If you have stray current in the marina, the worst thing you can do is bond all the immersed metal bits. What happens is the stray current flows into one of the metal bits, through the bonding and exits another metal bit. The metal bit where the current flows out will be eaten by electrolysis
. See here for a good, intro article: Grounding
You should consider de-bonding any metal through hulls that do not need to be electrically connected to anything else. Since your rudder post was not bonded, that implies stray current corrosion was not the culprit. Could it have been bonded in the past?
is unfortunately self-corroding in anaerobic sea water. You should have a dedicated zinc for the rudder, either on the shaft, or failing that in a nearby zinc mounted in the hull and connected to the shaft by a tinned braid or shaft brush. This zinc should not be connected to anything else so you reduce the chances of stray current corrosion....
Regarding crevice corrosion. Your photo
showing the band of pitting is pretty scary. I would do more than take a file to it - take a dremel tool with a pointed grinder, and just like a dentist going after a cavity, dig into some of the holes. You have to determine how deep they go. Just like a dentist, your goal is to get to the bottom of the cavity and clean it out. It's OK to remove material -after all, if it's pitted it's not adding much strength, and also, it's unlikely you are removing a large percentage using a dremel tool. I have done this on chainplates and found some holes penetrate several mm.
If the band of pitting does indeed extend deep into the metal, you can calculate how much strength you have lost
. Basically, you need to measure the depth
of the pitting, and use that to calculate the % loss in cross sectional area. Strength varies more or less with area (strictly speaking, it's also related to the distance from the center of rotation, but just ignore that for now).
Getting a welder to look at it seems like a good idea, although of course welded metal is often more corrosion prone.
The concern I would have on my own rudder shaft is that crevice corrosion has eaten away the metal inside the rudder where you can't see it....