FWIW, I have a IIfx power supply that was stored in a non-water tight shed for several years. The case is all rusty. Yet, it still works perfectly, fan and all. It was bad enough I sanded off most of the rust so it wouldn’t blow rust chunks on an unsuspecting logic board.
I have two IIsi power supplies that were stored with loving care. Neither one worked when I pulled them out last year. I recapped and restored one, the other continues to actively disintegrate.
Those IIfx supplies seem to be made of titanium. The guts, anyway.