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 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.

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.
• 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.

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, which is affected by half physical damage.
• 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, which is affected by half physical damage.

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

Erosion-proofing and repair

Many objects can be made erodeproof (or fooproof informally) and protected from erosion, and existing erosion 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.

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

• Successfully dipping a long sword and transforming it into Excalibur will remove any existing erosion on the long sword and make it 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 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.

Strategy

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). Additionally, blessed scrolls will bless the enchanted item for free, which (with high Luck) already provides adequate protection. 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. A cloak of protection may also be worth erodeproofing, since its base AC is 3.

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 Fire and Frost Brand), 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.

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.

In NetHack 3.6.0 and earlier versions, metal wands, rings, and tools such as stethoscopes and skeleton keys could rust and corrode, but this was purely cosmetic and did not affect their performance.

Variants

SLASH'EM

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