Aluminum Zincs?

Ken Cross

Ken Cross

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Oct 24, 2010
2,405
Hunter 30 Everett, WA
Last time I hauled my boat I talked myself into trying aluminum zincs. From what I read they should do well in salt water (but don't use them in fresh).

I pulled them off my prop shaft today and they were completly covered with hard growth. They were also eroded (Which tells me they work) but I was amazed at the growth. Now my question: How could they ever work? It seems like there needs to be something to keep the growth off of them.

Ken
 
CloudDiver

CloudDiver

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Sep 8, 2014
2,551
Catalina 22 Swing Keel San Diego
Ken, aluminum works well in salt water if they are scrubbed regularly and/or props are run daily on working boats. If AL sits too long it creates a hard oxide coating which actually protects it and allows marine growth to take hold which insulates the bare metal from the salt water even more. Keep them scrubbed clean and you won't have issues.
 
Will Gilmore

Will Gilmore

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Oct 19, 2017
7,744
O'Day 19 Littleton, NH
What would be the advantage of aluminum over zinc and why not use them in fresh water? I can see that they wouldn't likely do anything, but then galvanic action is minimized anyhow.

-Will (Dragonfly)
 
D

dlochner

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Jan 11, 2014
11,401
Sabre 362 113 Fair Haven, NY
Will Gilmore said:
What would be the advantage of aluminum over zinc and why not use them in fresh water? I can see that they wouldn't likely do anything, but then galvanic action is minimized anyhow.

-Will (Dragonfly)
Click to expand
Aluminum is recommended in both salt and fresh water. For more information check BoatZincs.com. Zinc will not work in fresh water as it is not reactive enough and it forms a hard coating the insulates it from the water.
 
thinwater

thinwater

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Mar 26, 2011
3,410
Corsair F-24 MK I Deale, MD
Not in fresh because you will be using Mg in fresh (zinc won't work either).

Actually, the main reason for the difference in growth is that aluminum is not toxic, zinc is. The oxide layer is also true and is the reason aluminum pits more than zinc (it is either active or protected). But Al is also effective without scrubbing, as you learned; I did a bunch of static testing. It just gets weird looking. The performance was actually not very different from Zn.
 
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Ken Cross

Ken Cross

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Oct 24, 2010
2,405
Hunter 30 Everett, WA
Gunni said:
How long ago did you change your aluminum anode?
Click to expand
I just pulled the aluminum off. They were in the water for a year. The diver said they were 75-80%. But when I looked at them after putting the boat on the hard, they were all encrusted and I would say at 50% of useful life. I'm putting zinc back on.
Ken
 
Gunni

Gunni

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Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
50% gone, or one year, which ever comes first is the rule for anodes life. Aluminum anodes work better than zinc when you get significant times of low salinity. In years when we have seen lot of rain and the salinity in our estuary is low, zincs salt over and quit working within weeks, aluminum does not. This year so far the upper Chesapeake has seen more rain than Seattle, and the Susquehanna River has brought more down from upstate New York, so we are practically freshwater. Aluminum anodes are necessary.

You don’t want to corrosion damage your prop and underwater metals so a workable solution may require that your diver is scrubbing the anode to keep it working. Salting over or bio growth are both concerns.
 
thinwater

thinwater

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Mar 26, 2011
3,410
Corsair F-24 MK I Deale, MD
Yeah, the Bay was not very salty this year!

My testing was done in Deale, MD in a high salinity year. The point, of course, was to test brackish water performance. The bottom line was that there was not much difference in performance and either would be fine.
 
JamesG161

JamesG161

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Feb 14, 2014
7,418
Hunter 430 Waveland, MS
I don't want to spoil the conversation, but Cathodic Protection works by the following electrochemical potentials.
GalvanicSeries.png

in sea water. Lowering the salinity lowers the potentials, but not the order in the list.
____
Since good sacrificial Anodes require that they sacrifice electrons faster than the protected metals, several factors determine the rate of metal loss of the Anode.
1) Water salinity
2) Water Temperature
3) Exposed Surface Area of the Anode.
4) All submerged metals must be electrically connected to the Anode.

A Zinc Anode and an Aluminum Anode are NOT interchangeable, even in sea water.
You must change the Surface Areas exposed to interchange them!
______
Encrustation of an Anode suggests other effects are in play.
A reputable Anode maker impregnates the pure metals and casts them to allow for surface Oxidation.

My first guess is...
Not enough Surface area of either Anode or you added more submerged metals to your boat.

If you are using a shaft "Egg" Anode, try adding 2 or more.;)
Jim...
 
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Roland5048

Roland5048

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May 12, 2004
1,505
Hunter Cherubini 30 New Port Richey
I just bought two "egg" shaft anodes from West marine. They felt much lighter in weight than previous zinc ones but are the same size. There are no markings other than the size and 'Made in China'. Could they be aluminum? How do I tell without going to a metallurgist?
 
jssailem

jssailem

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Oct 22, 2014
21,088
CAL 35 Cruiser #21 moored EVERETT WA
I heard on the radio that zinc is good to reduce the severity of a cold. So licking them to see if you stay healthy might be an option...

But I would ask the manager what material is being sold. There was a change over to Aluminum last year. I stopped buying them from our local WM. Then they started a change of ownership and new management may have returned to zinc zincs. So ask.
 
Will Gilmore

Will Gilmore

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Oct 19, 2017
7,744
O'Day 19 Littleton, NH
Licking aluminum has a completely different effect on you from shortening your cold, so that's another way to tell.

-Will (Dragonfly)
 
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D

dlochner

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Jan 11, 2014
11,401
Sabre 362 113 Fair Haven, NY
Aluminum anodes tend to be lighter in weight and color than zinc. They are slightly shinier and the gray metallic color is a lighter shade.

In the photo, from left to right zinc, aluminum, and magnesium. The zinc was in the water for more than 1 season, the aluminum for 1 season, and the magnesium 2 seasons.

IMG_0536 (2).jpg


The best way to determine how much has eroded is a before and after weight.

Edit: All three were in freshwater.
 
Roland5048

Roland5048

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May 12, 2004
1,505
Hunter Cherubini 30 New Port Richey
jssailem said:
I heard on the radio that zinc is good to reduce the severity of a cold. So licking them to see if you stay healthy might be an option...

Yea, next time I get a cold I'll forgo the pharmacy and just go to WM. :)

But I would ask the manager what material is being sold. There was a change over to Aluminum last year. I stopped buying them from our local WM. Then they started a change of ownership and new management may have returned to zinc zincs. So ask.
Click to expand
Thanks for the info on that. I did ask the clerk but she was clueless. When I see the manager, I'll ask her.
 
thinwater

thinwater

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Mar 26, 2011
3,410
Corsair F-24 MK I Deale, MD
James: Aluminum anodes are NOT aluminum. They are a specific MIL spec alloy that has potential very close to zinc. You can find the details on-line from CMP or others. In brackish water, after exposure, the potential and current were very close. I'm sure this will vary from year-to-year.
 
Charles Erwin

Charles Erwin

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Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
All you need is a 200 mv negative shift to protect the metal(s) involved. Very easy to measure. Either anode - aluminum or zinc - works just fine. Endless debate over the question is wonderful but there is no substitute for fact - in this case a simple, cheap measurement of potential will provide a reliable answer.
 
D

dlochner

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Jan 11, 2014
11,401
Sabre 362 113 Fair Haven, NY
Charles Erwin said:
All you need is a 200 mv negative shift to protect the metal(s) involved. Very easy to measure. Either anode - aluminum or zinc - works just fine. Endless debate over the question is wonderful but there is no substitute for fact - in this case a simple, cheap measurement of potential will provide a reliable answer.
Click to expand
See, there you go again. Letting facts and science get in the way of a good argument. ;)
 
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