You need R.H. of at most 15% to get an evaporative cooler to work well.
I can get a drop of about 20 degrees with a portable unit I've got at
that R.H., at 20% and above all I get is hot, wet air. Really good
only for the Southwest where the humidity is low for most of the hot
season. It evaporates water into the incoming air, so it increases the
R.H. inside, so, yes, there's a good chance it'll start equipment
rusting. Insulation, paint, partitioning and/or an exhaust fan(s)
located high up will work better.

Stan