Air doesn't come into the crankcase via the breather - rather
combustion products that leak past the rings vent out thru the
breather. Burning hydrocarbons generate CO2 and water. The net dew
point of combustion and blowby products is about 180 degF. The water
will condense in cooler sections of the crankcase. It is this water
that causes most corrosion - especially after combining with nitrogen
oxides and sulfur oxides which make acid.

Crankcase condensation happens from engine operation - not from just
sitting around. The real trick is to ventilate these residual
combustion products from the crankcase immediately after shutdown
before they all condense. Systems are now starting to be sold which
actively do this.

A lot of this moisture accumulation problem would go away if aircraft
engines had a positive crankcase ventilation (PCV) system like car
engines now do, but they don't for whatever reason. I suspect part of
the reason car engines now last so much longer is due to the PCV
system.