Considering Aluminum Anodes

I'm considering switching to aluminum engine pencil anodes.
6 months is about the max I can go. Wear rate seems unrelated to engine hours and the zincs most affected seem to be the ones that sit in raw water when the boat is not in use.
After 6 months a few of them will likely break off and lead to pulling off hoses and searching for pieces.

So considering cored aluminum anodes from Performance metals.

I have twin Cat 3208 NAs with 5 zincs on each engine.
I am in salt water 100% of the time.

Does 6 months sound like a short period of time for engine zinc wear especially if it's during my off season when the boat sits more?

Right now I have zinc anodes on the shafts/rudders/transom and everything is bonded. From reading, sounds like the aluminum anodes in the engines might protect the zinc anodes in the outside water and wind up depleting faster.
Anyone out there have experience with zinc on the outside of the boat and aluminum in the engines?

Thanks

Generally the engine anodes and hull anodes are considered to be in 2 separate bodies of water. They should not effect each other. So you can use different types of anodes, zinc or aluminum but not magnesium in salt water.

I use aluminum anodes throughout my boat as I regularly travel each year in fresh, brackish, and salt waters. The aluminum pencil anodes grow in diameter due to the aluminum oxide outer corrosion. As a result, they twist off just like zinc pencil anodes.

Ted

Ted,
Good to know on the twist off issue.
Sounds like no free lunch with the switch and you have to choose which devil you want to put up with.
If you were purely in salt water, would you prefer zinc or aluminum?
Thanks

ORIF said:

Ted,
Good to know on the twist off issue.
Sounds like no free lunch with the switch and you have to choose which devil you want to put up with.
If you were purely in salt water, would you prefer zinc or aluminum?
Thanks

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Purely in salt water I would probably still use zinc. My guess is that zinc will become environmentally unfriendly at some point, and expect aluminum to become the defacto standard.

On my next anode change, I'm considering switching the 1/2" heat exchanger anodes to 3/8". Would use them in conjunction with a 1/2" to 3/8" bushing. While I recognize that the anode will get used up faster, I'm hoping that I can pull the bushing out without twisting off the aluminum anode. We shall see. Nothing beats trying, except failing.

Ted

Fast erosion means they are doing their job. Why would you want to use something that offers less protection? IMO six months is not unusual in salt water. The galvanic half cell reaction potential of AL is much lower than ZN.

Zincs that twist off the brass plug and remain inside the engine can be removed by screwing a threaded rod coupling nut onto the zinc and pulling it out.

I can go 1 year on my diver dream zinks but only 6 months on my shaft and engine zinks. I have twin cat 3208’s with 5 zinks per engine.

If you think zincs are eroding too fast start checking for stray currents. If you haven't opened all your dock power cord connections in a while start there.

My boatyard suggested Al for our old boat that was stored in fresh water and only spent a few weeks per year in the salt. But in salt water they recommended Zn.

ORIF said:

Ted,
Good to know on the twist off issue.
Sounds like no free lunch with the switch and you have to choose which devil you want to put up with.
If you were purely in salt water, would you prefer zinc or aluminum?
Thanks

Click to expand...

Zinc in pure salt water. Readily available, inexpensive, function as designed.

******Right now I have zinc anodes on the shafts/rudders/transom and everything is bonded. From reading, sounds like the aluminum anodes in the engines might protect the zinc anodes in the outside water and wind up depleting faster.******

Won't work like that. The two systems are sufficiently isolated so one cannot take over the function of the other except maybe absolutely minimally. Turn it around, If you removed the engine zincs the external zincs would not be able to protect the engine parts from corrosion.

Proximity, an important factor, to the metal protected must also be maintained or the anodes lose their effectiveness.

That's why engine parts have their own anodes and do NOT depend upon the external anodes..

If the engine zincs are corroding faster than you like then you need to figure out THEIR cycle. They will not adapt to your cycle.

Of course ensure you don't have electrical problems that cause trouble but if the external zincs are lasting then it is unlikely the internal engine zincs have a problem like that. Some anodes, because of their size vs the size of the metal they protect are undersized to last long. That cycle must be figured out and allowed for.

The only way you can beat the anode life is add more to the circuit. Maybe another anode in another drain hole if available. If there is room then maybe a larger anode. But often there is no way short of a total redesign of the part to add pencil mass in many engine and gearbox coolers.

Yes, the hull anodes and engine anodes are 2 separate bodies of water. I use aluminum anodes on the hull, shafts, etc. I use magnesium anodes in the engine because that is what is made in the size I need for the engines. And it is perfectly ok to use 2 different anode types for the 2 different areas. Now I couldn’t use 2 different anode types on the hull, shafts and things immersed in the same body of water.

bayview said:

The galvanic half cell reaction potential of AL is much lower than ZN.

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That is incorrect according the the anode manufacturers. Al anodes are a specific alloy, not just any Al. For example Performance metals lists the galvanic potential of Zn anodes as -1.05 and Al as -1.1V Other manufacturers similar. If you are looking at a galvanic chart, typical aluminum alloys are around -0.6 to -0.7V, but that isn't the alloy used in anodes (MIL-A-24779). Aluminum anodes protect better than zinc, not worse.