Erosion
Erosion is an attribute that can apply to some items depending on their material. The performance of an eroded weapon (including tools that can be used as weapons) or piece of armor will be degraded - weapons will have their damage decreased and armor's intrinsic AC bonus will be reduced by one for each level of erosion (in neither case affecting enchantment, and never reducing intrinsic damage or AC below zero). This means a +0 dwarvish iron helm's AC is -2, while a thoroughly rusty +1 dwarvish iron helm's AC is -1. The following types of erosion exist:
Rust affects only items made from iron. It results from exposure to water (by potion, fountain, pool, etc.) or to a rust monster.
Corrosion affects copper or iron items. Exposure to acidic environments (potions or some monsters) results in this form of damage.
Burning affects any organic material or plastic, and results from e.g. fire traps.
Rotting also affects any organic material and most commonly results from exposure to brown puddings.
Items can suffer up to three levels of erosion, for example: rusty, very rusty, and thoroughly rusty. The types of damage do not add; rather the greater damage is used. Thus a very rusty corroded short sword has a -2 penalty rather than -3. An item can be both thoroughly rusty and thoroughly corroded, but it will never suffer more than 3 points of damage.
Preventing erosion
Many objects can be protected from erosion, and any existing erosion repaired. A metal object that is so protected identifies as rustproof, and an organic or plastic object identifies as fireproof. The procedure is the same for both types:
- A weapon (including tools that can be used as weapons) can be protected from erosion, and any damage repaired, by wielding it and reading a non-cursed scroll of enchant weapon while confused.
- Any armor can be protected from erosion, and any damage repaired, by wearing it, removing all other armor, and reading a non-cursed scroll of enchant armor while confused.
- Any armor can also be protected from erosion by wearing it, removing all other armor, and reading a cursed scroll of destroy armor while confused. This procedure does not repair existing damage.
If the scroll of enchant weapon or armor is cursed, this procedure instead strips the item of any protection from erosion, and does not repair existing erosion.
No erosion event will ever affect body armor worn under a cloak, nor a shirt worn under body armor or a cloak. One might choose, then, to wear a junk cloak such as a dwarvish cloak to protect a banded mail from rust.
History
Before NetHack 3.1.0, items did not have erosion as such; rather, damage operated by reducing the enchantment. Thus a rust monster attacked rather like a modern disenchanter, except that its attack was blocked by rustproofing rather than magic cancellation.
