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  • 1 Post By Classic30
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Old 05-30-2013
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Alkaline Rot

Leading on from Knotty's chainplate thread and just in case this helps anyone here:

We all know electrolysis is caused by dissimilar metals in salt water, but I discovered (the hard way!) what can happen when the dissimilar metals in question just happen to be bolted to a piece of timber.

If you'll pardon the mess and the crummy camera angles, here's the 'before' pic:



It turns out that what is happening in the above pic is that the bronze bolt above the stern gland has worked itself loose over time and sea miles allowing water to slowly leak along it, down the sternpost and onto the bronze stern gland fixed with stainless-steel screws. At some stage in the distant past the original bronze nut either failed or went missing and some well-meaning PO replaced it with a stainless one hardening it down onto the original bronze washer beneath.

Not good!!... According to reputable sources, including this USDA Forest Service Research Paper the dissimilar metals cause the sea-water to turn strongly alkaline and basically eat away at any timber it touches.

For me, the solution to the problem was simple: replace the nut and the other stainless fastenings on the stern gland with bronze versions, wash the area with vinegar (a weak acid) to neutralise the alkaline solution, slop on a couple of coats of CPES and all is good again.



Goes to show that, even if you think you've learnt all there is to know, there is always something new to learn.
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Last edited by Classic30; 05-30-2013 at 08:26 PM.
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Old 06-01-2013
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Re: Alkaline Rot

Yup, learned that one the hard way with my keel bolts on my wooden boat. Thought my oak keel was strong as the backbone of an OX when it really had the compression and tensile strength of coleslaw...

Who's idea was it to build boats out of wood anyway? Sheesh!

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Old 06-01-2013
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Re: Alkaline Rot

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Originally Posted by Hartley18 View Post
We all know electrolysis is caused by dissimilar metals in salt water, but I discovered (the hard way!) what can happen when the dissimilar metals in question just happen to be bolted to a piece of timber.
.
What you describe is not electrolysis, it is a simple galvanic reaction. Electrolysis is defined as "Chemical changes in a solution or electrolyte due to the passage of electric current".

Never hire a marine electrician who uses the term "electrolysis" to describe any corrosion on a boat.
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Old 06-02-2013
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Re: Alkaline Rot

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Originally Posted by boatpoker View Post
What you describe is not electrolysis, it is a simple galvanic reaction. Electrolysis is defined as "Chemical changes in a solution or electrolyte due to the passage of electric current".

Never hire a marine electrician who uses the term "electrolysis" to describe any corrosion on a boat.
Well, in that case it's fortunate I'm not a marine electrician.

I used the word "electrolysis" simply because that's a galvanic reaction that most people can understand - and used that to lead in to the discussion at hand.
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Old 06-02-2013
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Re: Alkaline Rot

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Originally Posted by Hartley18 View Post
Well, in that case it's fortunate I'm not a marine electrician.

I used the word "electrolysis" simply because that's a galvanic reaction that most people can understand - and used that to lead in to the discussion at hand.
Sorry, not trying to be picky but misuse of these terms is rampant on this and other forums and causes a great deal of confusion for those who don't know the difference.

Sorry again but electrolysis is not a galvanic reaction either. Galvanism refers to disimilar metals in an electrolyte and connected by a conductive medium (wire).
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Old 06-02-2013
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Re: Alkaline Rot

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Originally Posted by MedSailor View Post
Yup, learned that one the hard way with my keel bolts on my wooden boat. Thought my oak keel was strong as the backbone of an OX when it really had the compression and tensile strength of coleslaw...

Who's idea was it to build boats out of wood anyway? Sheesh!
Good point. What worried ME was that two different english, tall-ships trained, qualified shipwrights didn't know what was going on!

At least I'll never have to worry about polyestermites..
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