sledracr

Aluminum "zincs"?

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Just picked up a new set (shaft and strut) anodes from West Marine, and they felt "light",  Turns out they're aluminum, not zinc, and claim to be good for "brackish water".

Feh.

My boat "lives" in a brackish marina, but sails in saltwater.

Not sure if I should stick these on, or stock up on "real" zincs.

Thoughts?

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Buy the zincs for where it lives, not where it visits.

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Just went to the local Yanmar agent and they only stock genuine Yanmar aluminium "zincs" for the sail drive now - they come in 2 halves and have a stainless mounting bracket so that you can change the anodes without taking the prop off.

Wasn't too convinced with this idea, but they did have  non-genuine "proper zinc" anodes which were twice the price which I purchased. Changing times.....

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Aluminum seems to be replacing zinc as the anode of choice. See for example this. Slightly more protective than zinc, lasts a little longer, costs a little less. Pretty much every vendor is saying the same thing, so it isn't just one.

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I would worry about the electrical connection to an aluminium anode. Aluminium does not make connections well in less than perfect or “damp” conditions.

Does “lasting longer” indicate it is accomplishing less? I think so.

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It's longer lasting because it has more anodic capacity/ lb, not because it is less effective. About 3 times zinc, but that is deceptive as it is also about 1/2 the density. The claim is about 30% longer life in salt water for a given application.

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15 minutes ago, DDW said:

It's longer lasting because it has more anodic capacity/ lb, not because it is less effective. About 3 times zinc, but that is deceptive as it is also about 1/2 the density. The claim is about 30% longer life in salt water for a given application.

I saw that in the Cruising World piece. Very deceptive cut and paste from aluminum marketing materials. We do not buy and install anodes “by the pound.”

Presenting the numbers honestly an aluminum anode will be about 14% more active...doing a slighty better job or lasting slightly longer doing the same job.

Aluminum is better environmentally because they say the zincs have cadmium in them. I still worry about the electrical connection as aluminum is famously bad at humid electrical connections.

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5 hours ago, daddle said:

I saw that in the Cruising World piece. Very deceptive cut and paste from aluminum marketing materials. We do not buy and install anodes “by the pound.”

Presenting the numbers honestly an aluminum anode will be about 14% more active...doing a slighty better job or lasting slightly longer doing the same job.

Aluminum is better environmentally because they say the zincs have cadmium in them. I still worry about the electrical connection as aluminum is famously bad at humid electrical connections.

Not so daddle. For an anode of equal volume, say a Saildrive leg anode, the electrochemical capacity of the anode will be V (cm3) x 2.75 (g/cm3) x 2500 (mA/gm) the equivalent volume zinc anode will have capacity  V (cm3) x 7.1 (g/cm3) x 780 (mA/gm). Ie the aluminium anode has approx 25% more electrons available. In practical terms the aluminium anode is slightly more electronegative -1150mV vs Ag/AgCl ref electrode vs -1100 for zinc which results in a slightly higher current output for the aluminium anode resulting in a similar life for both anodes although it is possible that the aluminium actually has a shorter life with higher current output. Both materials will be cast onto a steel core with a resistance between anode and core being micro ohms so no damp issues. 

I don’t know where the advice not to use zinc in brackish water arises. I suspect that it may be the Saildrive leg marketing department. We have supplied thousands of tonnes of zinc anodes for saline mud and brackish water applications and in place of magnesium in low resistivity soil applications.

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41 minutes ago, frant said:

Not so daddle. For an anode of equal volume, say a Saildrive leg anode, the electrochemical capacity of the anode will be V (cm3) x 2.75 (g/cm3) x 2500 (mA/gm) the equivalent volume zinc anode will have capacity  V (cm3) x 7.1 (g/cm3) x 780 (mA/gm). Ie the aluminium anode has approx 25% more electrons available. In practical terms the aluminium anode is slightly more electronegative -1150mV vs Ag/AgCl ref electrode vs -1100 for zinc which results in a slightly higher current output for the aluminium anode resulting in a similar life for both anodes although it is possible that the aluminium actually has a shorter life with higher current output. Both materials will be cast onto a steel core with a resistance between anode and core being micro ohms so no damp issues. 

I don’t know where the advice not to use zinc in brackish water arises. I suspect that it may be the Saildrive leg marketing department. We have supplied thousands of tonnes of zinc anodes for saline mud and brackish water applications and in place of magnesium in low resistivity soil applications.

Okay, I was using the Cruising World numbers, which are significantly different than yours, rather than some official source. And musta made an arithmetic error.

The corrosion products brush off of zinc easily during bottom cleaning. Wonder if the same is true for aluminium?

I’m all for AL anodes if they are better. Was mostly commenting on the atrocious writing. Have not seen steel cores in sailboat anodes. 

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The aluminium anode will contain approx 0.02% Indium in the alloy. This is known as an activation addition which makes the normally insoluable oxide film soluable, therefore exposing fresh aluminium for oxidation and so on. Of course there are many trade secrets hidden behind the actual nominal compositions so all anodes are by no means equal. Was thinking in particular of the Yanmar Saildrive leg with a stainless steel insert because admittedly many shaft anodes do not have a cast in insert and do rely on the bolt connection.

Also in my previous post the capacity for aluminium/zinc should read mAhrs/gm not mA/gm

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On 6/16/2018 at 10:06 AM, sledracr said:

Just picked up a new set (shaft and strut) anodes from West Marine, and they felt "light",  Turns out they're aluminum, not zinc, and claim to be good for "brackish water".

 

EM6LRZ.jpg

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