Rudder head corrosion
Zinc Chromate primer will not provide an effective barrier between the Aluminum and Stainless (as you note,neither of which do well under water); but Del''s advice presents a good start.
Even better would be a nylon sleeve, separating the dissimilar metals (perhaps not practical?).
Further to peterpan2k, If the pintle fastenings are accessable from the inside, bond them and the rudder shaft, if possible, (using a shaft brush) to the ground bus.