Corrosion & electrolysis mixing bronze with alu

[Srah 1953]

I was re-reading various posts on feathering vs folding props. There was a reference to a Seahawk Australia website. Anyway, on that site, in a comment relating to a fixed prop where it recommends an aluminium fixed prop, it says that this "significantly reduces the potential problems of corrosion and electrolysis that can occur when a bronze propeller is fitted to an aluminium saildrive unit.".
Can anyone advise how serious this corrosion and electrolysis problem could be if I changed to a bronze folder or featherer? Currently have an aluminium fixed on Yanmar saildrive and there was quite a lot of pitting on the prop after a year - don't know why.
Thanks

[Stumble]

It really depends on exacally what alloys we are talking about. Different alloys have different electrical potential, and the further away they are from each other the bigger the difference.

Just as a guideline metals within -.15v of each other are considered galvanically comparable.

Aluminium alloys start at -1v on the galvanic chart while bronze stops at -.2v. This is worst case situation, and not what I would expect you have. The more likely situation is that the aluminium is in the -.9v range, and the bronze would be -.3v range. So a difference of .6v. This requires real thought as to galvanic isolation, and sacrificial anodes.

In practice, so long as your sail drive has a sacrificial anode you are probably ok. But the fact that you already have pitting on the drive leg makes me concerned that you already have an isolation problem that isn't being handled correctly.

[Srah 1953]

Thank you for this. I have read various articles and books which deal with this subject of electrical isolation but have never been able to get my head around it sufficiently that I can act on it. There was erosion (is that the right word?) on the saildrive anode but nothing excessive.

[Stumble]

Srah,

The details are pretty complicated, and way beyond me. But I deal with it to some degree professionally and have worked with a galvanic corrosion engineer on a number of projects.

The fundamentals are pretty easy, and don't really require a lot of understanding, just following the rules.
1) use the same metal whenever possible
2) where two different metals touch isolate them where possible with a non-conductive material in the middle.
3) sacrificial anodes are a good thing. But must have an electrical connection to what they are going to protect.

There are a few other rules derived from these.

1) NEVER use aluminium wire on a boat. It is so low on the galvanic chart that the wire can actually act as an anode. Which can then break the electrical connection to what we thought was the anode
2) loctite is your friend, use it on threads, but also anywhere else two pieces of metal touch
3) just because two things are aluminium, doesn't mean they have the same galvanic potential, so get out the loctite.