Water does crazy things to metal. I’m assuming the rack isn’t going to be made of top grade stainless so no matter what you’re going to have some form of rust or corrosion of the metal. Initially it’ll be to exposed metal surfaces like nuts and bolts, but once the finish/paint starts chipping it’ll start happening to those surfaces as well. Covering with a tarp would possibly help a bit, but humidity in the air will do similar things.
That being said you could probably get a few years or more out of it being somewhat diligent in keeping up with any chips or scratches of the finish. I personally wouldn’t be terribly concerned about winter due to the humidity typically being so low.
Is there any way you could store it inside when it’s not in use? Or are you using this to get through the quarantine/gym lockdown and then back to a commercial/school gym?
Another thing to consider is metal plates and the bar. All of my equipment has been stored in an unconditioned space inside and you’ll see, the bar is still discolored. I didn’t wipe it down after every use though, but even being out of the elements water still finds a way to corrode everything. So keep in mind bringing the plates/bar inside as well if you want them to stay semi decent for a while.
Edit: or be like flap and make a rack out of pressure treated lumber. A lot cheaper too I’m sure.