Mixing Alumin(i)um and Zinc Anodes

[Dsanduril]

Aluminum anodes seem to be all the rage. 50mV more negative than zinc, longer lasting (for the same physical size this is a bit of a red herring), more environmentally friendly, more active on boats that are on the move.

I can find aluminum anodes for my shaft no problem, but the specially shaped anode that goes on the J-Prop seems to be only available in zinc. I guess the first question would be, anyone seen a J-Prop anode in Al?

If not, then what is the problem with mixing Zn (on the prop) with Al (on the shaft)?

Our own Mr. D'Antonio wrote in Cruising World:

If you opt for aluminum anodes, be sure that the change is universal for all anodes used within the same bonding system.

Someone else says:

Different types of anodes may be used, just don’t intermingle them.

And Performance Metals (makes a lot of anodes) actually gives a bit of a reason:

Don’t Mix Anode Types. If you install different anode materials like zinc and aluminum, the more active anode (aluminum) will spend part of its effort protecting the less active metal (zinc). This will reduce the overall protection that you are getting. A classic mistake is adding a zinc transom anode to a Sterndrive equipped with aluminum anodes. Transom anodes are connected through the bonding system so make sure you install aluminum replacements.

In the real world, what actual difference does it make? If I put aluminum anodes on the shaft (and everything is perfectly bonded with 0mV potential difference) then won't a zinc anode on the prop sit there fat, dumb, and happy? And if for some reason I fail to change the Al before it is wasted, won't the zinc then take up the job (presuming it is not crusted over and inactivated)? What about pencil anodes in the engine? I don't even ask about their material, just get them from the engine dealer parts counter. Does it make a difference in the real world?

I guess I could understand if the total anode area I needed to protect things required the surface area of both the shaft and prop anodes, but IME I don't need all that surface area, it just allows me to go a little longer between anode replacements. And in the real world I suspect there is at least a few mV potential difference between my engine block and the prop hub.

Thoughts/experience/research?

[Tsuru]

I can't help with an answer but wonder about the same thing. Our boat is in fresh water but we sail in brackish and salt water. I use aluminum anodes but it's difficult to get aluminum alloy for every anode in the system (for example the pencil anodes for the raw water exchanger per your experience). Also the aluminum anode on our Maxprop seem to be a totally different mix of aluminum alloy than the prop shaft and strut. I've recently found that I can use Navalloy (aluminum voodoo mix) for everything except the Max prop so that's the way I'm heading.
I believe that all anodes are a mixture of elements designed to achieve protection. The point is you can use aluminum anodes but unless they are the same composition you could have the same hypothetical problem. I changed from zink to aluminum when our boat was in a busy salt water marina in Washington. The aluminum anodes showed much more action and would require change out twice per year where the zinks could easily go for more than a year. Interested in what the experts have to say...

[Shrew]

I was under the impression that, while they are all anodes, there are two variables at play here.

1) Where the metal rates in the galvanic series (nobility)

2) Where the metal rates in the galvanic series based on the environment.

While Magnesium, Aluminum and zinc are all low in nobility (anodic), they are not equal. Also, the relative position to one another in the galvanic series is based on the environment they are in (e.g. fresh water or salt water)

Saltwater: (list in descending order of nobility)
Aluminum
Uranium (pure)
Cadmium
Beryllium
Zinc plating (see galvanization)
Magnesium

In saltwater Aluminum is more noble than zinc.

I think the idea is to have all of your anodes working together.

[tkeithlu]

I'd be interested in knowing where you found the aluminum prop shaft anode. All of my anodes (14 of the dinner plates) are aluminum except the prop shaft, because I could not find one.

No, the mixture has not created a problem.

[Sailmonkey]

Originally Posted by tkeithlu

I'd be interested in knowing where you found the aluminum prop shaft anode. All of my anodes (14 of the dinner plates) are aluminum except the prop shaft, because I could not find one.

No, the mixture has not created a problem.

I get my Alu anodes from boatzincs.com.

Further to the question, my prop anodes are only available in zinc, and they’re EXPENSIVE! So if I can provide adequate protection for the prop and shaft with a pair of aluminum shaft anodes I’m willing to protect the prop anode for a time with the shaft anodes.

[Dsanduril]

Originally Posted by Shrew

...In saltwater Aluminum is more noble than zinc...

But only just barely (Zn reported at 1030 to 1050mV, Al at 1100mV), hence the question. The maximum difference between the two is only 70mV. Between the prop and shaft I would expect far less potential difference than that, but between the shaft and the engine block (where the pencil anodes are located)? I could easily see a bit of difference developing. If no difference then the Al on the shaft should protect everything (and your Zn anodes should last forever), if the difference > 70mV then the Zn in the pencils should protect the engine while the Al on the shaft deals with the underwater metals?

[Tsuru]

Originally Posted by tkeithlu

I'd be interested in knowing where you found the aluminum prop shaft anode. All of my anodes (14 of the dinner plates) are aluminum except the prop shaft, because I could not find one.

No, the mixture has not created a problem.

I've purchased 1" prop shaft aluminum anodes from West Marine, England Marine in Astoria, boatzinks.com, and others (seem to be easy to find). I purchased the Max prop aluminum anode from Max prop. As I recall you need to call and ask for them as they were not listed on line. I also seem to recall that aluminum Max Prop anodes are available boatzink.com.

[rslifkin]

Keep in mind when figuring out which anode is more noble, "aluminum" anodes aren't pure aluminum. They're an alloy of aluminum, zinc, and indium in most cases.

[Dsanduril]

Well, because of this thread I did find that Martyr lists my J-Prop anode in Al in their catalog. Now to see if I can find someone that actually has it (so far that's been a swing and a miss).

For the rest, I've found my metric shaft anodes in all materials at BoatZincs.com as others have noted. Defender carries the inch sizes and I'd guess that if I called and asked them they could get metric as well.

[RaymondR]

With the small differences in EMF produced I suspect the outcome might depend upon the length of the current path through the water. Some places with short current path the alloy protect the zinc, then a bit further a balance is achieved then further and no effect.

[Martkimwat]

I live close to one of the main manufactures of marine anodes in UK (M.G.Duff) I spoke to one of their technical reps about the difference between zinc and alloy anodes. Apparently Alloy anodes work much better in salt and particularly brackish water, than zinc. The reason zinc was always the norm was because for many years, zinc was cheaper than alloy. That is no longer the case. He stated that alloy anodes will work better and give longer protection for the same SIZE anode (not weight obviously) typically giving 25% longer protection. Like one of the contributors here, I could not get alloy nose cones for my Featherstream feathering prop, so they sold me a round billet of the particular alloy used for anodes and I turned up my own nose cone anodes for my Featherstream on my lathe.

[a64pilot]

The only effect of mixing anodes is that if they are all electrically bonded together, the aluminum ones will waste first. That means that they won’t last as long as if they were all aluminum.
I think you will still be protected though.
If they are not bonded together, I don’t believe it will matter.

[BruceS]

Must be a bit different terminology in USA?
Downunder we were advised to use zinc in salt water & magnesium (alloy) in fresh water.
If you have an alum boat or outdrive/outboard then will alum ones protect it?

[Dsanduril]

Zn for salt and Mg for fresh is the standard old advice. The Al anodes are newer, supposed to work better in salt, cost less (realistically the same), and last longer.

That's the catalog description, there are a fair number if caveats about keeping your anodes active (fresh surface exposed) when the boat sits still for a long time. This is true of all materials but the mechanisms differ.

And yes, works on Al hulls (better than zinc). The alloys used for hulls are different from those used for anodes, and the anodes are negative voltage compared with most hull alloys,

[bill good]

Tsuru, Do you have Galvanic isolator of some sort fitted to your boat?


regards