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Old 29-10-2014, 09:58   #1
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Electrolysis problem

Each year after fall haul out, I need to repair damage to my lead keel resulting from an electrolysis issue. Last spring, I completely stripped the keel, repaired the powdery areas with epoxy filler, applied four coats of barrier coat and finished with 2 coats of fresh bottom paint. Still the same problem although not as bad as previous years. Last spring I had a marine electrician look at the AC system and determined the problem was most likely from an incorrectly wired GFCI. I was getting a low voltage reading on the keel when the outlets were switched on. I replaced all three recepticles with new GFCI rewired as a precaution. The entire system is bonded to an aft keel bolt. Still the same problem. Since I am in a slip with power access, the only continuous electrical appliances on constanly are refigeration, battery charger and GFCI outlets. The bilge pump also checked out ok.

In addition to the keel, I am also getting some electrolysis on the bronze strut. The 2 zincs placed forward of the strut are barely being sacrified. The max prop zinc is about 50% sacrificed meaning its doing its job.

The receptacle outlet on the boat checked out OK. Totally perplexed. Any suggestions on what to look for next.
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Old 29-10-2014, 10:25   #2
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Re: Electrolysis problem

There is a difference between bonding & grounding. If you have an outboard engine, your electrical system shouldn't contact the keel, only the (-) back to the battery. With an inboard engine, the block is the ground. Check your AC wiring. Also read Calder's Boatowners Manual. Great material on stray currents and too long to explain here, that's why they write books. You could do some searching here on that topic, too, since it's come up before. Good luck.

Marine Grounding Systems | West Marine
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Old 29-10-2014, 10:30   #3
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Re: Electrolysis problem

Are you sure its a lead keel? Sounds more like iron.
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Old 29-10-2014, 10:32   #4
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Re: Electrolysis problem

A GFCI does not isolate underwater metal from shore power ground currents. All it does is make sure the AC in = AC out and cuts the power off if they don't balance, meaning some AC is leaking and could kill someone. It does not monitor DC current and doesn't even connect to the ground lead.

A Galvanic Isolator costs under $100 and is the first line of defense against electrolytic corrosion caused by the ground lead in your shore power supply.
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Old 29-10-2014, 10:34   #5
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Re: Electrolysis problem

The inboard auxillary is grounded to the engine block. It appears I need to check the entire AC system starting with the power source on the dock and work my way back.
Thanks
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Old 29-10-2014, 10:39   #6
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Re: Electrolysis problem

Thanks for the Isolator suggestion. A definite add-on.
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Old 29-10-2014, 10:49   #7
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Re: Electrolysis problem

If you are getting measureable voltage it should be relatively simple (which is not necessarily "easy" though it can be) to track down.

The way I am understanding this, you only have this issue when hooked to shore power, correct? If you are on battery with the inverter on, do you still have the same issue?

Have you tried hooking up to a different shore power outlet to double check it is not a dock related problem?

If I understand you correctly, there is only a voltage when the GFCI circuit is energised? If that is switched off at the breaker there is no measureable voltage on the keel? If that is the case, I'd physically remove all 3 outlets, well I guess you could just remove the 2 hot leads and leave the grounds connected but I am a wuss in my mature years and would simply spend the extra few seconds and remove all 3 wires on each and put a fold of electrical tape over the ends before proceeding. Anyway, switch it back on and see if the voltage has gone away. If it has then start checking out the breakers but if it comes back start looking for some splices in the line that are getting wet. You might have to do some digging and if you can track it down in this direction and locate some offending wires it may be simplest and best to simply run all new wire in that circuit.

Anyway, hope that is a start and post back what you find out.

Good luck!

Mark
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Old 30-10-2014, 10:20   #8
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Re: Electrolysis problem

Freedom 145-
Corrosion 101
Lesson 1: It takes three components to allow galvanic corrosion:
1. Dissimilar metals
2. Electrically connected
3. Immersed in the same electrolyte
Lesson 2:
AC rarely causes corrosion.
Lesson 3:
AC leakage into the water can be deadly in brackish or fresh water.

The preventative measures on the keel last spring would have eliminated Point 3 in Lesson 1 above. The corrosion you are experiencing on the keel is most probably not caused by AC. The leakage of AC into the water is your biggest problem and must be corrected ASAP.

As to the corrosion on the keel, lead is very noble and is not likely to galvanically corrode. I agree with OneStepCSY, and I too suspect the keel is made of steel/iron. If it is truly lead, than I would suspect stray current corrosion.

The end result is you will probably need the services of a competent marine corrosion technician to sort this out.
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