Aluminium anodes in fresh water??

[bluedragon]

It's anode time again :-((

Well, for most of you it's no problem - seawater / zinc is a proven combination. But for those of us berthed in fresh water behind a barrage and sea lock (Cardiff Bay) it's a nightmare. Magnesium for freshwater, that's clear advice from Mg Duff and others, but go out to sea for even 2 weeks and it's mostly gone. 4-5 weeks, all gone in my case and it leaves a hard mineral deposit all over the metalwork it's protecting. There is also some evidence on my saildrive that over-protection is occurring (bubbles in the drive paint). I'm nervous now about using Mg anodes again.

Changing the anode to Zinc before leaving for summer cruise (and then back again) is the "advised" solution, but to avoid the hassle and cost a lot of boaters here seem to think Aluminium alloy anodes are the magic answer for fresh water as they are suitable for "brackish" water. However, this is not always confirmed by some of the reading I've done. The problem with Zn is that supposedly the surface becomes "passive" after a week or so in fresh water, and then it doesn't work at all. But how do the Al alloy anodes perform in this respect? Is it immune to the pacified effect in fresh water? Or is it just a slower process, allowing for more time in fresh water before going into salt? This is not clear to me from literature sources (opposing views) and local anecdotal advice / evidence is also confusing.

Do we have any real experts here on the forum who can help?

Thanks.

PS - before somebody suggest it, the idea of using hanging anodes connected to internal bonding won't work with a saildrive will it? The leg is electrically isolated from the engine I believe.

 

[ianj99]

It's anode time again :-((

PS - before somebody suggest it, the idea of using hanging anodes connected to internal bonding won't work with a saildrive will it? The leg is electrically isolated from the engine I believe.

Attach the wire from a hanging electrode to the sail drive unit rather than the engine. This will then be equivalent to bolting it to the sail drive unit with the advantage of being able to lift it out when not needed.
Ian

 

[VicS]

It's anode time again :-((

Well, for most of you it's no problem - seawater / zinc is a proven combination. But for those of us berthed in fresh water behind a barrage and sea lock (Cardiff Bay) it's a nightmare.

My thoughts:-

It would be interesting to ask MG Duff for their expert advice.

I often wonder how important cathodic protection is in fresh water ....? You could always dangle a Mg anode if necessary.

I think i would try aluminium anodes

 

[bluedragon]

Attach the wire from a hanging electrode to the sail drive unit rather than the engine. This will then be equivalent to bolting it to the sail drive unit with the advantage of being able to lift it out when not needed.
Ian

Not quite sure where the electrical isolation occurs? Is it where the drive connects to the engine, or just the lower leg? Need to have a look at this.

 

[Tranona]

It is between the engine and the gearbox. So connecting to the gearbox is a path to the saildrive housing.

 

[bluedragon]

My thoughts:-

I think i would try aluminium anodes

I probably will, but my main concern is not so much having protection in the fresh water (I could put a Mg anode on for the winter anyway), but if I fit an Al alloy anode now, will it still be active in seawater in say June having sat in fresh for two months?

 

[bluedragon]

It is between the engine and the gearbox. So connecting to the gearbox is a path to the saildrive housing.

Maybe this is the answer. But would a hanging anode (from say the transom) replace the need for leg anode completely I wonder? The internal wiring would be quite straight forward to an external terminal, but then what wire to dip into the water? Copper, tinned copper, something else?

 

[VicS]

I probably will, but my main concern is not so much having protection in the fresh water (I could put a Mg anode on for the winter anyway), but if I fit an Al alloy anode now, will it still be active in seawater in say June having sat in fresh for two months?

I think that is something to ask MG Duff.

You could of course get a silver/silver chloride electrode and monitor the status of the cathodic protection.

http://www.galvatest.com/

 

[bluedragon]

Both good ideas. The Galvatest unit is certainly worth a try. I think I'll get one.

I see MG Duff supply hanging anode kits - probably easier than making one up myself. They say they protect saildrives, but I'll have a word with them. If it avoids the hassle of taking the prop off every year (Volvo 120 SD) it's worth it on those grounds alone.

 

[Quandary]

Anodes

I have a similar problem, boat with saildrive in Crinan Canal, Volvo Penta so no aluminium leg or prop. anodes available, 26 weeks in water, about half of which is in the sea with trips up to four weeks. I use zinc anodes plus a zinc hanging anode which is wired back to the casing of the saildrive. The prop anodes stay clean and effective degrading in about 6 months, the saildrive anodes degrade very slowly and I reckon they are covered in a passivated coating after about 3 weeks in the fresh water. The hanging anode is zinc and is deployed in salt water or where there is shorepower, it needs cleaning with rough sand paper every two weeks (wire brushing does not take off the passivation) The prop is polished bronze, the leg is Propshield over the original VP enamel, this lasts 2-3 seasons.
So far (touch wood) no evidence of of corrosion and the prop and leg are still shiny.
Neigbours in the canal with conventional prop. shafts tell me that aluminium is durable and effective.

 

[viva]

On the Vilaine (freshwater)I hang a magnesium anode. However most of the French only have a zinc anode. Indeed the local chandleries only sell zinc. When I have asked in a chandlery for magnesium I have been told it is not available only zinc is used!