Erosion

Erosion is an attribute that applies to items in NetHack, and makes it possible for items of a certain material to degrade in quality, which in turn can potentially affect their performance.

Description

There are four types of erosion recognized by the game.

Rust affects items made of iron, and is caused by:

• Water damage
• The bite of a gray ooze, or the touch attacks and passive attack of a rust monster (which all use the AD_RUST damage type)

Corrosion affects items made of copper or iron, and is caused by:

• The potion of acid, when thrown or dipped into
• The acid breath weapon of a yellow dragon, the passive acid attacks of a green mold, spotted jelly or ochre jelly, or the engulfing attack of an ochre jelly
• The bite and passive of a black pudding (which both use the AD_CORR damage type)

Burning affects items made of wood, leather, cloth, and plastic, and is caused by fire damage from most sources, as well as a passive effect of attacking certain fire-element monsters.

Rotting affects items made of wood, leather and cloth, and is caused by the bite attack of a brown pudding (which uses the AD_DCAY damage type).

There are three levels of erosion possible, with each instance of erosion adding one level, i.e. banded mail can become "rusty", then "very rusty", then "thoroughly rusty". An item that is at the maximum for a particular type of erosion cannot be further eroded that way, though it can still be subject to other applicable types of erosion, e.g. a thoroughly rusty banded mail can also be corroded.

Erosion does not affect body armor worn under a cloak, and does not affect a shirt worn under body armor or a cloak.

The following information pertains to an upcoming version (3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.

Per commit e9c58c2f, damage to glass objects is treated as erosion rather than immediately destroying the objects: a glass item can be cracked, very cracked at 2 levels, or thoroughly cracked at 3 levels, and any damage to a thoroughly cracked glass item destroys it. The chance of glass items being damaged is also raised from 5% to 10%.

Item effects

Erosion has particular negative effects on items from specific object classes.

• Eroded weapons and weapon-tools have their damage decreased by 1 for each level of erosion, to a minimum of 0: a short sword that is very rusty has a −2 penalty to damage. This penalty is doubled for weapons that do double damage against certain monsters.
• Eroded armor has its base AC reduced by one point for each level of erosion, to a minimum of 0: a +0 dwarvish iron helm grants 2 points of AC, while a thorough rusty +1 dwarvish iron helm grants 1 point of AC (0 from base item + 1 from enchantment).

Penalties from erosion are not cumulative, and are applied based on the highest level of erosion for that object - e.g., if the very rusty short sword from the above example becomes corroded as well, it still has has a −2 damage penalty rather than −3. Erosion does not affect an item's other enchantment-based effects, e.g. a thoroughly rusty +2 helm of brilliance still provides +2 to intelligence and wisdom.

A user has suggested improving this page or section as follows:

"Specifics on damage penalties for erosion."

Monster effects

A few monsters in NetHack are capable of being damaged by erosion attacks:

• An iron golem (including a hero in the form of one) that is subjected to rusting from water damage or being hit by a rust-inducing attack takes damage equal to its current HP.
• A wood golem (including a hero in the form of one) that is subjected to decay from a brown pudding's attack takes damage equal to its current HP.

A hero in golem form that dies this way is returned to normal form, even if they have unchanging.

Erosion-proofing and repair

Objects can be made erodeproof (or fooproof informally) and protected from all forms of erosion, and existing erosion on erodible objects can be repaired - certain materials are also considered to be inherently erodeproof, such as silver, metal and dragon hide. The process for making items erodeproof varies with the object class:

• A hero reading a non-cursed scroll of enchant weapon while confused will erodeproof the currently wielded weapon and repair any existing damage. Doing so with a cursed scroll will remove any erodeproofing.
• A hero reading a non-cursed scroll of enchant armor while confused will erodeproof a random piece of currently worn armor and repair any existing damage to it. Doing so with a cursed scroll will remove any erodeproofing from a random piece of armor.
• A hero reading a cursed scroll of destroy armor while confused will erodeproof a random piece of currently worn armor without repairing existing damage to it.
• Any item can be wished for as erodeproof, even if they are made of a normally-erodeproof material.

The word used to describe an item as erodeproof varies with the item's material: rustproof for iron items, corrodeproof for copper items, and fireproof for items vulnerable to burning; though the description "rotproof" is recognized by the game, all materials that can be rotted can also be subjected to burning. The "fixed" status exists for crysknives and is intended for displaying how likely they are to revert to a worm tooth (¹⁄₁₀ for a fixed crysknife versus always for a normal one), but shares the same bit in the object data structure as erode-proofing, and a wish for a "fixed" item will set this bit. An erodeproof item is protected from all erosion, regardless of if its description mentions that form of erosion.

There are other methods that can make items erodeproof, repair erosion or otherwise offer protection from it:

• Dipping a long sword into a fountain with a ¹⁄₆ chance, or else crowning while wielding one as a lawful hero, will transform it into Excalibur, repairing any erosion and making the sword rustproof.
• An erodeproof item that is polymorphed into another item retains its erodeproof status.
• Erosion can be removed from a wielded weapon as a boon from successful prayer.
• Grease applied to an item blocks all forms of erosion, but has a chance to dissolve after each instance of blocked erosion.
• Dipping a rusty and/or corroded item into a potion of oil removes one level of rust or corrosion from it.
• Blessed items have a luck-dependent chance to resist erosion.
• Artifacts given as gifts from sacrifice and crowning are always made erodeproof - a wielded weapon that is enhanced as part of the crowning process (i.e. if the hero cannot be given the normal crowning gift) is repaired and made erodeproof.

A user has suggested improving this page or section as follows:

"Specify the erodeproofing methods that ID the proofing."

The following information pertains to an upcoming version (3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.

Per commit e9c58c2f, glass items can be made erodeproof, and those items are referred to as crackproof or tempered.

Strategy

A hero should erodeproof their primary weapon of choice at minimum to avoid damage penalties, if it is not already made of an erodeproof material; a character focused on two weapon combat should erode-proof their offhand weapon as well if applicable. However, most characters' primary weapons of choice are artifacts, which come erode-proofed if obtained via sacrifice, crowning or explicitly wishing for an erodeproof artifact weapon (but not if that artifact is randomly generated or acts as their quest artifact, such as The Sceptre of Might for Cavepeople) - such weapon are unlikely to lose erode-proofed status, barring carelessness with a scroll of enchant weapon.

Conversely, making armor erodeproof may often be lower priority for many characters, depending on the current stage of the game and the armor in question: in general, this is best saved until a hero has settled on a long-term set of armor and enchanting it as far as they can do so safely or desire to - afterward, any excess scrolls can be used to repair erosion and proof armor as needed. High luck and blessed armor (which usually occurs when reading blessed scrolls of enchant armor) can also reduce the odds of any one piece of armor being eroded, and a cloak will protect both body armor and shirts.

Fireproofing boots is generally worthwhile to avoid destroying them should the hero accidentally walk into lava - other armor with low base AC is somewhat lower-priority, due to how AC penalties for erosion function, e.g. many helms, most cloaks except the cloak of protection, and all gloves provide a base AC of 1. While erode-proofing the hero's suit of armor is a good idea, it is also unlikely to be necessary if the aim is to wear dragon scale mail or even a mithril-coat in the long term, since both types of armor cannot be eroded normally.

It is a common habit for players wishing up items to request those items fixed or otherwise erodeproof as a form of precaution, even if the risk of that item being eroded is unlikely to come up.

History

The modern erosion system is introduced in NetHack 3.1.0. In NetHack 3.0.10 and previous versions, erosion occurs in the form of reducing that item's enchantment - a rust monster in these versions is more similar to a modern disenchanter as a result, except that its attack is blocked by rustproofing (which exists in those versions) rather than magic cancellation.

In NetHack 3.6.0 and earlier versions, including some variants based on those versions, wands, rings, and tools such as stethoscopes and skeleton keys can rust and corrode, but this does not affect their use.

Variants

SLASH'EM

In SLASH'EM, rust may also be removed from a weapon by applying a whetstone while standing over a water source.