Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 28-11-2012, 14:41   #1
Registered User

Join Date: Jan 2010
Location: Ireland
Boat: Jeanneau Sun Odyssey 409
Posts: 81
Corrosion & electrolysis mixing bronze with alu

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
__________________

__________________
Srah 1953 is offline   Reply With Quote
Old 28-11-2012, 15:23   #2
Marine Service Provider

Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,940
Re: Corrosion & electrolysis mixing bronze with alu

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.
__________________

__________________
Greg

- If animals weren't meant to be eaten then they wouldn't be made of food.
Stumble is offline   Reply With Quote
Old 28-11-2012, 16:13   #3
Registered User

Join Date: Jan 2010
Location: Ireland
Boat: Jeanneau Sun Odyssey 409
Posts: 81
Re: Corrosion & electrolysis mixing bronze with alu

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.
__________________
Srah 1953 is offline   Reply With Quote
Old 28-11-2012, 16:41   #4
Marine Service Provider

Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,940
Re: Corrosion & electrolysis mixing bronze with alu

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.
__________________
Greg

- If animals weren't meant to be eaten then they wouldn't be made of food.
Stumble is offline   Reply With Quote
Reply

Tags
corrosion, electrolysis

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 10:34.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.