Hi,

I'm no expert, but I've been studying
electrochemical reactions including stray current
corrosion due to problems with the rudder post on
a boat that we recently acquired.

Since the steel is very clean under the bubble, I
would guess that you are correct about it being an
electrolysis type reaction. Otherwise it would be
a rust bubble.

The "Marine Electrical and Electronics Bible"
lists the following under Corrosion System
Troubleshooting:

Paint stripping off keel and hull:
Hull overprotected (too many anodes)
Severe electrical leakage to hull
Copper based antifoulant

The reason that the problems show up around the
welded areas is because that is where you are more
likely to have pinholes in your paint.

Don W.

wrote:
Hi, I have put some pictures he
www.photobucket.com/
Search for images named "sabina113"

I find when I take the yacht out of the water each year I have bubbles
in the paint mainly where things have been welded to the inside of the
hull. The bubbles are filled with salt water which is quite strong and
possibly acidy. When the bubbles are opened there is very clean steel
underneath. It doesn't seemed to have corroded the steet alot. The
zincs last a year ok.
I guess this is some sort of electrosis or stray current erosion but
was hopeing for a second opinion.
Also, if anyone looks at the pictures, have you seen a driveshaft
thrust bearing setup like in the picture. There is a small brass bung
on top where the thrust bearing is. Do you put oil in there?
Thanks
asdex