TomeNET

(5.10a) Equipment damage from elemental attacks                                 
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Armour can get damaged when an elemental monster attacks you or when you get
hit by elemental magic. Blocking an attack with a shield may damage it in the
same manner. Weapons may take damage if you hit an elemental monster with them.

Armour (and shields in archaic server version) getting damage means a reduction
of their +AC bonus by 1 point each time, while weapons getting damaged means a
reduction of their +damage bonus by 1 point each time.
AC bonus won't drop lower than half the negative amount of the base AC.
Damage bonus won't drop lower than -10.

If the player has resistance to the element, this will cut down the probability
of any equipment damage. Having double resistance (permanent + temporary) will
cut it down even further.
To completely prevent any equipment damage, you need to become immune to the
element, or the equipped item needs to ignore the element. In most cases, items
that grant resistance to an element also ignore it.

The following table lists the elemental damage taken by various item types.

                    Acid    Fire    Water
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Wooden boomerangs   x       x
Metal boomerangs    x               x

Swords              x               x
Axes                x               x
Blunt                       x
Polearms                    x

Leather caps/Hats   x       x
Metal caps/Helmets  x               x
Iron crowns         x               x
Crowns
Leather shields     x       x
Shields             x               x

Boots               x       x
Gloves              x       x
Gauntlets           x               x
Cesti               x       x       x
Cloaks              x       x
Soft Armour         x       x
Metal/steel Armour  x               x
Mithril/adamantite
DSM Armour

Also, disenchantment can disenchant your equipment if not resisted, lowering
either (+hit, +dam) or [+armour class] boni on your equipped items, down to +0.
Even artifacts are not safe! Only if you have disenchantment resistance or the
item has the IGNORE_DISEN flag (ignore disenchantment) then the item is safe.
Ethereal ammunition is especially susceptible to disenchantment.

Last but not least, electricity damage can also discharge items in your
inventory or even your equipment, if the item doesn't ignore or resist
electricity.
This mainly concerns magic devices, but also amulets!
Ethereal ammunition is also susceptible, similar like to disenchantment.
Items that ignore electricity are safe, and if you have double
electricity resistance (or immunity of course) then all items are safe.