Aluminium anodes

Caladh said:
I’ve inadvertently bought an aluminium hull anode for this season. Has anyone any experience that they’re ok? I’ve read Vics page on Anodes. I also have a new zinc anode on the rudder stock.…just want some piece of mind. Ta
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Aluminum anodes are recommended in waters with somewhat low salinity - brackish waters like a harbour with freshwater flowing in.

I have been using them in a situation like that.

They are a bit more expensive.

I have two on my volvo penta saildrive - one next to the prop, and one on drive housing. The one on the prop goes very quickly, which I take to mean that it is doing the job it is supposed to do.
 
Aluminium anodes are commonplace now and are virtually the same potential as zinc. As suggested they are more tolerant of variable salinity, but the main driver for their use is lower cost and "greener" manufacturing process for the alloy.
 
Tranona said:
Aluminium anodes are commonplace now and are virtually the same potential as zinc. As suggested they are more tolerant of variable salinity, but the main driver for their use is lower cost and "greener" manufacturing process for the alloy.
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mmmm….lower cost ? The one I bought was £5 more than the zinc. However no one has said don’t do it, so I’m happy !
 
One thing that i read with respect to aluminium anodes is that it is critical to have exactly the right alloy, and to be careful about buying from unknown sources.

The implication was that it is more critical than with zinc anodes,

I don't know if this is true...
 
Laser310 said:
One thing that i read with respect to aluminium anodes is that it is critical to have exactly the right alloy, and to be careful about buying from unknown sources.

The implication was that it is more critical than with zinc anodes,

I don't know if this is true...
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My guess is that it is critical. It takes very careful additions of small amounts of alloying elements to achieve the correct characteristics. Any old aluminium would not work, particularly for saildrives. Whereas zinc is more anodic and probably less composition critical.
 
Caladh said:
mmmm….lower cost ? The one I bought was £5 more than the zinc. However no one has said don’t do it, so I’m happy !
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It is complicated comparing the prices for the same amount of protection.
You have to take into account the relative atomic mass (RAM), better known to the oldies as "atomic weight", and the different valencies

Zinc RAM =65.4, valency 2
Aluminium RAM = 27.0, valency 3

This gives vey different electrical capacities for the two materials. According to the data collected by Vyv Cox aluminium anodes have an electrical capacity of 2700Ah/kg while zinc anodes have an electrical capacity of 780Ah/kg
This means that an aluminium anode with a mass of only 290g will have the same capacity as a 1kg zinc anode.

You also need to take the different densities into account because in order to fit the existing mounting studs an aluminium anode with the same dimensions as a zinc anode will be much lighter. an aluminium anode the with the same dimensions as a 1kg zinc anode will have a mass of only approx 380g and consequently will have an electrical capacity approx 1.3 times that of the zinc anode.

All rather theoretical I know

There are some interesting comparisons on this page Alloy Specifications
 
Laser310 said:
One thing that i read with respect to aluminium anodes is that it is critical to have exactly the right alloy, and to be careful about buying from unknown sources.

The implication was that it is more critical than with zinc anodes,

I don't know if this is true...
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the composition of both aluminium and zinc anodes is critical.

The important element in the composition of aluminium anodes appears to be a small percentage of indium.

Zinc for anodes is a very high purity In particular its has very low concentrations of iron, copper and lead.
A small percentage of cadmium in the mix is important to make the low levels of some other elements tolerable

Both are usually to US military specifications

See again Alloy Specifications for details of the compositions
 
I have just fitted aluminium anodes and have been using them over a period of at least 10 years with no corrosion issues.
I change the anodes every two years although they would probably last longer . The boat is on fresh water with occasional salty water trips.

Mixing anode types on the same boat may not work well - eg the aluminium anodes might sacrifice themselves to protect the zincs.
 
MartynG said:
I have just fitted aluminium anodes and have been using them over a period of at least 10 years with no corrosion issues.
I change the anodes every two years although they would probably last longer . The boat is on fresh water with occasional salty water trips.

Mixing anode types on the same boat may not work well - eg the aluminium anodes might sacrifice themselves to protect the zincs.
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If your anodes are lasting that long, perhaps it's a sign that they're not working very well. Magnesium is recommended for fresh water, not aluminium, although I don't know how magnesium works with occasional salty.
 
PeterBoater said:
If your anodes are lasting that long, perhaps it's a sign that they're not working very well. Magnesium is recommended for fresh water, not aluminium, although I don't know how magnesium works with occasional salty.
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Magnesium is ok for inland lakes but will last days if immersed in brackish water. Aluminium is now No.1 choice for all waters. It is better than zinc in fresh/brackish water because it does not develop a passive hydroxide film. Also see VicS' earlier post about its effectiveness.
 
PeterBoater said:
If your anodes are lasting that long, perhaps it's a sign that they're not working very well. Magnesium is recommended for fresh water, not aluminium, although I don't know how magnesium works with occasional salty.
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The anodes are working fine thank you .
Aluminium anodes are suitable in fresh or salt water .
 
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