Anodes both Zinc and Aluminium

[BobE]

I'm told that I should have aluminium anodes on my boat at Christchurch.
So far I've not used one. (10 years plus)
Unfortunately the boat spends most of her time moored up in the "brackish" waters.
I have the usual Bukh type zinc pencil anode in in the diesel, it sacrifices, and is replaced annually.

Question is could one cast a "Mixed" anode of Aluminium and Zinc or add some Aluminium shavings to the molten Zinc or even place some shavings in the mould in advance?

The two metals are next to each other at the Anodic end of the table. (Only about 0.5 V apart in the worse case) so they should both sacrifice and hopefully not effect each other?
Does the water temperature of the engine increase the sacrificion (Spelling??) of the Pencil anode?

Anyone know if this could cause some unforeseen problem?
BTW I've never had to replace the prop shaft Zinc anodes but always replace the conical prop Zinc anode which sacrifices slightly each year...

Do I really need a Aluminium anode somewhere on the boat?

Comments thoughts etc. most welcome.

Cheers Bob E...

 

[VicS]

I'm told that I should have aluminium anodes on my boat at Christchurch.
So far I've not used one. (10 years plus)
Unfortunately the boat spends most of her time moored up in the "brackish" waters.
I have the usual Bukh type zinc pencil anode in in the diesel, it sacrifices, and is replaced annually.

Question is could one cast a "Mixed" anode of Aluminium and Zinc or add some Aluminium shavings to the molten Zinc or even place some shavings in the mould in advance?

The two metals are next to each other at the Anodic end of the table. (Only about 0.5 V apart in the worse case) so they should both sacrifice and hopefully not effect each other?
Does the water temperature of the engine increase the sacrificion (Spelling??) of the Pencil anode?

Anyone know if this could cause some unforeseen problem?
BTW I've never had to replace the prop shaft Zinc anodes but always replace the conical prop Zinc anode which sacrifices slightly each year...

Do I really need a Aluminium anode somewhere on the boat?

Comments thoughts etc. most welcome.

Cheers Bob E...

Aluminium anodes are recommended for brackish water. they are also suitable for salt water

Anodes are made of alloys of very specific compositions.

Zinc anodes usually to a US military specification which crucially is very low in iron content and importantly also contains a small amount of cadmium.

Aluminium anodes are usually to a spec which contains some zinc and importantly a small % age of indium.

You should not mix anodes of different types

I have no idea what would happen if you tried to cast anodes that are a mixture of zinc and aluminium.
If you must cast your own be sure to obtain alloy of the appropriate composition .

Cannot say if you need any other anodes without knowing details of the boat and materials of underwater metal or alloy parts but if you have not needed them in over 10 years you almost certainly don't need them.
The stern gear you say has its own anodes.

 

[BobE]

Thanks Vic....
So if I melt down old anodes which are readily available in the scran bins and skim the slag off the top I should have about the correct Zinc composition...then maybe i can experiment with a sort of layered pencil anode to see if there is a discernable difference??
What do you think??
Cheers Bob E...

 

[VicS]

Thanks Vic....
So if I melt down old anodes which are readily available in the scran bins and skim the slag off the top I should have about the correct Zinc composition...then maybe i can experiment with a sort of layered pencil anode to see if there is a discernable difference??
What do you think??
Cheers Bob E...

Iron contamination is the risk when melting old zinc anodes to recast. The spec is only 0.005% max and that's only tolerable due to the small cadmium content. If they contain too much iron they don't work!