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Old 25-03-2019, 08:59   #1
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zinc versus aluminum galvanic protection

We sail out of Deale, MD about mid-bay on the Chesapeake and it was advised at a seminar recently that aluminum "zincs" were a better option for us than zinc "zincs" because of the lower salinity of the water. Three questions: at what point does this become true, if in fact it is true? And doesn't salinity vary from Spring with the heavier run off of fresh water to late summer? Last, if one of my "zincs" is aluminum and in close proximity to another that is zinc am I setting off a bad situation?
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Old 25-03-2019, 09:17   #2
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Re: zinc versus aluminum galvanic protection

Aluminum anode alloy provides more protection and lasts longer than zinc. It will continue to work in freshwater, and is safe for use in salt water. Aluminum is the only anode that is safe for all applications. Don’t mix zinc anodes on the hull with aluminum anodes on a sail-drive. The aluminum anodes will protect the zinc anodes, as well as the outdrive.
https://cdn.shopify.com/s/files/1/22...es/ABYC_09.pdf
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Old 25-03-2019, 15:31   #3
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Re: zinc versus aluminum galvanic protection

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Originally Posted by GordMay View Post
Aluminum anode alloy provides more protection and lasts longer than zinc. It will continue to work in freshwater, and is safe for use in salt water. Aluminum is the only anode that is safe for all applications. Don’t mix zinc anodes on the hull with aluminum anodes on a sail-drive. The aluminum anodes will protect the zinc anodes, as well as the outdrive.
https://cdn.shopify.com/s/files/1/22...es/ABYC_09.pdf
That's what I was told, as well.

But I was also told that using the zinc pencil inside the sail drive was OK even when you had an AL sail drive anode
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Old 25-03-2019, 22:31   #4
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Re: zinc versus aluminum galvanic protection

Dunno about that?

The corrosion potential ranges in volts taken from ABYC Standard E2 are:

Zinc, -0.98 - -1.03

and

Aluminium alloys -0.76 - -1.00

which would have the less noble zinc protecting the more noble aluminium.
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Old 26-03-2019, 01:46   #5
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Re: zinc versus aluminum galvanic protection

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Originally Posted by RaymondR View Post
Dunno about that?

The corrosion potential ranges in volts taken from ABYC Standard E2 are:

Zinc, -0.98 - -1.03

and

Aluminium alloys -0.76 - -1.00

which would have the less noble zinc protecting the more noble aluminium.
I was wondering how effective aluminium anodes could be in protecting aluminium?
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Old 26-03-2019, 02:07   #6
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Re: zinc versus aluminum galvanic protection

Quote:
Originally Posted by RaymondR View Post
Dunno about that?

The corrosion potential ranges in volts taken from ABYC Standard E2 are:

Zinc, -0.98 - -1.03

and

Aluminium alloys -0.76 - -1.00

which would have the less noble zinc protecting the more noble aluminium.
Aluminium alloys ain't aluminium alloys! or so the advertising clowns would tell you.

What GordMay was pointing to was MILSPEC 24779 aluminium, which is 99.8% aluminium.

And it's supposed to deliver between -1.05 volts and -1.15 volts (when tested with an Ag/AgCl half cell).

So it's not just scrap window frame aluminium alloy, old pop cans, or that bit of mast you cut off. Only works if it is made to spec.

People who know more than me reckon the Al anodes are more environmentally friendly than Zn ones.

You can probably read more at: boatzincs.com/pdfs/Mil-Spec%2024779B(SH).pdf
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Old 26-03-2019, 04:32   #7
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zinc versus aluminum galvanic protection

I’ve switched to aluminum anodes for use in Galveston bay TX. Our marina basin is more fresh than salt most of the year, rendering the zinc anodes useless. The aluminum delivers protective potential no matter the water salinity that we experience here.
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Old 26-03-2019, 05:58   #8
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zinc versus aluminum galvanic protection

Quote:
Originally Posted by RaymondR View Post
Dunno about that?

The corrosion potential ranges in volts taken from ABYC Standard E2 are:

Zinc, -0.98 - -1.03

and

Aluminium alloys -0.76 - -1.00

which would have the less noble zinc protecting the more noble aluminium.


I have hung an aluminum anode “Guppy zinc” and had a zinc prop anode on.
The zinc no longer wastes away when the aluminum anode is in the water.

So far for me, I it seems the aluminum anodes are more “active” and waste away much more evenly.
I’m not often in brackish water, but see no upside to zinc, I’m changing over to aluminum
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Old 26-03-2019, 10:07   #9
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Re: zinc versus aluminum galvanic protection

Two cents...

When working for West Marine, we always used to say to "zinc for saltwater, aluminum for brackish water, magnesium for freshwater". This changed somewhat when Canada Metals started promoting aluminum anodes for all waters. I was never sure if this was a change in their composition that made aluminum different than before, or what. Certainly there is a movement against certain metals being allowed to concentrate in harbors in California, like copper and cadmium (found in some anodes).

I changed to aluminum ball zincs on my Alerion about four years ago (in saltwater) and did not see any deterioration in protection, but I really can't say if they lasted longer than the previous zinc versions.

Chuck
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Old 26-03-2019, 11:08   #10
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Re: zinc versus aluminum galvanic protection

We sail out of Annapolis, MD which is just up the road from Deale. We switched over to aluminum a number of years ago for the reasons a number of folks on here have mentioned.
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Old 26-03-2019, 11:45   #11
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Re: zinc versus aluminum galvanic protection

I too am all aluminum for cathodic protection.
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Old 26-03-2019, 11:57   #12
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Re: zinc versus aluminum galvanic protection

Quote:
Originally Posted by Chuck Hawley View Post
Two cents...

When working for West Marine, we always used to say to "zinc for saltwater, aluminum for brackish water, magnesium for freshwater". This changed somewhat when Canada Metals started promoting aluminum anodes for all waters. I was never sure if this was a change in their composition that made aluminum different than before, or what. Certainly there is a movement against certain metals being allowed to concentrate in harbors in California, like copper and cadmium (found in some anodes).

I changed to aluminum ball zincs on my Alerion about four years ago (in saltwater) and did not see any deterioration in protection, but I really can't say if they lasted longer than the previous zinc versions.

Chuck

We are leaving Lake Ontario (magnesium anodes) for the St. Lawrence and (by fall) Nova Scotia, where we will have the hull redone for barrier coat and the change from ablative to hard anti-foul topcoat. We have gone to aluminum anodes at the counsel of CMP. We have a steel sailboat and proper protection in this regard is a priority. I was told pretty much the same story concerning the grade of aluminum and its ability to "protect".

SS pads welded to bare mild steel and SS bolts that will carry the "Martyr Diver's Dream" aluminum alloy plates. YMMV.
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Old 26-03-2019, 12:04   #13
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Re: zinc versus aluminum galvanic protection

Quote:
Originally Posted by NightSky46 View Post
We sail out of Deale, MD about mid-bay on the Chesapeake and it was advised at a seminar recently that aluminum "zincs" were a better option for us than zinc "zincs" because of the lower salinity of the water. Three questions: at what point does this become true, if in fact it is true? And doesn't salinity vary from Spring with the heavier run off of fresh water to late summer? Last, if one of my "zincs" is aluminum and in close proximity to another that is zinc am I setting off a bad situation?

As luck would have it, I did a series of long-term tests for Practical Sailor mag in Deale, MD (across from the fuel dock), using multiple brands, aluminum alloy and zinc.

Bottom line: In Deale you'll be fine either way, though it may depend on EXACTLY where you are, since salinity varies considerably according to location in the creek and rainfall. Aluminum is a little better.

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Old 26-03-2019, 12:48   #14
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Re: zinc versus aluminum galvanic protection

The mistake most people make in researching "aluminum" anodes is looking at "aluminum" on a galvanic scale.

"Aluminum" anodes are an alloy of aluminum, indium and even a little zinc. I have not yet seen a galvanic table include this alloy.
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Old 26-03-2019, 12:51   #15
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Re: zinc versus aluminum galvanic protection

Quote:
Originally Posted by S/V Alchemy View Post
We are leaving Lake Ontario (magnesium anodes) for the St. Lawrence and (by fall) Nova Scotia, where we will have the hull redone for barrier coat and the change from ablative to hard anti-foul topcoat. We have gone to aluminum anodes at the counsel of CMP. We have a steel sailboat and proper protection in this regard is a priority. I was told pretty much the same story concerning the grade of aluminum and its ability to "protect".

SS pads welded to bare mild steel and SS bolts that will carry the "Martyr Diver's Dream" aluminum alloy plates. YMMV.
As a Certified Corrosion Analyst I'd have serious concerns about welding stainless steel to mild steel as they are so far apart on the galvanic scale. Your hull will become the anode to the stainless cathode. Crevice corrosion in the stainless is also highly likely.
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