T&T: Can you over zinc a boat?

[Arild Jensen]

> -----Original Message-----
> From: Robert Kassal

>
> My diver thinks our boat is over zinc'd. This is a 48 foot boat with long
> shafts and it has three zinc's per shaft, one each on the rudders and trim
> tabs. The latter are probably a size too big.
>
> During the barnacle removal, the diver hit a 5/16" SS bolt (that
> attaches a SS swim platform support to a SS transom bracket) and it broke
at
> the head. The post mortem shows a large pit into the bolt right at the
head. I know this
> bolt has only been in the water 8 months. Could this be the result
> of too much zinc?
>
> Bob



REPLY
Yes it is possible to over zinc a boat and yes that can be damaging.

However, your description of the bolt head snapping off, does not sound like
it resulted from over zincing the boat.
More than likely it was localized corrosion due to incompatible metals or
crevice corrosion as Rodger Wrona suggested.

To determine if your boat is over zinc'ed, measure the galvanic potential of
the bonding wire at several points; closest, mid-way and furthest from the
zinc on your transom. The reading should be around negative 750 - 950
millivolts relative to a silver chloride half cell reference.

In the absence of a bonding system measure the individual parts. Proper zinc
application should raise the galvanic potential 200 millivolts more negative
than its natural galvanic reading in full salt water. Brackish water will
require some kind of compensation for salinity dilution.
If this sounds complicated, best leave the job to someone with experience in
this matter.
Its too easy to jump to erroneous conclusions based on incomplete data or
mis-corrected galvanic readings.

best of luck

Arild