Why do ferrite magnets not rust? Because they're ALREADY MADE of rust!
Unfortunately, ferrite magnets are not very strong. It was the availability of
super-strong rare-earth magnets that made permanent-magnet alternators
a technology suited for a dedicated hobbyist, rather than a dedicated engineer.
And it was permanent magnet alternators that made wind power practical
in areas with far less wind than previous, electrically-excited, designs.
Electrical field excitation consumes a substantial amount of power. Further,
the slower the genny is turning, the more power is consumed. At low
wind speeds you don't break even, so the cutin speed is high. But during
low wind - when your batteries are already down - is when you need your
generation the most. With permanent magets you paid the excitation power,
once, for a fraction of a second, at the factory. In the field, every bit of
generation is available for charging or driving loads.
For an environment this hostile you might want to consider doing a motor
conversion on one of the sealed sort of motors, with the cooling fins, that
are used on pumps in hostile environments. That way you could use rare
earth magnets with minimal protective coating, and if you are careful about
the joints where the motor is reassembled after conversion, the shaft seal,
the wiring entry, the integrity of the paint job or coating, and (if present)
the filter on the breather hole, you can use the resulting device in a
corrosive environment without having one in its innards.
= = = =
I really don't like using aluminum on things that get stress. Unlike steel,
which doesn't get appreciable stress-hardening and fatigue until it's
stressed up near its yeild strength, aluminum gets metal fatigue with
a very small amount of stress. Its life is thus limited - even if long -
in structural uses. (That's why commercial aircraft have to be retired
after a certain amount of air time, and you occasionally hear about
things like the top of the cabin peeling off on planes that were used
too long.)
Of course, using (non-stainless) steel in a corrosive environment,
or wood near an ocean shore, where protective coatings are required
and degradation once they're breached is very rapid, is asking for
trouble in short order. So maybe aluminum IS a better choice..