Corten Steel Yacht ???

Sorry for sticking my nose in but, I installed C.A.P.A.C. systems on yachts and ships back in the 60s for Englehard . You're not protecting your steel hull so much as your bronze thru hulls, which corrode at 350 millavolts.Zinc will protect your hull fine.I had a Van de Stadt I bought in Holland, made of corten for six years in salt and fresh water, used Zincs, no problem. Call me at772. 801 4166 and I'll explain the no b.s. theory of marine corrosion to you. Stick with your zincs.
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Old 05-06-2015, 12:08   #12
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Re: What Anodes for my Corten Steel Yacht ???

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Originally Posted by thinwater View Post
Actually, they use aluminum anodes (the anode is a different alloy than the hull or outboard). Use zincs and the warrantee is gone.

The only advantages I can see for zinc are...
* they are traditional. Matters to many people.
* they used to be cheaper, but not any more.
* they are more available, but that is changing rapidly.

... and none of them are actually advantages.
Aluminum hulls simply do not use aluminum anodes...unless you want the entire hull to be sacrificial.
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Old 05-06-2015, 12:52   #13
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Re: What Anodes for my Corten Steel Yacht ???

There is some understandable confusion about aluminium anodes. Aluminium is a complex alloy with quite different properties depending on its composition. The marine grade aluminium alloys used on the skin of an aluminium boat are selected for their resistance to corrosion. They are quite different to the aluminium in your home, they are even significantly different to the aluminium in your mast. The latter is chosen primarily for strength rather than resistance to corrosion.

Aluminium anodes are an alloy which is not much use for anything but sacrificing itself to other metals.

As i noted above you should not change the anode material on a metal boat without careful consideration. All the aluminium boats I have seen in salt water have been designed (and use) zinc anodes. Many aluminium hulls actually have very little anodic protection. Usually it is just installed where different underwater metals are used.

Aluminium anodes could be used on aluminium boat as the galvanic potential of the aluminium anode is quite different to the aluminium used in the hull construction.
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Old 05-06-2015, 13:32   #14
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Re: What Anodes for my Corten Steel Yacht ???

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Originally Posted by noelex 77 View Post
Changing the anode material needs to considered carefully on a metal boat, although aluminium is quite close to zinc so it is generally OK.

Zinc is -1.05v
Aluminium -1.1v
Magnesium -1.6v
AH, in the galvanic series,

Aluminum is is -1.0v to -.077v depending on alloy mix.

Zinc is roughly -1.05V

In other words Aluminum is slightly more noble then zinc, not the other way around. Least on the data I have.
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Old 05-06-2015, 13:51   #15
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Re: What Anodes for my Corten Steel Yacht ???

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Aluminum hulls simply do not use aluminum anodes...unless you want the entire hull to be sacrificial.
I'd suggest a little reading first.
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