Magnesium AND Zinc anodes on boat...

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I'm in fresh water and have magnesium anodes on my 290DP drive - great.

However i'm just after copping on that my trim tabs have Zinc ones. Had spent about 3 months in the water and no visible nasties (there's very little electrical activity from external sources, never had a problem before at my pier) - do I just change the trim tab anodes to magnesium ones and forget about it, or should I worry??

 

[nonitoo]

Cannot imagine you will have many worries.

I was always told that :-

Magnesium for fresh water
zinc for sea water
aluminium for both sea and fresh water.

Quite prepared for others with greater knowledge to add to this though.

Tom

 

[VicS]

Nonitoo is not quite right.
Magnesium anodes for fresh water because zinc or aluminium will become "passivated" and therefore ineffective.
Magnesium are far too reactive for use in salt or even brackish water and will soon become corroded away.

Aluminium in brackish water because Mg is too reactive and because zinc will be passivated and ineffective.

Aluminium or zinc in salt water. The aluminium is a special alloy containing a small percentage of indium and has the same performance, or slightly better even, than zinc anodes.

Yes in fresh water you should change your trim tab anodes to magnesium

MG Duff's website is worth reading

 

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Thanks folks, just to remind you that my only concern is the zinc on the trim tabs, the leg is fine.

Will get Magnesium anodes before she goes back in the water!