Best anodes for sea water: zinc or aluminum

[MapisM]

I recently learned, based on some documentation from a couple of anodes producers, that there are anodes made of an aluminum alloy which are allegedly better than the traditional zinc ones in just about every way (strictly talking of sea water):

- more longlasting, while offering the same level of protection if not a tad better;
- significantly lighter - about half the weight of equivalent zinc anodes - obviously an advantage particularly on moving parts like rudders and shafts;
- slightly cheaper - though the price difference is nowhere near enough to be relevant for the comparison.

Now, I am ordering zinc anodes at the moment, on the "if it ain't broken, etc." basis.
But the above got me thinking, because it sounds too good to be true - to the point of wondering why zinc anodes are still produced at all.

So, I thought to check the forum consensus, if any.
Wadduthink, folks?

 

[Andy Cox]

Mapis, my understanding is that zinc is for salt water and aluminium should be used in tidal brackish water.
Andy

 

[MapisM]

Yup, that used to be my understanding, too.
Otoh, according to the documentation of two producers (FONP and Tecnoseal, which are the ones with a better reputation, at least down here), it's the other way round - sort of: zinc is no good in brackish water, because over time it can develop a sort of "crust" on its surface which restricts its protection capacity, while alu doesn't.
BUT, and it's a big but, according to them both materials are perfect for salt water, with alu arguably even better, on the basis of what I previously summarized.
Mind, both specify that when talking of alu anodes, they are not actually made of plain vanilla aluminium, but it's an alloy specifically designed for the job.

 

[Fishtigua]

I know that Mercruiser outdrives are designed for alloy anodes only, never zinc. However brand new Volvo outdrives often come with ali anodes that don't last 5 seconds in saltwater and you must check/change them before launching a new boat.

 

[Greg2]

We used aliminium when moored in salt water and they were fine. Always fit them now for a mixture of fresh and salt cruising.

 

[Murv]

When I last bought anodes, I visited the supplier in person (I wanted zinc)
He eventually convinced me that aluminium was the way to go and I finally gave in.
The new anodes are a lot larger, lighter and cheaper and after 18 months I'd choose them again.

 

[rlw]

From personal experience on outdrive they seem to work better in brackish water. However I found on salt water they struggled to last the season with the exception of the big bar anode under the drive. If used on a shaftdrive I would increase the anode size and keep a close eye on wear rate. In summary I think they work better but wear faster.

 

[dpb]

I know that Mercruiser outdrives are designed for alloy anodes only, never zinc. However brand new Volvo outdrives often come with ali anodes that don't last 5 seconds in saltwater and you must check/change them before launching a new boat.

In thirty odd years of outdrive ownership, mostly Mercruiser, I never knew that!!! Have always used Zinc and they seem to have done their job.
So how does an aluminium anode protect an aluminium drive?
I shall have to look into this some more.......

 

[Portofino]

Longevity should not really be a decider or price with shafts in the Med .
After all solid gold will never need replacements it’s inert .


There in lies the problem .For the anode to do its job it needs to be more electrochemical active by a considerable margin than the metal it’s proporting to protect .
Zinc , pure zinc (99.9 % ) seems to last long enough if sufficient weight in most shaft drive set ups , about 1 year. Rule of thumb about 1/2 worn to have enough critical mass to continue depleting enough protection.

Of course you can blend metals they are called alloys .That word is not to be confused or interchangeable with Aluminium.
So you may blend your zinc with say a less electrochemical active metal say Aluminium so,s to reduce the depletion rate thus lasting longer obviously.
But the protection is from the release of the correct sub atomic particles in sufficient quantities.

Sufficient quantities being the important part of the equation.

If by using an alloy of blended zinc with [ insert your metal (s) ] to extend the life - there’s no argument there on the longevity points, but what about the efficacy?
Will there still be sufficient quantities of the relevant subatomic particles released to protect your stearn gear ? that’s the killer Q , only answerable next time you lift your boat and inspect or worse still 2/3 years down the line the damage ( that’s started from day dot with the new alloys ) manifests itself when it’s too late .

Personally we are in the same situation as you salty Med and shafts I would stick with pure zinc .
Btw there’s a case to stay with zinc more in a SALTY med vs ave sea salt concentration like the Channel , the medium ( inc salt conc ) is more amenable to electrochemical reactions .

You have the added worry of balancing the electrochemical effects of your copper coat with your anodic protection. Good luck with that .As said before I fail to understand how in one breath the copper is wrapped up in resin thus rendered inert and in the other it’s constantly exposed to give antifoul properties? Huh !

So wearing my Chemistry hat I would think you need to think more going in the other direction of a more electrochemical reactive substance, since Zinc is about as good as it gets then bigger zincs and or a more frequent change interval say every 6 months .
Because you want to over flush everything with MORE subatomic protection , just in case the case there’s a reaction between the copper and any metals it’s in contact with .Sort of huge dose to snuff out any adverse reactions from acres of copper plastered on your hull ,

 

[Andy Cox]

I don't think I'd be swayed to go for aluminium. I'd rather stick with zinc, which I think is the best option where my boat is and accept I will change them annually.
Andy

 

[BruceK]

Aluminium has been shown to be the best. The market has yet to catch up.

I am in brackish water with the topmost layer being fresh at ebb and get a calciferous coating on my zincs so did some research a while back. You do need to change all anodes to aluminium if that is the direction you take though.

https://www.cruisingworld.com/how/zinc-and-aluminum-sacrificial-anodes

https://www.absolutemarine.co.nz/image/catalog/documents/anodes/truth_aluminum_anodes.pdf

 

[MapisM]

You do need to change all anodes to aluminium if that is the direction you take though.

Yup, that's what I learned, too.
Which is also the reason why I'm not switching to alu yet, at least for this season: I have a few peculiar anodes in my boat for which I still have a few zinc spares that I'd rather not throw away.
Btw, since you raised this point, do you (or anyone else) know if the need to use the same material for all anodes applies also to internal anodes (engines/genset and their exhausts systems), or is it sufficient to use the same material for all the external anodes (hull+u/w gear)?

 

[BruceK]

That rule / advice is for all anodes on the same bonding system. So I would say yes.

 

[NBs]

Hi,

I have always had 99.9% zink anodes and here the salt content is only 0.6% vs Atlantic 3.5% and they have worked brilliantly, that is, they corrode electrochemically. For a new season, I get the Performance metal anodes they market to fit all types of water and even work in salt water more economically than zink and give better protection, time to show or have you experienced technoseal anodes with a patented metal alloy?

"The Only Anode That Works in All Types of Water

The aluminum alloy used in Navalloy anodes is very different from normal aluminum. It includes about 5% zinc and a trace of Indium, which prevents the build up of an oxide layer.

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.

Better Protection

Navalloy® has a higher protection voltage than zinc.

Longer Life

Navalloy® lasts up to 30-50% longer than zinc."

https://performancemetals.com/


NBs

 

[Momac]

My boat with VP outdrives is in fresh water with very occasional salty water trips and usually a two week holiday spent mostly on salty water
The aluminium outdrive anodes have worked well with no apparent corrosion on the outdrives.
I changed the leg anodes last in March 2018 which was after 2 years use .
I bought them from Solent Anodes.
https://www.solentanodes.co.uk/collections/aluminium-volvo-anodes

.

 

[TheOrs]

So how does an aluminium anode protect an aluminium drive?

They are different alloys. The aluminium the outdrive is made from is designed to resist corrosion, the anodes are designed to corrode at the correct rate for the application.

Aluminium is the standard for Mercruiser and is what you usually get unless you specify otherwise. In my case I bought a boat with a mix of zinc and aluminium and found the aluminium worked whilst the zinc calcified. (As soon as I realised I had a mix I changed out the zinc for aluminium.)

 

[MapisM]

For a new season, I get the Tecnoseal anodes they market to fit all types of water and even work in salt water more economically than zink and give better protection, time to show or have you experienced technoseal anodes with a patented metal alloy?

Tecnoseal anodes (zinc version) is what I used last season, and they worked brilliantly. I suppose the version you are referring to is the alu ones, whose part numbers in their catalogue ends with "AL" (as opposed to no letters for zinc, and "MG" for magnesium).
I'm not sure if Tecnoseal managed to get any patent on their own alu alloy, but pretty sure they are not alone in offering alu anodes.
"Alu" always meaning a special alloy of course, obviously different from any structural alu parts used in outdrives or whatever.
Anyway, fwiw, material aside, here in IT Tecnoseal stuff is considered the best, and I don't think you will get wrong with that.
I will be interested to hear about your experience!

 

[MapisM]

I bought them from Solent Anodes.
https://www.solentanodes.co.uk/collections/aluminium-volvo-anodes

As far as I can tell from the website you linked, they are also selling Tecnoseal stuff. :encouragement:

 

[QBhoy]

Hi
Pretty definitively;
Zinc are for sea water
Magnesium are for fresh water
Aluminium are for brackish or a mix of both and are also what the OEM will play it safe with, as in with merc and VP outdrives. They will not offer the same protection as a zinc in SW, for certain.

 

[MapisM]

They will not offer the same protection as a zinc in SW, for certain.

Well, that sounds more like your opinion, rather than something certain.
Not that I have any more "certain" knowledge, mind - otherwise I wouldn't have posted this thread, to start with...
But according to some anodes producers, alu is as good as zinc also in salt water.