Galvanic Corrosion and Zinc's

[shrew]

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

I wet slip my boat in salt water. It's an Bravo III IO. I install new zinc's in the spring. I launch in May. I need to replace my zinc's again in July. When I pull hte boat in early October the zincs are ready to be replaced again. It seems to be affecting the zinc's on the outdrive moreso than the zinc's on teh tabs, which only need to be replaced once in the spring.

This seems to be very frequent. Where would you start looking? At stray current in the marina, or stray current on the boat?

Any thoughts?

 

[dingbat]

Re: Galvanic Corrosion and Zinc's

http://www.mercstuff.com/bravo3corrosion.htm

Lots more reading about corrosion issues on Bravo III Drive

http://www.google.com/search?q=Brav...ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7ADBF_en

 

[sasto]

Re: Galvanic Corrosion and Zinc's

Hey, shrew.

Are you hooked up to shore power at the marina? Do you have an isolator?

Hope it is something that can be solved vs. something you have to live with.

Good Luck!

 

[MainShip36]

Re: Galvanic Corrosion and Zinc's

Hey, shrew.

Are you hooked up to shore power at the marina? Do you have an isolator?

Hope it is something that can be solved vs. something you have to live with.

Good Luck!

Can you give a little more information on the shorepower hookup? How does it effect zincs? I have a shorepower hookup and use it to keep the batteries charged.

Does the shorepower do anything different to the engine zincs -vs- hull zincs?

Thanks.

 

[1fishbone]

Re: Galvanic Corrosion and Zinc's

Ask the other people if they're having the same issue.

Some areas are more prone to it than others.

You should check the grounding system on your boat, engine and out-drive.
You should also be leaving you drive UP, out of the water.

FYI years ago...you could see the galvanic action glow around some boats!
YEARS ago...when wood and steel was used!

 

[Adjuster]

Re: Galvanic Corrosion and Zinc's

Your zincs wear in relation to the size of the area they are protecting. Therefor zincs on your tabs will last longer than zincs on your drive.

If your shorepower hookup is not grounded correctly or other incorrect issues it could be sending stray current through your boat or even through the ground(dirt) then into the water then into your boat. Anyway electricity can make a circuit it will and this will eat your zincs.

I was replacing my zincs too often. My solution, while docked I bolted the wire of one of those grouper zincs to the lifting eye of my outdrive. That completely stopped the outdrive zincs from being used up. I simply lifted the grouper zinc out of the water and placed it on my swim platform while underway. I bought an old driveshaft donut zinc from a marine swap shop. Clamped it onto a stainless steel cable and hooked that cable to the negative of my batteries. Once again just lifted it out of the water while underway. I never had to change a zinc again on my boat.

 

[shrew]

Re: Galvanic Corrosion and Zinc's

You should check the grounding system on your boat, engine and out-drive.
You should also be leaving you drive UP, out of the water.

I'll ask around the marina and recheck the grounding system. It's an IO so I can't raise it out of the water (I should have been more clear on that).

Hey, shrew.

Are you hooked up to shore power at the marina? Do you have an isolator?

Hope it is something that can be solved vs. something you have to live with.

Good Luck!

Yes, it's connected to shore power except when I'm out. 2 x 30A shorepower connectors each with a dedicated shore power cord. My panel is split with 1 side for house (charger, fridge, outlets, stove) the other side for AC. When I leave the boat for the week I leave both the main for the AC side as well as the AC breaker off. Only the fridge and charger circuits are on on the house side. I don't believe I have an isolator.

 

[shrew]

Re: Galvanic Corrosion and Zinc's

Hey, shrew.

Do you have an isolator?

There is a Mercathode on the engine.

 

[sasto]

Re: Galvanic Corrosion and Zinc's

Some boats join the neutral and negative together in the boats panel, some rely on the marina's panel for this. I would run a continuity check between your AC neutral and negative to your boats DC ground..and to your outdrive. AC system should be independent of your DC.

Good Luck!

 

[sasto]

Re: Galvanic Corrosion and Zinc's

Shrew....check this out, much better at explaining it than me.

www.yandina.com/electrolysis.htm

 

[TilliamWe]

Re: Galvanic Corrosion and Zinc's

There is a Mercathode on the engine.

Not the same as an isolator on the shore power. You need to make sure your Mercathode is working, but you also NEED an isolator.

You aren't using Magnesium anodes in saltwater, are you?

B3s are prone to corrosion, so get it under control. Have you removed the props to check the rear bearing carrier? That's where the drive will fail first, if it corrodes too much.

 

[shrew]

Re: Galvanic Corrosion and Zinc's

Ok, I seem to have a bunch of homework to do. This is my priority project for the winter. I do use Magnesium and I've done enough reading recently to now be aware that they are not appropriate for salt water use. West Marine and Defender seem to only sell Magnesium sets for the Bravo III, and both are located in a seaside city. I'm in the process of locating a true Zinc set for a Bravo III.

I'm looking at Galvanic Isolators now. They seem pricey, but only until I consider the cost of a replacement Bravo III outdrive.

 

[ChampionShip]

Re: Galvanic Corrosion and Zinc's

There's a reason that I/O's aren't all that awesome for salt water applications- having to use about 50 different anodes is one of them...... But at least Mercruiser has a Seacore thing now to help a ton with this application.

Sounds like you have the wrong anodes, but even still I'd add a few. Trim tabs like suggested, but also a big, solid block or 2 mounted to the transom should help a bunch. I would also be asking the marina to do a test in the water for stray current. Someone's boat near yours could be putting the electricity into the water for you which would explain part of why your zincs get chewed up so fast.

 

[Ike-110722]

Re: Galvanic Corrosion and Zinc's

see the following http://newboatbuilders.com/docs/CorrosionOnBoats.pdf

There are various types of corrosion. In your case it could be one of two things galvanic corrosion or stray current corrosion. I would check for stray current corrosion first. First make sure the electrical system on your boat is wired correctly. If you don't have a galvanic isolator install one. If you already have one have it checked for continuity. It should not pass DC current. If it does it is no good. These things are just diodes and sometimes they fail.

If it's not your boat, then it can be the shoreside power. The best way to know is have an electrician check for stray current in the water around your boat. If there is current in the water then have the tech find where it is coming from. If its coming from your boat, you have a wiring problem. If its not you it could be any of the boats around you, or the shore power.

Oh by the way if there is current in the water this is extremely dangerous. Do not let your marina ignore it! http://www.boatus.com/seaworthy/kritz.asp

If it's not shore power then it's someone else's boat near you.

If its not that then it could be galvanic corrosion. Dissimilar metals in contact with the water can create a slight current that corrodes the less noble metals such as aluminum and zinc, if you have zincs. By the way they may not be zinc or they maybe should not be zinc. Different metals require different anodes. Everyone calls them all zincs but they are different alloys. check your owners manual, and if you don't have it, check with the outdrive manufacturer.

By the way, your anodes are doing their job. They are supposed to eat away rather than your outdrive or prop. But they are going rather fast so check it out.

 

[TilliamWe]

Re: Galvanic Corrosion and Zinc's

... West Marine and Defender seem to only sell Magnesium sets for the Bravo III, and both are located in a seaside city. ..

They are silly for doing that. But the magnesium anodes are about twice as expensive as the aluminum anodes. And that's what you need.

 

[Hank496]

Re: Galvanic Corrosion and Zinc's

Couple of comments:

We have BIIIs, and moor our boat in salt water (New England May - October). The recommendation from the local Mercruiser service shop is for aluminum anodes. FYI Merc sterndrives are shipped from the factory with Alu anodes.

If you have an older BIII that does not have a prop zinc, get one of the modification kits and install it!. The kit is a replacement outer prop nut that has a frame that the prop zinc can then be screwed into.

Check the wire on the mercathode. It is located on the bottom of the transom ring. The wire should be clean as in not painted. On a lot of boats the mercathode unit is flush or almost flush with the hull. If it does not get masked over prior to bottom paint, it can get painted over. Then it 'works', but is completely ineffective. If the wire is damaged, or painted replace the unit, or clean the wire (as needed).

Finally, it is ugly, but a good solid coating of drive anti-fouling paint will do wonders. I have been using Tri-lux. It is great for keeping growing crud and barnacles off the drive. It will also protect the aluminum because all of that crud latches onto the drive paint and breaks it down, cracks it, eats it etc. and next year the salt water will get into the aluminum.