In brief: they are not dissimilar steel materials, galvanizing is a coating just as paint is a coating. They prevent corrosion through different processes though, so the interface may cause gaps in your corrosion protection. I'd recommend painting over the galvanized steel in the vicinity of the connection. This will both ensure a uniform protection at the interface and also shield the galvanized area so it doesn't get "used up" too quickly by the lesser protected painted region.
Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA) Structural Engineer (IL)
American Concrete Industries
