Bottom Paint Corrosion

[Guest]

Anyone else seeing excess corrosion on their bottom paint?

I'm in a marina with a history of stray current problems, but I'm seeing patches in the middle of the paint as well as corrosion on the edges, and I hang a zinc fish over the side as well as maintaining the zinc on the saildrive. I go through about 75% of the Yanmar zinc in a year.

I've had the electrical system thoroughly inspected & tested- no problems there. Balt-o-plate on the bottom. Salt water marina (Puget Sound).

Thanks,
Eric

Corrosion

[Posted by: Eric]

[Guest]

I've had the same problem, Balto-plate paint, and in the same place. I'm still trying to devine the problem. Talked with TPI, and there not sure either. I have a problem with my Galvonic isolator, part of the system shorted out in the lazzaret, and is being replaced now, supposedly from the rudder bearings leaking. I will keep you posted if that is it. Hull #37-ecclesiastes.

[Posted by: Michael Thompson
]

[Guest]

We had a similar problem at the aft end of the keel bulb. However, the paint (Baltoplate) turns darker and loses some of its antifouling properties.

Our yard thinks that the problem is insufficient thickness of the barrier coat. Impurities in the lead react with the copper in the paint.

However, there is another possible explanation. I saved an old message from the Sailnet C&C owners' list back when I owned my old boat. A reliable and knowledgeable contributor the list, who is a professor of Oceanography in his day job, said:

"... Chemical oceanographer speaking here. Organic rich mud gets anoxic (no oxygen) and the bacteria produce hydrogen sulfide, the stinky smell of
rotten eggs. This hydrogen sulfide diffuses into the water, but only a few feet, where it can react with the copper in the bottom paint forming CuS,
copper sulfide. This is not an electrostatic process, but rather theformation of a very stable compound (actualy chemical bonds). Hence, the
need to sand it off (or scrape). CuS is not toxic, so you will get growth on these spots. Typically you see this problem on the bottom of your keel
since it is close to the mud, and if it's shallow, in the mud. Now, it is possible that when they drove the piling they stirred up a lot of mud, and
the associated hydrogen sulfide, which reacted with your bottom paint (and that of the boats around you). A light wet sanding with 400 or 600 grit,even done underwater (I do this all the time...it's really easier than on land!), will remove the CuS "tarnish".Hope this helps,..."

Will explore and post when I learn more.


[Posted by: Dan Grossman
]

[Guest]

My yard felt that the issue was with impurities in the lead, but was adamant that the epoxy barrier coat was of sufficient thickness. They did comment that the lead was oxidized under the epoxy and that this could mean the keel was not properly prepared or stored prior to epoxy application. I'm pretty sure that the patches in the middle of the keel (not visible here) are due to impurities. However I am not sure about the corrosion on the edges.

I talked to an engineer about grounding the paint, but this evidently is a bad idea.

Wet sanding would not help this problem; the paint has actually broken down in the dark spots.

[Posted by: Eric
]

[Guest]

Has a definitive cause been discovered for this??? We had VC Offshore sprayed on at commissioning and six months later we're seeing the same thing. Our diver says he's seen this on a number of J105 keels that have VC Offshore or Baltoplate (similar paints from Interlux). We've also burned through the zinc in six months and the prop was in perfect condition. The dealer that commissioned the boat pulled it and touched up the paint on the keel -- hopefully a little thicker coat will do the trick.

[Posted by: Christian
]

[jerry]

Looks like I get to join this club. Just had the boat hauled for lower rudder bearing work (another story to come later), and saildrive maintenance. Discovered electrolysis on the keel. I had a dehumidifier connect to the boat's AC running in the boat all winter and no problems. My last zinc lasted close to 18 months. Suspect relatively new and quick. I have the boat dived, inspected and cleaned monthly. No mention of this problem 1-2 months ago, but am double checking with the diver. Appears to be hitting the rudder bearing and trailing edge of the keel, but also leading edge and at bottom to a lesser degree.

I'll start my homework and try to troubleshoot. According to my yard here in Seattle, they have seem this in several of the J-109 and feel the keel barrier coat is too thin when it comes from Pearson. They recommend a thicker epoxy. My bottom paint is Trinidad (former owner took off Balto for a lower maintenance cruising paint). Eventually will have Balto

Any new ideas, solutions that will point me in the right direction? Any guess how long it would take to get to this level? If bad stray current in the marina, is the electrolysis days, weeks, or months?

Thanks

"Rudder Bearing Electrolysis"
"Trailing Edge Electrolysis"
"Close Up"