Aluminum Alloy "Zincs" Better than Zinc "Zincs"?

[fstbttms]

That is what I meant. Wouldn't you want a more active anode than the material you were trying to protect?

I suppose so. Is it your contention then that these aluminum anodes are not suitable for saildrives?

 

[mitiempo]

It doesn't seem logical somehow. Why would an anode of the same material dissolve instead of what it is trying to protect?

 

[mitiempo]

I googled "aluminum saildrive anodes" and got a hit on SA with the same concerns I posted here, so I am not alone wondering about their effectiveness - I see you posted there as well Fast.

I also read a Yanmar Advisory concerning saildrive protection and their aluminum recommendations.

Still seems odd somehow, but it apparently works.

My thoughts are to keep underwater metal to a minimum and as noble as possible - a stainless shaft and a bronze prop with several isolated through hulls of good quality bronze is plenty.

I would never buy a boat with a saildrive - they seem to have all the disadvantages of an outboard without the advantages. Easy installation for the builder though.


**I still think having both a zinc and an aluminum anode will only mean the zinc erodes to protect the aluminum. At least the prop won't.

 

[rardiH36]

My thoughts are to keep underwater metal to a minimum and as noble as possible - a stainless shaft and a bronze prop with several isolated through hulls of good quality bronze is plenty.

**I still think having both a zinc and an aluminum anode will only mean the zinc erodes to protect the aluminum. At least the prop won't.

If aluminum lasts longer, as its place on the galvanic series suggests, the zinc will deplete just as fast if it remains because it is more active or anodic.

Thanks all for the new input.

The above cut-and-pastes from a couple of the posts sent me back on to the internet to look for where I might have misinterpreted my recently discovered understandings.

I found a table in a ABYC document that does list that the category aluminum alloys is below Zn for corrosion potential. Mild steel, brasses, bronzes, etc are lower on the list still. So since Zn sacrifices to protect these metals, the rankings support the theme that a Zn cathode will sacrifice for an Al one.

But also I've found other information in manufacturer sites supporting that aluminum anodes are higher on the metal ranking than Zn. Maybe this has to do with the specific chemical specification for anode grade aluminum. Has 5-6% Zn and a trace of indium added that is cited as adding significantly to the sacrificial capacity of the aluminum anode. So aluminum anode as a sacrificial metal doesn't seem to be the same animal as just the generic category aluminum.

I've attached some pictorial references from a site called performancemetals.com. Seems to support that the aluminum cathode is more active (less noble) than zinc, but a lot less active than magnesium which isn't recommended in saltwater at all ... too active. Also that the more active metal (which will be the aluminum anode in my case if I decided to go the aluminum anode fish route) will protect several types of metals in tandem on the boat that are less active (more noble). So this would imply that the Al anode should sacrifice to help protect a shaft/collar zinc. (But with the electrical potential of Al = -1.10v being relatively close to Zn = -1.03/1.05v, the sacrifice rate of the aluminum for the Zn may not be too severe?)

My analytical capacity for all this rests only on long forgotten learning's from freshman/sophomore basic chemistry and physics. So certainly I still might be missing a big something that has got my interpretation backwards.

 

[mitiempo]

The list I included in post #16 is more detailed than your first attachment above. It shows magnesium, zinc, and then many different aluminum alloys. They are very close though.