Rust is a possible state for iron-made items in ADOM, which often reduces their usefulness and may even cause them to fall apart completely.
Rust may be caused in several ways. Exposure to water, for instance by swimming, water traps, or rain, may rust the PC's worn equipment. In the case of water traps or rain, this can be prevented by wearing a hooded cloak. Rust and other water damage to equipment cannot be avoided while swimming; however, items in the inventory will be safe if the PC is carrying a waterproof blanket.
The other common cause of rusting is the rust monster, as well its higher brethren, the large rust monster and the giant rust monster. Instead of attacking the PC directly, they may touch any of his/her worn and exposed pieces of equipment, rusting them; if they touch an already rusty item, it will be destroyed.
Items can also be intentionally (or accidentally) rusted by dipping them into uncursed potions of water or cursed oils of rust removal. Most items in the water dragon cave that can rust will be rusty by the time the PC reaches them.
While many things can cause items to rust, rusty items will not be destroyed without the influence of a rust monster - so intentionally rusting your items is not a way to get rid of them (except to speed up their destruction by rust monsters).
Rusty weapons quickly become useless; not only will a rusty weapon only deal half damage, it may also break, causing another halving of the damage (thus causing a broken rusty weapon to only deal a quarter of the usual damage). If the weapon in question happens to be cursed and cannot be removed, the PC is stuck poking foes with it, possibly spelling their downfall depending on their ability to use ranged weapons or spells and how quickly they can get rid of the liability. One solution to this problem can be to provoke a rust monster, which can destroy the weapon. Creatures that disarm, like the ratling fencer, will also prove helpful.
Rusty armor is less problematic: The only immediate result is an Appearance drop (shining armor looks more badass, presumably). The only danger in continued usage of rusty armor pieces is to the armor piece itself, should a rust monster appear to destroy it completely. This may be a desired result. Rusty jewelry also doesn't have negative effects. You should remember to unequip what you want to keep in case of rust monsters; losing an amulet of free action in this manner may come back to haunt the player later.
Rusty wands function like normal wands, with the exception that rusty wands of lightning or wands of ball lightning should not be used. These items are not properly insulated while rusty, and will cause the PC to be shocked (causing slight injury).
All iron items can be cleaned of rust by dipping in a non-cursed oil of rust removal. A blessed oil of rust removal will rustproof an item that is not rusty. Rust can also be removed with the smithing skill simply by selecting that item to be improved — the rust will be removed instead. All iron-made artifacts are also completely immune to rusting, even though rust monsters will still touch them in vain; in this manner, they function as protection for your other, actually rust-susceptible equipment. The elemental gauntlets are one example of an iron-made artifact.