90% humidity is high
As mentioned, it's great for "root cellars" but do you want to store your firearms in a "root cellar"
I recently purchased part of an estate sale, which was mostly ammunition and reloading supplies, including powder, bullets and primers.
The stuff had been in a basement just as you describe for appx two years, after the fellow passed away.
His widow took all of his "hunting stuff" down to the basement because it made her sad when she looked at it.
All of this stuff was at least a decade or more old, before it went down to the basement.
She said it was all fine before she took it down.
I believe her, the other stuff, such as clothes, firearms in a safe, leather slings, backpack, etc, were still in the room where they stored all sorts of things, upstairs, along with his loading bench. All of this stuff was fine.
All of the paper boxes, containing bullets, some unopened, were damp, and some of the bullets were discolored. One "paper" container of H4831 was damp as well, but the others were in plastic bottles, so OK, but the lids were starting to get rust specs on them. I threw out the powder in the damp pressed paper container.
All of the primers were in plastic trays, but the paper sleeves were all damp, and the glued joints had separated. I marked all of the trays with the proper primer classifications and just taped them all together in a couple of sleeves. I tested one from each tray yesterday, and they're fine.
OP, your firearms and tools may not rust quickly, but they will rust, as will your firearms in such storage areas.
At 90% humidity, I'm surprised there aren't any mold issues.
IMHO, from past experience, I wouldn't be storing anything firearm related in such conditions.
