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Old 15-05-2015, 11:59   #16
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Re: Over-Zincing. Any horor stories from too much protection?

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Originally Posted by fstbttms View Post
While certainly not proof, I do see burn-back far more frequently with high copper hard paints than with other paints. Black Pettit Trinidad comes to mind. Can't ever remember seeing it with a white paint, for instance (which will usually have the lowest copper content in a given product line.)
Do we think "burn back" is...
  • debonding of the paint, possibly due to hydrogen generation under the paint (I would bet that is what happened on the aluminum boat, but that should not apply for FRP boats or bronze fittings).
  • deactivating of the paint due to protective electron flow. The zinc is donating electrons to the copper particles, reducing them to Cu and preventing release into the water. If zinc protects your bronze prop from dissolving, wouldn't they have the same effect on paint if it is near a bonded fitting?
  • Do we see burn back on copper-free paints? You hinted it seemed Cu related, so I'm guessing not.
Sounds like a good cases for barrier coating (or at least several coats of a good primer) on metal that is bonded. If not bonded, it should not matter so long as it is a Cu alloy. But even then if depends on what other metals may be connected to the pipe.


And obviously we keep Cu bottom paint away from aluminum.

FastBottoms: Do you see more grown on bonded through hulls, or non-bonded (the fitting itself)? Just wondering. Someone suggested that the electricity somehow attracted growth, but I'm thinking it may just reduce the amount of Cu ion available to repel them.
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Old 15-05-2015, 14:43   #17
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Re: Over-Zincing. Any horor stories from too much protection?

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It's spring any everyone is rushing around to replace tired anodes. I saw a couple boats in the yard that were clearly the other way; damage due to over protection.

Examples?
You only gave 1 example so far in the thread and that was a special case of an aluminum boat with mag anodes that had been in fresh water. So when you say "a couple of boats" and "clearly" just what do you mean?

I wander the yard in the Fall after boats have been hauled and have never seen any boats with signs of any over zinc problem.
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Old 16-05-2015, 16:09   #18
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Re: Over-Zincing. Any horor stories from too much protection?

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Originally Posted by donradcliffe View Post
This year I did the bottom of the YC race committee boat, and there was quite a bit of 'burn-back around one thru-hull. I disconnected the bond on the thru-hull and will report on the results next time the boat is hauled.

The missing bottom paint around the SSB ground plate may also be due to connecting the SSB ground to other boat grounds.
The SSB radio ground is connected to the ship's ground, I'm not sure what you mean as it needs a ground - can you elaborate?

As for the SSB ground connected to the bonding/anodes it would be, but via the Engine block which has both grounds.
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Old 16-05-2015, 16:47   #19
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Re: Over-Zincing. Any horor stories from too much protection?

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Originally Posted by sailorboy1 View Post
You only gave 1 example so far in the thread and that was a special case of an aluminum boat with mag anodes that had been in fresh water. So when you say "a couple of boats" and "clearly" just what do you mean?

I wander the yard in the Fall after boats have been hauled and have never seen any boats with signs of any over zinc problem.
Look more closely. If you do not believe it exists, what exactly were you looking for? That is the purpose of the thread.

Caustic rot in wooden boats can be very serious. One boat has loose through hulls with rot around them and a local pH of 11.

Burn-back, as discussed.

----

That makes 3, concrete well proven categories. I said several. But I wondered if there were more. I did not want to lead the discussion, I wanted to see if there were things I did not understand.
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Old 17-05-2015, 04:49   #20
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Re: Over-Zincing. Any horor stories from too much protection?

Speaking to a Yanmar dealer it seems that they no longer use zinc , only aluminum annodes. Wonder how it will work out

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