sacrificial anodes

[sv.antea]

recently I made rounds of local chandlers in search of 1" zinc for my prop shaft.
All were out of 1" zincs. West marine had some very large zincs, but all their smaller ones were aluminum alloy anodes Martyr brand. So I bought one and installed it. At first it worked ok, though I noticed my Martec was less bright now. After two or three months (I clean almost every week or as needed for racing) while there was still more than half of the anode left, the prop started to turn brown, then green, which is what normally happens when a zinc is mostly depleted or gone. I replaced the anode with yet another aluminum on, benefit of the doubt sort of thing, and inspected the removed one. From the looks of the inner surface, it appeared the anode lost contact with the shaft. There is no contact device there unlike some brands of zincs had in the past.
Had anyone out there had better experience with these aluminum anodes? Does anyone have the scoop on whys, wherefores and such?

[rbk]

Did you change all your annodes to aluminium? Ive got the same setup and recently switched to aluminium (not enough time yet to give you feedback) but have not had any contact issues with the matyr prop nut annode. Make sure there’s contact and tighten down good is all I can think of.

[jamhass]

I think it goes
Aluminum is for brackish water
zinc for salt
magnesium for fresh

[sv.antea]

Am I jumping to conclusions? Jamhass, that's what I always read. But the disappearance of zinc anodes from the chandlery shelves, and why would West Marine in Tampa suddenly start carrying only anodes meant for fresh water? It made me think the new aluminum anodes were some new alloy suitable for salt water, while zincs were being phased out out of environmental or cost reasons. The folks at WM did not know why the anodes were aluminum, of course. Someone here on CF should know.

[billknny]

Originally Posted by jamhass

I think it goes
Aluminum is for brackish water
zinc for salt
magnesium for fresh

Actually, that is not true... Except for the part about Mg anodes being best in fresh water.

Aluminum (of the proper alloy) makes an acceptable sacrificial anode in salt water for non-aluminum boats. The important thing is do NOT MIX anodes. Use all Al or all Zn. If you mix them the Al will rapidly corrode in the presence of Zn anodes in the same connected system. and not provide useful protection.

Note that Al anodes are not just hunks of pure Al, or random Al alloys. They are special alloys just for this purpose. The proper Al alloys will be slightly (very slightly) more protective than Zn.
-----------------
Have I made note enough that it has to be the right Al alloy? Do not go bolting random chunks of Al to your boat and assume you are providing galvanic protection.

[mvAlegria]

Are you familiar with https://www.boltdepot.com. They have an enormous inventory of all kinds. They should have the Zinc you are looking for.
They may also be able to answer your questions.

[Bruce K]

Boatzincs.com. In the US but ship to Aus. Have everything!!! Al anodes are actually superior to Zn, in salt, must be the right alloy and all must be Al

[mvAlegria]

Sorry, you are right, I meant boatzinc. Com