I used to think that stainless steel was a magical material that wouldn't corrode, or that would corrode very slowly. It's so much superior to other materials that there is little reason other than cost to use choose otherwise. Indeed, in many marine stores, all of the fasteners offered for sale are stainless steel. Bronze, the previous magic metal for boats, has become difficult to buy.
This is unfortunate, for there are many places on a boat where bronze, despite its lesser strength compared to stainless steel, is a superior choice. One of these places on an Alberg 30 is the fasteners that hold the rudder shoe on the aft end of the keel.
Some years back, I pulled the rudder shoe to check the pin on the end of the rudder shaft. That pin is a different discussion, one I've described elsewhere. When putting things back together, I decided to install new fasteners. The old ones were a little chewed up and it seemed like a naturally good thing to do. Things got a little hectic, however, with repairing the rudder pin and trying to ready the boat for launching. When I went to buy a new 2-1/2" bolt for holding the rudder shoe, the store was out of stock. Other stores I checked carried only stainless, so that's what I used.
I didn't give it another thought until five years later, when I pulled the rudder shoe to inspect the pin. When the stainless steel bolt came out, I was amazed. It looked like a worm had burrowed through it like an apple. Maybe stainless steel really is magic. Hocus, pocus and it disappears! This is an advanced case of crevice corrosion.
Crevice corrosion occurs in stainless steel and other metals when the metal is covered, but immersed in a corrosive liquid such as seawater. Normally, stainless steel forms a protective oxide coating that protects it. In a small wet crevice, however, the oxygen becomes depleted and cannot restore this coating. As the affected area grows, chlorine ions (from the salt in the water), migrate into the crevice and build up a concentration. The chlorine ions make the solution acidic and corrosive, attacking the protective coating and the metal underneath.
There are more detailed descriptions available. You can check http://www.hghouston.com/n032298.html and http://www.ymp.gov/documents/amr/23066/23066.pdf for more detail. Or, you can just learn the lesson that I learned: use bronze fasteners to hold the rudder shoe.