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Old 26-06-2016, 09:32   #16
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Re: Magnesium anode for cast iron

Originally Posted by mike66 View Post
So for my education, I misunderstood the intent of the OP to protect the keel from corrosion. Keeping the salt water away from the cast iron is probably the best way to do that. I assume in a fiberglass or wooden boat with a cast iron keel that there is still a need for anodes tied into the bonding system to protect thru-hulls, rudders, etc. Is the keel also tied into the bonding system, and if so, would it matter if the anodes are placed on the keel or some other location, like hanging over the side or on the transom?

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There are two schools of though on bonding of through holes, etc. Bonding works well at preventing galvanic corrosion, provided all connections and wire are clean and have low resistance and the anode is replaced before it becomes ineffective, all the time.

My problem with Bonding (I'm in the camp that says don't do it) is that over a relatively short time the connectors, wire and posts will corrode enough to provide a high resistance at one or more locations. Generally from condensation in winter. Then it's bad as any cheaper brass will start to dezinc.

Also if the anode is not replaced soon enough the less noble metals in the bonded circuit will start to sacrifice themselves right away which also can increases resistance..

Bonding does work, but only when everything is new. Once connectors starting increasing resistance, it can create and cause galvanic corrosion in some cases. As I try to reduce failure modes as much as possible with that engineering thingy, I prefer not to bond any through hulls on the boat

While there is a distance issue with anodes being too far from the metals being protected, on a boat you would rarely come close to that. Underground pipelines, generally are protected by anodes placed 25-50 feet away from the piping. So throwing a zinc fish overboard, that is properly connected to the engine for example, would provide galvanic protection. The key is having a low impedance connection to the engine (prop). That is not always the case even with a bonded system.

The poor mans (or woman's) impressed current system would be to connect a piece of steel (say a 1-1/2" angle iron 10" long to a 14 gauge wire that was connected to the +side of the battery and tossing it over the side. This assumes that the engine is connected to the negative ground. Of course you would have no way of knowing if it worked or not, but odds are it would work pretty good.

Note: Bonding a non-encapsulated iron keel would not prevent the iron from rusting when in contact with water. Only an epoxy barrier coat or zinc chromate for a small touch up would do that.

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