Carbon and aluminum swap electrons, which significantly weakens and corrodes the aluminum, especially in a marine environment. Salt water accellerates the process. A white powder is the result of the corrosion. The HT aluminum mast bases are weakened over time and will break due to contact with the carbon mast. Its best to take them off and paint them, or replace them with carbon or SS mast base.

A thin layer of HDPE works

There are a number of primers that work too.

Dont use aluminum rivets in carbon, only SS rivets.