I only have a 10' CB Montgomery dink but have had several centerboard boats and others with external lead or cast iron keels.  All used in saltwater.

The biggest centerboard was approx 12'x4'x1100lbs.  Steel plate clad in wood like a sandwich...steel edges were exposed and painted with coal tar epoxy.  No rust problems after 4 years of ownership.  The boat was 30yrs old and the board original at that time (1969). This one had a new SS cb wire which let go after 2 years without zinc protection (I didn't know better at the time) so I changed it to dacron by splicing 3-strand (hard lay for less stretch).  No problems after converting to dacron and I had a stop knot in the line to prevent the board from dropping too deep.

Next...cast iron keel on a 30'r. Coal tar epoxy and zero rust problems.  Any small breakthroughs were touched up at the annual haul and bottom painting.  This keel was 2500 lbs and bolted to the hull.  I bonded the keel bolts with an external zinc and had no problems (3 yrs).

Another one: Lead keel...3500 lbs.  Just too week to use for a centerboard but the iron keel bolts were bonded to an external zinc.

In summary: I would always use a sacrificial anode (zinc for saltwater and magnesium for freshwater) if leaving a boat in the water for more than a week or two at a marina.  Corrosion is a minor problem compared to electrolysis.  Stray currents can be real bad from all the boats grounding their own gear.  For trailer sailers simply using modern epoxies to coat your boards (steel or cast iron) will 99% take care of the problem.  Dacron line is the only way to go if you have room to splice it...it won't deteriorate in the water even a little bit. Otherwise, if you use wire just change it out every few years..wire that looks good can be brittle and break easy so visual inspections aren't accurate. This really isn't a big deal to take care of so rest peacefully and enjoy your Monts.

Regards,
Bill