Aluminum anodes not preventing corrosion

[ScottRhodes13]

Hello everyone, I'm looking for opinions on aluminum anodes. Everything I read on the internet says they're great (in salt water), but my experience has been different.

Ever since installing aluminum anodes on my prop shaft and rudder last summer I've noticed a build up of white powder (presumably galvanic corrosion) on the rudder and through hulls. I asked the yard manager at the shop where I keep my boat about this, and he says that he won't install the aluminum zincs, even if people ask for them.

I keep my boat on the hard most of the time, but it can spend a week or two in the water at times, even in winter. It's in salt water, and never connected to shore power. I have not had this problem with zinc anodes.

Have others had good experience with aluminum anodes? I'd like to use them if I can.

[AKA-None]

Hmmm I’d always thought al anodes e we re for fresh water not salt? But I could be wrong

[RaymondR]

The galvanic potential of zinc is -0.98 to -1.03 volts and that for aluminium alloys -0.76 to -1.00 volts. The consequence of this is that you need a greater surface area of aluminium anode to generate the same amount of protective current. You might just need more aluminium anodes.

[Simi 60]

Maddox anodes?

https://www.marineprotectionsystems..../maddox-anodes

[Sailmonkey]

Originally Posted by AKA-None

Hmmm I’d always thought al anodes e we re for fresh water not salt? But I could be wrong

Aluminum are good for salt as well as brackish water.

Magnesium is for fresh water.

[LittleWing77]

Originally Posted by ScottRhodes13

Ever since installing aluminum anodes on my prop shaft and rudder last summer I've noticed a build up of white powder (presumably galvanic corrosion) on the rudder and through hulls. I asked the yard manager at the shop where I keep my boat about this, and he says that he won't install the aluminum zincs, even if people ask for them.

When one considers the ultimate replacement costs of your rudder's metal components (and rebuild) and replacement of all of your thru-hulls and/or potential thru-hull fail in the meanwhile, I would immediately return to zinc anodes without delay.

Respectfully, Scott, this is one of those times I do not understand why a boat owner would insist upon using an ineffective option which is causing/going to cause exponentially worse long-term damage to key components of his boat, just to economize.

Pay attention to your Yard Manager !

Kindly meant,
LittleWing77

[Pelican_38]

OK, that is very odd. Like you, I understand aluminum anodes are superior to zinc in many applications. I agree that white powder does seem to indicate galvanic corrosion.

I currently have zinc, but was planning on shifting to aluminum on my aluminum vessel per recommendation of BoatZinc and my yard. Aluminum anodes are definitely not ineffective as mentioned above. I wonder if the comment about having the correct mass of anode is right. Do you have access to a corrosion reference electrode (a silver/silver-chloride half cell)?

[fstbttms]

Originally Posted by AKA-None

Hmmm I’d always thought al anodes e we re for fresh water not salt? But I could be wrong

You're wrong.

[rslifkin]

Keep in mind, aluminum anodes aren't just aluminum. They're typically Navalloy, which is an alloy of aluminum, zinc, and indium IIRC.

[fstbttms]

Originally Posted by ScottRhodes13

Ever since installing aluminum anodes on my prop shaft and rudder last summer I've noticed a build up of white powder (presumably galvanic corrosion) on the rudder and through hulls.

Are your thru-hulls bonded to the anodes in any way?

[NPCampbell]

If your boat is spending a majority of the time on the hard then the white powder is possibly normal atmospheric oxidation of zinc. Zinc oxide is a white powder. Galvanic corrosion requires an electrolyte solution to complete the circuit between the dissimilar metals.

[fstbttms]

Originally Posted by NPCampbell

If your boat is spending a majority of the time on the hard then the white powder is possibly normal atmospheric oxidation of zinc. Zinc oxide is a white powder. Galvanic corrosion requires an electrolyte solution to complete the circuit between the dissimilar metals.

What zinc? He's using aluminum anodes. Also, this doesn't explain why he's seeing the white powder on his rudder and thru-hulls.

[Dsanduril]

Originally Posted by RaymondR

The galvanic potential of zinc is -0.98 to -1.03 volts and that for aluminium alloys -0.76 to -1.00 volts. The consequence of this is that you need a greater surface area of aluminium anode to generate the same amount of protective current. You might just need more aluminium anodes.

"Aluminum" anode is really a misnomer, a proper "aluminum" anode is an aluminum-indium alloy with a galvanic potential of -1.1V, greater than the -1.05V generally reported for zinc alloy anodes. Maybe the OP has aluminum rather than aluminum-indium anodes? We've been using "aluminum" anodes for about 3 years and show no signs of any problems and slightly improved life (that latter is hard to measure because we replace once every year or so, and that is almost always as a preventive maintenance task when doing other work rather than a completely wasted anode that needs replacing).




source

[NPCampbell]

Originally Posted by fstbttms

What zinc? He's using aluminum anodes. Also, this doesn't explain why he's seeing the white powder on his rudder and thru-hulls.

My experience is with cars in which I see white oxidation all the time due to nuts and bolts being made of zinc. However, that is a good point, zinc wouldn't be used below the water line for anything but brass and that oxidizes blue/green.

[ScottRhodes13]

Wow! Thanks for all the replies so soon.

I don't remember what brand of anode these were, but I bought them from a reputable dealer, so I'm sure they're the right composition.

The through hulls and rudder are bonded together. I will need to investigate, but I think they're bonded to the engine ground, too. I'm not happy with the bonding, it's just a wire connected to the fitting with hose clamp. I know that's not recommended, but don't see a better way. I keep the battery switch off when docked, so don't think there's a stray current problem.

The suggestions to consider the size of the anodes make sense. I will look into a corrosion reference electrode -- I haven't heard of those. As one responder mentioned, corrosion can cause expensive problems, so I hate to experiment just to see what works.

The guy at the boat yard said that when the aluminum anodes are out of water much of the time, they oxidize and form a coating that prevents them from working as efficiently as they should, so that could be my problem.