Erosion in NetHack 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 (specifically, pick-axes)) 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. Erosion does not affect an item's other enchantment-based effects; a thoroughly rusty +2 helm of brilliance still provides +2 to INT and WIS. The following types of erosion exist:
Items can suffer up to three levels of erosion, for example:
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.
Other erodible items
Metal wands, rings, and skeleton keys can rust and corrode, but this does not affect their performance. Also, if you are polymorphed into an iron golem, rust traps will kill your monster form (even if unchanging, unless you have the half physical damage extrinsic). All iron golems suffer this fate. Similarly, a wood golem can be rotted and destroyed.
Preventing and curing 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.
- Successfully dipping for Excalibur will remove any existing erosion on the long sword and make it rustproof.
- Any wishable item can be wished for in an unerodable state. (This is achieved either via using an adjective such as "rustproof", or more commonly, "fixed", which is intended for crysknives but shares the same bit in memory as erosion proofing.) For many items, a wish is the only way to erodeproof the item; this can be an easy way to identify wishes in dumplogs (e.g. fireproof bags of holding).
If the scroll of enchant weapon or armor is cursed, this procedure instead strips the item of any protection from erosion, and has no effect on 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.
Objects can be temporarily protected from rust or corrosion by greasing them. An event which would normally rust or corrode the item will instead have a chance of removing the grease. Also, one level of rust and/or corrosion can be removed from a weapon by dipping it into a potion of oil.
The prayer boon does not erode-proof your wielded weapon, though it does repair existing damage.
In most cases, using scrolls of enchant armor to erode-proof armor is a waste of resources. Since erosion only affects the object's base AC, and most items don't have more than 1 or 2 base AC, scrolls are usually better spent increasing the enchantment of your gear (unless you are unable to enchant your gear safely any more and don't have enough markers to make use of blanking the surplus scrolls). Many forms of body armor grant 3 or more AC, but usually the only suits you should consider enchanting are dragon scale mail and perhaps mithril-coats, neither of which can erode. Using extra scrolls to fix erosion is an option, however. It might also be worthwhile to fire-proof speed boots, jumping boots, or water walking boots, as any of these will be destroyed should you accidentally step into lava. This applies particularly to Valkyries whose quest contains lava in abundance.
In contrast, you generally will want an erode-proof main weapon, particularly since the erosion penalty is doubled for double-damage weapons. However, it is rarely necessary to do the erode-proofing yourself: artifact weapons from sacrifice will always be erode-proof, while those from wishes can be explicitly wished for erode-proof. However, in the case of a good weapon found randomly in the dungeon (for example one of the Brands), it is worth spending a scroll of enchant weapon to make it rustproof. Also, cavemen using the Sceptre of Might will also want to make it rustproof, as it is not generated so.
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.