Fresh to Salt and Mix 'n' Match Anodes?

[S/V Alchemy]

I have a steel sailboat (pictured) with a simple galvanic isolation in the shorepower ground (Yandina) and little evidence of any corrosion problems at my YC in freshwater Lake Ontario. My anodes are made of magnesium and consist of two teardrop-shaped ones through-bolted onto my aluminum rudder (itself separated from the steel mounts by Delrin washers), and a magnesium prop anode on my Variprop. All these anodes still have plenty of surface left.


Here's the somewhat complex issue. We are going down the St. Lawrence next summer and will be in briny water past Quebec City and salt thereafter. I am going to weld or through-bolt plate anodes made of zinc onto the hull to provide sufficient "coverage". In addition, I am upgrading from the Yandina galvanic isolator to a Mastervolt isolation transformer.

1) Can I mix and match these anodes in freshwater without consequence for a couple of months? Zinc anodes work in fresh water, but not as well as magnesium.


2) Should I get magnesium plate anodes and swap them out at haulout in Nova Scotia for zinc anodes everywhere?


3) Any issues foreseen with an iso transformer installation over the winter with zinc on the hull and magnesium on the prop when hooked to dock power in a basin that isn't, via some readings I've taken, known to be "leaky"?


Thanks in advance. I never have simple problems.

[GordMay]

... Should I get magnesium plate anodes and swap them out at haulout in Nova Scotia for zinc anodes everywhere? ...

I would think so.

However, you might ask CMP/Martyr: Contact | Martyr Anodes
Canada Metal (Pacific) Ltd
Phone: 1-604-940-2010
Fax:1- 604-952-2650
Email: sales@canmet.com

Magnesium Anodes have a much higher driving voltage* than zinc or aluminum, making them highly suitable for use in fresh water, they will, however, become very active in salt water where they will probably only last a matter of months. Protected surfaces can build up a layer of off white calcareous deposit which will be difficult to remove.

Magnesium anodes are not designed for prolonged use in sea water and if you are taking your boat into a salt water location for more than seven days (Fourteen days in any one year) you should consider changing the anodes.
➥ About Corrosion | Martyr Anodes

* Magnesium generates -1.6 Volts (i.e. negative 1.6 volts)
Aluminum alloy generates -1.1Volts
Zinc -1.05 Volts

[S/V Alchemy]

Thanks, Gord. That reflects some other advice not to mix and match and that the better course would be to go all zinc in fresh water and have slightly less protection until I get to salt, which will eat the Mg jiffy quick.


Putting in the isolation transformer will be a help, of course.

[mickt243]

You would be better off replacing all your anodes with “aluminium” ones.... they’re not *just* aluminium, they are a specific alloy (mil spec blahblahblah) that works in fresh or salt.

Leaving magnesium anodes in place in salt water may “overprotect” your steel hull (can cause paint blistering etc)

[fstbttms]

[a64pilot]

I have mixed anodes for a short while, aluminum and zinc.
What seems to happen was only the aluminum anode was “Active” meaning the zinc didn’t waste away at all. I think the least noble metal wastes away first, and the next in line won’t until the least is almost wasted away.
This was in brackish water in the St John’s River.
The zinc anode formed a white calcium coating on it as well, which seems to insulate it, and make it ineffective, even when you go back to salt.
I have since switched only to aluminum anodes as I’m sometimes in brackish water in the ICW, but mostly all salt.

In truth I don’t understand why anyone uses zinc anodes even if they are only in salt water