I don't use rust monsters. They are extinct.
But there is a Rust, a Crack, and a Rot spell common enough that does destroy nonartifact magic items that are metal, stone, or wood respectively.
Bronze items are immune to all three spells.
EDIT:
this is a major plot point.
The mage guild who make magic items put a bounty on rust monster heads as no one would by an item which could be rusted. Rust monsters were hunted to extinction.
But with no rust monsters, there was saturation and devaluation.
So the Iron Mages invented the Rust Ray spell to destroy maigic items.
Then rival guilds tried to corner the market with wood, bone, stone, and crystal items. The Iron Mages invented spells for them too.