Aluminum Anodes

[Alec]

I recently moved my boat to a new marina where the water is brackish and it seemed logical to switch to an aluminum anodes. Aluminum shaft anodes are easy to find but I've not been able to find an aluminum pencil anode for the heat exchanger. I'm concerned about mixing the two. Or does it matter?

 

[centerline]

technically, it wont cause any harm in mixing the two, other than the area that does not have the less noble anode may not be as well protected... you should be able to find aluminum, magnesium and zinc anodes in equal configurations, but you may need to turn to the internet to find the exact type/configuration for your application, as few retail outlets will carry all of the options made...

 

[justsomeguy]

I would think you could manufacture your own aluminum anodes.

At least, I think you could.

 

[JamesG161]

I would think you could manufacture your own aluminum anodes.

If you can find a source of stock shapes of anode grade aluminum, sure!
Scrap aluminum masts/hulls would not work though.
Pure aluminum is useless as an anode material because it immediately forms an oxide layer, like my anodized mast.
Add a dash of zinc and a pinch of indium to prevent aluminum from "scabbing over" and you have a sacrificial Aluminum anode.

BTW zinc anodes are not pure zinc, either.

I'm concerned about mixing the two. Or does it matter?

Not for corrosion protect, but they "sacrifice" at different rates. Don't touch them together in the same service or the zinc will sacrifice the aluminum faster too.
Jim...

PS: Indium has a reputation of "Screaming" when bent. Such a sissy metal!

 

[JamesG161]

I'm concerned about mixing the two. Or does it matter?

These guys have USA made zinc, aluminum, and magnesium anodes. They might know about your pencil needs.
http://www.boatzincs.com/
Foreign made zincs are suspect.

BTW Aluminum works in salt water too. All fresh water does is conduct less galvanic current and thus the slower rate of loss of the sacrifical anode. Magnesium would be preferred in any service but in salt water and the cost of magnesium is so high, zinc or aluminum is more economic and available.

But, you can put a zinc pencil and aluminum shaft anodes at the same time on your boat.
Jim...

 

[Alec]

Tha

 

[dhays]

These guys have USA made zinc, aluminum, and magnesium anodes. They might know about your pencil needs.
http://www.boatzincs.com/

BTW Aluminum works in salt water too. All fresh water does is conduct less galvanic current and thus the slower rate of loss of the sacrifical anode. Magnesium would be preferred in any service but in salt water and the cost of magnesium is so high, zinc or aluminum is more economic and available.

But, you can put a zinc pencil and aluminum shaft anodes at the same time on your boat.
Jim...

I think the OP was going to be in brackish water so I think Magnesium would be a bad idea. Performance Metals is another online source of aluminum anodes. I have not used them myself, only the above mentioned boatzincs.com.

Jim, what about aluminum pencils in the heat exchanger and after cooler (don't ask) and zincs on the exterior? Would the aluminum engine anodes be working to protect the exterior zincs? I ask because I am thinking of using a fresh water flush of the raw water system but am concerned about the engine zincs becoming passive while sitting in the fresh water in the engine between uses.

 

[JamesG161]

Jim, what about aluminum pencils in the heat exchanger and after cooler (don't ask) and zincs on the exterior? Would the aluminum engine anodes be working to protect the exterior zincs? I ask because I am thinking of using a fresh water flush of the raw water system but am concerned about the engine zincs becoming passive while sitting in the fresh water in the engine between uses.

I am not sure of what is protecting what in your case and a lecture on galvanic corrosion is not what you wanted.
But...
Any sacrificial anode will protect any metal ( more Noble), it is "electrical contact" with until the anode is essentially gone.

I don't understand how you have an engine zinc and an aluminum zinc in the engine raw water system.

Most sacrificial zinc alloys are designed not to become "passive".

Water of any non-distilled, salinity is just like shorting out a battery (galvanic corrosion). The more electrical current passed by the water, the faster that an Anode "dissolves". Fresh water passes less electrical current than sea water. Distilled water passes NO electrical current, at least for a while.

If you want to send me a private message, we can figure it out and then post our conclusions here for others to perhaps benefit.
Jim...

 

[Charles Erwin]

David

Even if you flush with fresh water use zinc in the heat exchanger or after cooler. Just check them it once every 4 months or so to verify they are not scaled.

There will be no interplay between those anodes located in the motor cooling system and those on the shaft, prop or anyplace else outside the motor.

Charles

 

[dhays]

Thanks Jim and Charles.
My question was poorly worded. I am looking at setting up a fresh water flush of the Raw water system that passes through the heat exchanger and aftercooler. There are two anodes in the aftercooler and one in the heat exchanger. The idea is to flush out the salt from the raw water side so that fresh water is sitting in the aftercooler and heat exchanger rather than fresh.

My concerns to were two-fold. The first is whether sitting in the fresh water would cause passivation of the zinc anodes. If so, I was thinking of using Aluminum anodes. They would like work better than zinc anyway and work fine in fresh water. They are more expensive than zinc, but likely by about the same as the increased lifespan so probably a wash. Unless they are more prone to fall apart when being removed....

The second concern was whether there was any problem with using Aluminum in the engine and zinc on the shaft, prop, and strut. From what you both say, it doesn't sound like that will be a problem.

 

[JamesG161]

I think I understand your desired direction.
1) You want to Fresh water flush your engine raw water cooling system.
2) You think Aluminum might be better than zinc in fresh water.

There is NO unexpected corrosion from Aluminum/Zinc anode mix, on the metals they are intended to protect!

Mix away!!!

Probably this is unwanted advice (my boat is about same specs as your)...

I wouldn't flush the sea water system with fresh water unless your down time would allow barnacle growth. Flushing will have negligible effects on galvanic corrosion and would probably increase overall corrosion by the dissolved Chlorine gas used for municipal water treatment. Chlorine loves to attack Aluminum.

Check the electrical continuity between each of your exchanger, after cooler and engine. Use a volt meter in Ohms mode. If they all show low resistance of say less than 10 ohms, your shaft zinc does them all. If not, then that explains the separated anodes (they wouldn't significantly effect each other in this case).

Lastly... Pacification of Anodes.
From a reputable source.... NOPE!
Pure zinc and pure aluminum and pure steel (all pure metals), pacify themselves sitting around in open air.
The do NOT pacify when exposed in water to galvanic (electrical type) corrosion.

I have heard of many on this forum who clean and polish their zincs.
Why? Shrugs.

I recently opened and inspected my engine exhaust riser (14 yr old cast iron). It had no wear and had iron corrosion. My helper reached for a wire brush to clean the inside. STOP! Don't clean off the protective barrier from sea water and acidic exhaust gases. THAT IS NOT GALVANIC CORROSION! I was protected from galvanic corrosion by my shaft zinc.

Jim...

 

[Gunni]

Why worry about your engine anode status, replace them every season at commissioning. And anodes will passivate themselves as the resistivity of the water increases (salt -brackish-fresh). Couple that with the higher electropotential of magnesium or aluminum and the solution to having your anode salt-over and passivate itself is an anode that is more electrically active (Aluminum, or in fresh, magnesium) We see this effect very commonly in brackish water where zinc anodes sit useless on their mounts, looking more like white blobs, than anodes. If it isn't dissolving, it isn't working. A working anode looks metallic, and corroded.

 

[Charles Erwin]

Electrical continuity between a motor block and its intercooler, heat exchanger, oil cooler is largely irrelevant because the shaft anode is not immersed in the 'same' body of sea water as those in the sea water side of cooling units. Since the mass of sea water in the motor cooling system is essentially isolated from outside sea water - galvanic protection offered by an external anode is nearly zero. In other words, if external anodes offered any serious protection to the cooling system components there would be no reason for separate cooling system anodes.

In addition, the alloys used in these coolers are typically very resistant to galvanic activity - and the comparative amount of dissimilar metals in these units is very small. Thus - cooling jacket anodes can be viewed more as guards than active warriors. The normal replacement interval at 50% loss typically takes 2 or 3 years.

Magnesium anodes in motor cooling sea water - that is when you are using the boat - would not be a good choice. Aluminum is not so good sitting in chlorinated city flush water. So zinc is probably the better choice.

In any case - check cooling jacket anodes every so often and replace as your own wear rate dictates. I really doubt flushing will have any significant impact on internal anode wear rates.

Edit: Likewise there is little practical difference in the choices of internal anode alloy recipes. But I would avoid magnesium since there will be plenty of exposure to seawater
.
Charles