Thing is, the seatpost depends on close contact/friction with the steel seat tube for it to stay put and not slipping down from the rider's weight and shock. I'm wondering why anyone never cam up with a sort of thin isolator sleeve made out of a material that will not react with both metals and provide the needed grip to keep the seatpost securely clamped on to the frame. smaller diameter seatposts can be obtained to accomodate such sleeves. Seizing aluminum seaposts on steel frames had been an age-old problem with that seems to have escaped solution before the end of the steel frame era.
On a related topic, the seatpost on my 84 Peugeot PSV is held on to the frame with a Allen head jamb bolt mounted on what really is a waterbottle boss, about an inch and a half below the top of the seat tube, between the fastback seat stays. The proprietary JPR aluminum seatpost that came with the bike has a groove machined on to the back side of the seatpost tube where this jamb bolt presses on inside the seat tube and keeps the post/saddle in place and straight. I'm not sure if the setup will avoid the corrosion/seizing problem, but I was always leery of this seatpost anchorage design by Peugeot that seems to have been done on only one or two model years (1984-5?) on their Vitus and maybe Reynolds 501 tubed bikes. I was thinking that maybe I and others here can use the anti seizing compound that mechanics put on spark plug threads to prevent them from seizing/galling on to aluminum engine heads??
Chombi
84 Peugeot PSV