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  #11  
Old 10-16-2011
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But my guess is that the rotating shaft would provide a wiping action that would polish the bond between the carbon and SS. My shafts are 1 1/2" in diameter. It would take many lifetimes to weaken such mass. So there may be a reason for making a trade off between potential corrosion and providing a better shaft bond.
Rotating or not, the reaction is electrochemical and continues as long as there is contact. I have seen shafts rendered useles in a few weeks due to this issue. I also find it curious that one would try to defeat corrosion by introducing an element known to cause corrosion.

To anyone interested in this topic I highly recommend "The Boat Owners Guide to Corrosion" by Everett Collier. Everett manages a pretty good explanation in only 300 pages.
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Originally Posted by Fstbttms View Post
Of course it is possible to over-protect a boat with zinc. When creating a galvanic cell (such as when protecting an underwater metal part with an anode) the water around the part becomes alkaline. Many anti fouling paints are softened and blistered by alkali. Hydrogen bubbles may also form under the paint surface, causing it to lift off. The more anodes you use, the worse the effects. I have seen many, many examples of anti fouling paint being damaged like this.
As previously stated ... anodes should never be in contact with anti-fouling paints. Without contact there can be no electrochemical reaction.
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Wooden boat are a whole different world however most corrosion and woode decay is caused by the fasteners themselves. Anodes are there to protect metals (not wood) and should not be in contact with the hull or bottom paint (metal boats are another animal). This is why I/O manufacturers will deny warrranty if antifouling paint touches their drive assemblies.

I strongly recommend Everett Colliers book or The ABYC Corrosion course.
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Originally Posted by boatpoker View Post
Rotating or not, the reaction is electrochemical and continues as long as there is contact. I have seen shafts rendered useles in a few weeks due to this issue. I also find it curious that one would try to defeat corrosion by introducing an element known to cause corrosion.

To anyone interested in this topic I highly recommend "The Boat Owners Guide to Corrosion" by Everett Collier. Everett manages a pretty good explanation in only 300 pages.

Just maybe the problem is not as sever as you make it out to be. When a carbon brush rides on a SS shaft, there is very little voltage between the bonding carbon and the SS shaft. If there is no voltage difference, there will be no corrosion. Of course if there is an electrolyte between the two different metals, there will be a voltage difference.

Ships commonly use carbon brushes but usually the shaft has a copper collar upon which the brushes ride.
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[QUOT]Every little voltage between the bonding carbon and the SS shaft.[/QUOTE]

Interesting, thats the first time I have heard someone contradict the Noble scale
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Originally Posted by boatpoker View Post
[QUOT]Every little voltage between the bonding carbon and the SS shaft.
Interesting, thats the first time I have heard someone contradict the Noble scale[/QUOTE]

If you're going to quote me, quote correctly please!

" When a carbon brush rides on a SS shaft, there is very little voltage between the bonding carbon and the SS shaft. "

I am dismayed someone would believe that a voltage difference could exist between two dissimilar metals when they are tightly bonded together. It is my understanding that galvanic corrosion is caused by the interaction of two dissimilar metals connected in an electrolyte such as salt water.

Last edited by foggysail; 10-16-2011 at 08:57 PM.
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IF your marina is a HOT marina, depending upon the time you want to have your boat dived and zinc replaced, will depend upon HOW many zincs to put on. With 2 zincs, I go about 4 months, 3 zincs I can get a bit more ie 7 months and one will be dang near gone! ie literally! My max prop does not have a tip zinc from what i can tell, so the extra zinc is liked by a lot of folks in my marina, recommended by the local repair place at the marina to stave off excess damage.

The how much frankly will depend upon the marina etc!

Also there are some SS prop paints you can put on the shaft to stop Barnicle/mussel/shell type creatures from attaching themselves. Obviously do not paint the zincs or place the zinc over the paint........

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Originally Posted by foggysail View Post
Interesting, thats the first time I have heard someone contradict the Noble scale
If you're going to quote me, quote correctly please!

" When a carbon brush rides on a SS shaft, there is very little voltage between the bonding carbon and the SS shaft. "

I am dismayed someone would believe that a voltage difference could exist between two dissimilar metals when they are tightly bonded together. It is my understanding that galvanic corrosion is caused by the interaction of two dissimilar metals connected in an electrolyte such as salt water.[/QUOTE]


Humidity is sufficient to complete the circuit. In over 2500 inspections I do not think I have seen more than a handfull of dry bilges under stuffing boxes, even with dripless type seals.

PS. The "quote" you complain about is a direct cut & paste from your own post.
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Originally Posted by boatpoker View Post

PS. The "quote" you complain about is a direct cut & paste from your own post.
You better read it again, I used the word "VERY" not "EVERY"

Yes, humidity can result in condensation. But getting back to two tightly bonded metals with different galvanic corrosion voltages. The bond will prevent a voltage difference between the metals and I believe you know that.
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Old 10-17-2011
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Originally Posted by boatpoker View Post
As previously stated ... anodes should never be in contact with anti-fouling paints. Without contact there can be no electrochemical reaction.
Read my post again. Anodes do not have to be in contact with the anti fouling paint to damage it.
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