Another possible cause is that your "safe" is actually a Residential Security Container, like most purchased from sporting goods, gun, or big box stores.

Virtually all RSCs simply have sheets of drywall between the inner and outer steel walls, and cheap MDF on the interior for mounting the lining. The drywall and the MDF can both off-gas formaldehyde into the RSC, making corrosion even worse when humid. Here's one article to start, but search around, there are a lot: https://thegunzone.com/can-a-gun-safe-with-formaldehyde-be-fixed/

Real safes are extremely heavy, and very expensive. For one that can hold long guns, probably $15,000 to start these days. But they have much better protection, actual fireproofing that will protect important paper as your house spends two hours burning down around them, and UL ratings for how many minutes it takes a pro with power tools to cut into them (like UL-T30 means it would take someone with a carbide saw or drill 30 minutes of time with the tool on the safe, not including breaks, to cut an 8" hole to remove the contents). At the very top end, also how much explosives it will resist (IIRC, the top UL rating resists six charges of 8oz of Nitroglycerine, but you're talking $60-70,000 plus but those kinds of safes, they are really meant for jewelry & precious metal stores).