Electrolysis

[Egregious]

Just got Polly P. hauled out for bottom paint, and once they sanded everything there was some minor*pitting on the shafts and struts*and other evidence of galvanic corrosion, but not bad.* The zincs on the transom were all but gone, and the shaft zincs were gone on one side and degraded on the other.* The guys in the yard say that the most common cause of this is a bad bilge pump leaking current into the bilge water.* Also of course it could be the Sea Ray Sundancer two slips down that never leaves the dock and seems to be running the HVAC 24/7 while she is buttoned up for the Winter.

The zincs on the port side shaft were new as of June '10 and were gone while starboard zincs were still there but needed replacing.* I think maybe the port side strut is not bonded to the aft transom zinc properly?* Of course I will check this, but I didn't have time as I was preparing for cold weather on the hill* and other business.

OK people, please give me the wisdom this board can give about galvanic corrosion and what too look for, how to fix it, et al.

Thanks

Woody

 

[Marin]

In our marina zincs, particularly the smaller shaft zincs, tend to be pretty much gone or seriously degraded in six months, so your experience is not unusual for us. But it may be unusual for your marina. Our marina is pretty "hot."

Uneven degredation can be caused by bad connections to the zinc. The amount of current is so small that it doesn't take much to stop it. So if you are getting uneven zinc degredation one cause (of many) can be a poor connection between the zinc and the metal it's fastened to. For example even a tiny bit of corrosion on the bonded bolt holding a transom zinc can isolate the zinc from the bonding system. So it's a good practice to always clean the surface (or bolt) a zinc is going to be mounted to.

When the boat is out is the best time to check the continuity of everything. When the boat is in the water this can fool you since the water itself will provide continuity between the fittings that are undewater. Have a shop (or do it yourself if you know how) check the continuity of the entire bonding system with the boat out of water. This will show you if there are any breaks in the bonding network that need to be repaired.

 

[Egregious]

Marin wrote:

Our marina is pretty "hot."

Is it worth it to get a "fish" (I think that is what it is called) which is essentially a zinc that is grounded and you put it in the water in your slip, and it takes most of the sacrifice in the case where other boats are making it hot.

 

[O C Diver]

My marina is pretty hot. Have gone to an over size rudder zinc and doubled the shaft zinc. Unfortunately my bonding system didn't save the through hull fittings, seacocks , and brass piping. Ended up replacing all of it this sping. The brass pipes were eaten 75% of the way through. Now both through hull systems have heat exchanger zincs plumbed into the systems.

Moral of the story: If you see pitting in your shafts and rudders, don't forget to check your through hulls and seacocks.

Ted

 

[Marin]

A lot of people in our marina, including us, suspend a zinc on a heavy cable (battery cable) about eight feet down with the boat end attached to something tied into the bonding system. In our case the cable is clamped to one of the bronze rudder posts. One reason for doing this in our marina is that Bellingham Bay has a high fresh water input from the Nooksak River and a couple of smaller streams, one of which comes out right next to the marina. So often you can dip your hand into the water and have no salt taste at all. The zinc-on-a-wire gets down into the saltier water underneath.

I don't know of anyone who spends money on the "fish." Everyone I know simply uses old zincs, usually the big transom zincs. The dive shop many of us use has hundreds of them that they recycle from time to time. They are usually about 2/3 degraded. So we get them for free, hang them on the wire, and when they are down to about nothing we get another one.

An advantage of the zinc-on-a-wire is that you can pull it up and see how things are doing. If it starts disappearing at a faster than normal rate, or stops disappearing, you know you have something to check out.

When we go out we simply pull the zinc up and put it and its cable into a bucket on the aft deck.* My wife made a big "STOP-ZINC" sign that we hang on the shifters whenver the zinc is deployed.* This reminds us or the diesel shop not to put anything in gear until the zinc is hauled up.

With regards to protecting your boat from stray current getting into the water from another boat nearby, I've read that the only way to effectively protect yourself is to install an isolation transformer.* A galvanic isolator will protect the boat from stray current in the ground wire of a shorepower connection but does nothing to protect the boat from current in the water.* But.... isolation transformers are way expensive so most boats don't have them.* Hanging a zinc in your slip connected to the shorepower ground won't do anything to protect your boat from current in the water.* And a current leak into the water can do major damage in a big hurry.* It will eat props, through hulls, you name it, real fast.* This is one reason I go out of my way to pull someone's shorepower cable out of the water if I notice it's slipped in.


-- Edited by Marin on Wednesday 22nd of December 2010 10:06:14 PM

 

[Phil Fill]

*
I check for stray electricity several times per year, and have*a diver clean/check the zings twice a year.***Stray electricity can come though the water, the shore power and/or your boat.* Most high amp items like heaters, water heaters, chargers, and incorrect wiring produce some stray electricity.* *You can ask your marine to check for stray electricity or a marine electrician. First find the source. Its the milliamps that will shock/kill you, not the volts.* Last year our marina went on a stray electricity hunt, my boat was marked as it had 0.06 amps the marina allows 0.04 amps.* It was an out let that I wired back wards.* The big*proiducer was a steel commercial they where welding on.* It can cost a bundle to find and fix stray.** So before you ask the marine you might want to make sure your boat is not producing as the marine will make you correct/fix it,*****

 

[Egregious]

Marin,

please tell my why you connect the zinc to your boat's bonding system instead of grounding it to a shore power ground.**They are both serving as a ground as I understand it, but what is the difference?

I've heard that you can take a simple multi-tester and put the red lead into the water and the black lead to a reliable ground and if you got current, even if it is small, then you can determine how "hot" your slip is...** Does this make sense?

Thanks for your help.

Woody

 

[Marin]

Egregious wrote:

Marin,

please tell my why you connect the zinc to your boat's bonding system instead of grounding it to a shore power ground.*

Because that's what our marine electric shop told us to do.* This was seconded by the dive shop that does all the underwater work on the boats in our marina.* And it's how*all the hanging*zincs are connected that I've seen on the boats in our marina. I've never seen one connected to the shorepower ground, always to the boat's bonding system.

As I understand it, the zinc needs to be in*the boat's bonding system to provide any protection.* But*my dog*knows more about eletricity and electrolysis than I do, so all I can do is follow the advice of the pros.* You'll have to pose your question to one of them (the pros, not my dog).

I have no idea how one checks for current in the water around a boat.

-- Edited by Marin on Wednesday 22nd of December 2010 10:28:25 PM

 

[RickB]

O C Diver wrote:Unfortunately my bonding system didn't save the through hull fittings, seacocks , and brass piping. Ended up replacing all of it this sping. The brass pipes were eaten 75% of the way through.

Moral of the story:* Don't use brass pipes in seawater systems.

*

 

[RickB]

Phil Fill wrote:You can ask your marine to check for stray electricity or a marine electrician.

We don't get many marines around here but there are lots of Coast Guard guys hanging around. I don't know how many of them are trained to check for stray electricity though.

Most of the marine electricians here are fairly careful and don't stray into the water very often. I guess if they did and there were no marines around, the Coast Guard guys would pull them out.

 

[Phil Fill]

I have two Grouper Anode, fish, zinc on the Eagle besides the big main zincs, shaft, rudder and bow thruster zincs,* One is *connected to the bow thruster and the other is connect to the engine and gen set. **
*
The galvanic Isolators are one route but electricity will take the least pass of resistance.* First you have to find out the source.* You should have your zincs checked and check for stray on a regular bases.* Many drives will check the water before they suit up.* It only takes a small amount of milliamps.

 

[skipperdude]

sounds to me lik you need a Galvanic isolator that should help a lot.

They are not expenseve as a matter of fact I built my own.

I used a piece of aluminus and two bridge rectifiers wired together and run to the ground of the shore power.

SD

 

[C lectric]

Egregious

http://www.yachtwork.com/report-corrosion.htm

Try this article on Scott Fratcher's site for a primer. To really check your boat out thoroughly will take more reading and effort but it may help. Thee are several books on the subject.

 

[Egregious]

Haha I shouldn't have called it "electrolysis" since that is what your wife gets to keep the hair from growing on her legs!

"Galvanic Corrosion..."

Either way it has pitted by solid stainless shafts in some places.

Now, what would cause that?*

 

[RT Firefly]

Hiya,SS in some places?* Probably crevice corrosion.

http://en.wikipedia.org/wiki/Crevice_corrosion