I kept my safes in unheated garages for years. Never had any issues with rust, pitting etc. However, where I lived it was always relatively low humidity with the exception of south central Texas. I kept the long guns in silicone socks. Key to that type of storage is to ensure there are no major internal temp swings in a very short span of time. That’s a bigger concern during winter than summer.
Sold off the safes recently and moved everything to the basement after finishing a vault. Rebar reenforced concrete walls on all sides, roof and floor. Sealed concrete before paneling interior with poly coated knotty pine. Currently I have no humidity mitigation in play. I do have power in there and could run a heater and/or dehumidifier if the need arises. Long guns no longer in socks but uncovered standing in racks.
So, looking at my wifi weather station monitor this is what the current readings are.
68 degrees indoors upstairs with 43% humidity
Outdoors temp is 69 with 46% humidity.
I have three humidity monitors in the vault.
One on the outside of the Vault made by Hornady with a remote sensor located in middle of vault. That gives me temp and humidity readings such as current, average over 12/24 hrs and lifetime Max. Just looked at the readout and it shows….internal temp is 67 degrees with 42% humidity. Lifetime max and minimum humidity readings since finishing vault (5 months maybe) are %50 and 38%.
I also have a cheap temp and humidity gage in the $20 range at one end… That currently reads 67 degrees with 58% humidity.
At the other I have a high dollar West German hygrometer tuned by a good friend for accuracy some years ago. Came out of a surgical unit that was decommissioned. That currently reads 68 degrees and 54% humidity.
Again, barring crazy high humidity I think the key is to negate severe temp swings in a very short time frame.
I wipe down the blued guns every couple of months with a soft WHITE cloth and some Ballistol. That helps show me if any light oxidation is taking place. Haven’t noticed anything yet.