aluminum anodes

[jim rosenthal]

This year I decided to use aluminum anodes; I've always used zincs in the past. My understanding is that not only the anodes but the paint on the props, shafts, struts and rudders should be compatible with the aluminum anodes. There are aluminum paints out there, but if anyone has recommendations as to what to use, I'd like to know about it. Thanks, in advance.

 

[IOLANI]

Interesting post I just got done changing the anodes on my bow thruster . I could read the number on what was left of the old ones and they were magnesium designed for fresh water. I called the mfg and he said just add an A suffix to order the same in Aluminum.

 

[Canuck Dennis]

I tried them before and found the did not last as well as zinc and that is in the PNW. I also used zinc and hung an aluminum anode overboard, it wore out quicker that the zincs.
This was tried on both my aluminum Yacht and fiberglass cruiser,
YMMV.

 

[bostonhatteras]

Look at the galvanic scale of conductive materials in seawater. Corrosion potential, in volts, of these anode materials are; aluminum alloy -0.76 to -1.00, zinc -0.98 to -1.03. Very close together. Copper is -0.30 to -0.57. By comparison, cupronickle which is your heat exchanger, is -0.21 to -0.28. No issue with any bottom paints whatsoever if if correct surface area of anode. The copper is more cathodic than either the zinc or aluminum. Obviously the aluminum and zinc are not identical. Without getting into the weeds of the voltage math, the surface area of the two anodes has to be slightly different to provide the same level of protection. Many things affect anode wastage including the actual boat installed on, nearby boats, water composition, turbulence, boat sits or is active. To just say aluminum isn't good misses the entire argument of why its used in the first place. A few big benefits of aluminum over zinc are; less weight, far less expensive, does not deposit cadmium toxic heavy metal in the water, can be left on between seasons and a huge one if you go between fresh and salt water often is that they will not passivate and become inert. As for the passivation, if you spend any time in fresh water the surface of the zinc will passivate, in other words, develop a protective coating and then will offer no protection for your underwater metals whatsoever. This passivation happens to zinc in simple atmospheric humidity and that why reusing zinc after extended dry storage can't be done, you'd need to sand away the passivation from the surfaces to expose unpassivated zinc. Aluminum can go between fresh and salt without issue and can be left on over dry storage without issue. Back to paint, overprotection with zinc creates a sodium hydroxide solution around the zinc and this is what blows the paint off the surface. The zinc acts literally as a paint remover within its close proximity. This also is why most wood boats have little to no zinc anodes and do not bond underwater metal...delignification.

 

[bostonhatteras]

I tried them before and found the did not last as well as zinc and that is in the PNW. I also used zinc and hung an aluminum anode overboard, it wore out quicker that the zincs.
This was tried on both my aluminum Yacht and fiberglass cruiser,
YMMV.

How long and where attached? Perhaps the aluminum was protecting more than the zinc was. A hard electrical path is necessary for protection to occur. If any anode isnt electrrically bonded it wont do anything because its not part of the system, it will just sit there and not wear away. Perhaps the aliigator clamp for the aluminum was a better connection than the zinc had.

 

[GCSI]

We’ll be fresh water for approximately 6 moths this year. Currently sitting in salt water and plan on migrating mid May.

Given that my zincs are about due, thinking of changing to aluminium now (trusted diver) and then switching back to zinc when we return to salt water.

Thoughts??

 

[SKYCHENEY]

We've run zinc in freshwater for the last 50 years on boats. Most in the Great Lakes do the same. Not an issue.

 

[SEVEN]

We’re in very light brackish water and I tried aluminum zincs and they lasted less than six months. I switched back to regular zincs and they last well in excess of two years, but I change out every year just to be safe.

 

[Captain Ralph]

We have been in fresh water for 5 years now. Still use standard saltwater zincs. My partner's Hatt is 10 feet away from us. He went to the freshwater alloy zincs 2 years ago. All is still looking great.
Of course both boats are properly bonded, on the same shore power, no stray currents between us, thus we may see more years on what we have even with different zinc material in use.

IMO; zinc life depends on what how well your next (few) marina boats are maintained and wired properly.
We have been next to some meat heads when our zincs lasted monthly or less.
That is when a good diver that knows of these things pays him/her self off quickly.
10+ years on the Huckins dock, we knew quickly when a new arrival had issues.

 

[bostonhatteras]

We've run zinc in freshwater for the last 50 years on boats. Most in the Great Lakes do the same. Not an issue.

salt water is an excellent conductor, fresh water is a poor conductor. in fresh water, the electrolyte component of the corrosion cycle is largely absent.

 

[Eddieclemons]

Here on the Cumberland River in Tennessee, Zinc anodes do not work. This is my opinion based on seeing many boats come from the salt here after purchase and when the owner does finally get them changed, the Zincs have passivated and are covered in white oxide rendering them useless. Aluminum is great if you bounce back and forth, but won't be as active as magnesium.

 

[Len R]

I've always ben told NOT to use aluminum along with zinc together on the same boat. Something about them not reacting well together. That is probably the issue with the running gear paint.