The carbon fibers react with the aluminum. You'd think the clear coat on the seat post would prevent this. But my carbon post in an aluminum frame did eventually make some white corrosion in the frame, and the post itself was showing discoloration and a rough, bumpy clear coat.

My LBS told me to put a bit of grease around the top of the post to seal out water. The post itself was dry. I mostly did keep the top sealed, and didn't ride in wet conditions very often, but it didn't prevent the corrosion.