Which anode - both?

[Graham_Wright]

My sailing boat spends roughly half her time in brackish water and half in sea water.
My underwater fittings are not electrically connected (by wire but maybe by dampness admittedly).
I have been told to fit aluminium anodes for the brackish water.
Should I also fit zinc for the salt?
Would they react to each other.

I though I understood all this

 

[Tranona]

Magnesium will tolerate either. What sort of anodes and what are they protecting?

 

[vyv_cox]

Magnesium is only suitable for fresh water. Its lifetime would be very brief in seawater.

Aluminium anodes are a clever alloy that has the same galvanic potential as zinc ones. You only need aluminium ones, giving several advantages with no negatives.

 

[Graham_Wright]

Magnesium is only suitable for fresh water. Its lifetime would be very brief in seawater.

Aluminium anodes are a clever alloy that has the same galvanic potential as zinc ones. You only need aluminium ones, giving several advantages with no negatives.

I have loads of aluminium (6082T6). Which alloy is best?

 

[Graham_Wright]

Magnesium will tolerate either. What sort of anodes and what are they protecting?

All underwater fittings are 316 stainless save for the propeller which is "bronze".

 

[Tranona]

All underwater fittings are 316 stainless save for the propeller which is "bronze".

OK, but what is the anode wired to?

Sorry about the brain fade - Vyv is of course right about using aluminium.

 

[Graham_Wright]

OK, but what is the anode wired to?

Sorry about the brain fade - Vyv is of course right about using aluminium.

There is no room on the prop shaft to fit an anode there (except for a small one). The rudder sits on a stainless shoe on the skeg and that would be a convenient position. Should I electrically connect the shoe to the engine?

 

[Tranona]

No. The only thing that needs an anode from what you say is the prop/propshaft to prevent dezinfication of the prop. If you don't have any exposed shaft then either a prop nut anode or a hull anode bonded to the shaft via the gearbox (with a bridge over a flexible coupling if you have one) are the alternatives. No need for any anodes on stainless but ensure all fastenings are well sealed so that damp cannot get in and possibly start crevice corrosion.

 

[VicS]

I have loads of aluminium (6082T6). Which alloy is best?

Its a special alloy containing amongst other alloying elements, zinc and a small % of indium and very low maximum concentrations of several other elements

Usually to U.S. Military Specification MIL-DTL-24779C , or similar, which has an electrode potential of -1.09 V vs an Ag /AgCl reference electrode which is slightly more negative than zinc anodes