Galvanic corrosion is caused by dissimilar metals below the water line.
Zincs are bandaids, they do not correct the problem. You should be worried,
because your throughhull bronze fittings will fail. It is just a matter of
time and from your description of your zincs, it won't be long. Bonding your
throughhull fittings will even make the corrosion rate faster. I suggest you
locate the cause of the galvanic current and eliminate the problem. I would
not be surprised if you find that the throughull fittings are the cause of
the problem, not the effect. For instance, there is no reason not to replace
the fittings with plastic fittings. You already run the fire risk anyway, as
your hull is plastic. The use of metal throughulls adds no additional value.
There are some exceptional high quality plastic ball valves used in the
corrosive chemical industry, which would make the entire problem go away.
Steve

"cvj" wrote in message
...
I recently had to install a Galvanic Isolator in order to reduce zinc loss
on my boat. I have zinc anodes both on the propellor shaft (3 small zincs
on a Volvo Sail Drive) as well as a large separate zinc wired to the
engine. Prior to the installation of the Galvanic Isolator, the propellor
zincs would be "gone" after only 1 1/2 month.

But now I am concerned that the bronze seacocks (4 years old) may also
have been subject to some galvanic erosion - and I am wondering how you
check the integrity of the bronze seacocks without hauling the boat out of
water? Is this something that can be done by a diver?

And in general - how "paranoid" should one be about the possibility of a
bronze seacock failing without prior warning? I have purchased the
recommended soft wood plugs....but that still does not abate the worry....

Any thoughts.....

Clus