My experience with internal frame corrosion differs greatly from what others have stated here. The first frame I built was in 2001. I commuted 30 miles a day in all weather including winter on salted roads using no fenders. Frame tubing was 70's era 531. Lugged construction with no vent holes and no treatment to the internals of the tubes. Around 15 years later I hit the bike with a car and ruined the rear end. Removed seat and chain stays and replaced them.
The removed stays had only a patina of surface corrosion much like a bare tube gets sitting on the shelf.
Perhaps I am lucky or something, but I have not had corrosion issues inside frame tubes on any of my commuter bikes. Keep in mind the roads around here are heavily salted during the cold months and corrosion of components is something I have observed, but not internals of frame tubes.