Galvanic Corrosion

George ( C&C 40 ) · Post by **George ( C&C 40 )** » Sat Feb 11, 2006 11:45 am

Hi All,

For the two years I have owned my boat I have been plagued by the necessity of replacing my shaft zincs every month or so. Since I scuba dive the boat every two weeks to keep her race-ready this is not a huge problem. However it is a pain in the butt, and the reason for the zincs being eaten away continues to elude me.

The boat has a very nice Xantrex Truecharge 40 charging system which features full galvanic isolation at all inputs and outputs. I've also installed a spiffy new galvanic isolator right at the point that the shore power enters the boat. I'm pretty handy with circut testers and digital multimeters and have checked every A/C circut, D/C circut, the galvanic isolator itself, the dock wiring, boats in close proximity to mine, and even stray current in the water at several points around my end of the marina. Also, I should point out that the shorepower A/C system and the boats D/C system do not share a common ground. A/C uses the shore power ground and D/C is grounded through the engine block.

I'm posting about this problem now because I saw something when I had the boat hauled for the winter this past Wednesday. The paint on the lead keel / centerboard trunk was coming off in an odd pattern. I used a good lead metal primer before the bottom paint went on too. The pattern reminded me of galvanic patterns I have seen on badly corroded props in the boat yard.

Here's a picture...

Is it possible that a combination of aluminum mast step, aluminum mast, aluminum spacers on the keel bolts, stainless steel keel bolts, washers, and nuts, lead keel, and saltwater in the bilge could be generating current capable of stripping my shaft zincs? The keel/mast/rigging complex is not bonded to the engine block and is therefore separate from the D/C system. The connection would have to take place through the salt water between the shaft and keel.

It never occured to me to test for current generation in the keel so it is one thing I haven't checked for. Has anybody ever heard of something like this happening? Any other ideas would be welcome too.

George Jones
C&C 40 "Delphinus"
Wavelength 24 "Poco Loco"

George ( C&C 40 ) · Post by **George ( C&C 40 )** » Mon Apr 10, 2006 11:24 pm

Hi again all,

The problem is, I believe, solved! It occured to me that what appears to be galvanic corrosion might actually be stray current corrosion if it was very very intermittent. Pure stray current corrosion would, of course, melt the zincs, and then the prop off the boat in a matter of hours. However, a few minutes of stray current corrosion could masquarade as galvanic corrosion if it was spaced out over a month or two. Stray positive DC current in the bilge water would connect through the keelbolts to the lead keel through the salt water, to the prop shaft, to the DC negative ground on the engine block. This would produce the galvanic "flowers" I saw on the lead keel as the prop zincs were stripped towards the keel. Sure enough, one of the tests showed that the bilge pump motor casing was putting 5.5 volts of positive DC current into the bilge water whenever the pump operated. I've replaced the pump and now there is no more current in the bilge water.

George Jones

C&C 40 "Delphinus"
WL 24 "Poco Loco"

Post by **dasein668** » Tue Apr 11, 2006 7:23 am

Nice catch! Did you track this down yourself?

George ( C&C 40 ) · Post by **George ( C&C 40 )** » Tue Apr 11, 2006 7:50 am

Hi Nathan,

Yeah, I figured it out myself. Took two years to do it though so I'm not sure how high a grade I can give myself for this one. Nice to have it solved finally.

George

Figment · Post by **Figment** » Tue Apr 11, 2006 7:58 am

I remember scratching my head over the original post, thinking what a frustrating pain it must be to chase down stray voltage like that. Congrats on what is certainly a rum-worthy victory!

Post by **dasein668** » Tue Apr 11, 2006 8:01 am

George ( C&C 40 ) wrote:Took two years to do it though so I'm not sure how high a grade I can give myself for this one. Nice to have it solved finally.

Well, if you are a marine electical engineer by trade, not so high... otherwise, I'll give you an A- on that one! ;-)

Post by **Tim** » Thu Apr 13, 2006 7:30 am

It's amazing how such a seemingly small thing can create such havoc!

There's no doubt that there are many, many boats with similar problems, thanks to old electrical equipment and/or poor wiring. Reason #436 for why one should do the job right in the first place!

Good work on the diagnosis. I don't think I have enough patience to track down a little thing like that.