Probably there is a wide variety of corrosion environments. Some
severe, and some quite passive. Surely the coastal areas have to be
more severe. Temperature variations will cause a doubling of corrosion
about every 20 deg F rise, so summertime is when it is most severe.

My 172 engine has set for the 3 winter months a couple of times (in MN)
with no visible sign of corrosion in the cylinders. Another never-run
C-85 engine in my hangar was assembled over 6 years ago but is still
not showing any signs of corrosion in its cylinders. It is turned over
only yearly. I have to resist running it though until the rest of the
J-4 project gets done.

FWIW - Rust (ferric oxide) is non-magnetic. Ferrous oxide (black form)
is though.