I've had a 3x6x5/8 zinc bolted to my transom for at least 10 years and have noticed that it's not dissolving like the zincs on my OB motor.
So I checked it to see if it was making contact with the hull surface.
Using an ohm meter, I've discovered that the surface of this old zinc was non-conductive when holding the probes on the surface. It seems as if the zinc had formed an insulating surface on it.
My question is: Is this a common occurrence with zincs in general or a sign of a low quality or incorrect alloy blend zinc?
This is a trailer boat. The brand is Marshall Mfg.
Back view of the zinc
![Image](http://i213.photobucket.com/albums/cc255/sdsaw/DSCF1201.jpg)
Close up of the surface. Counter bored the back to clear the mtg. stud washer
![Image](http://i213.photobucket.com/albums/cc255/sdsaw/DSCF1202.jpg)
This how I have it installed. Thru bolted threaded rod which is conducting to the hull with nylon lock nuts with flat washers.
![Image](http://i213.photobucket.com/albums/cc255/sdsaw/DSCF1200.jpg)
Rejuvenated zinc (cut the top surface to expose fresh zinc and ground all other surfaces)
and used lock washers to dig into the surface and annually check for continuity.
![Image](http://i213.photobucket.com/albums/cc255/sdsaw/DSCF1213.jpg)