Ring of resist corrosion
An amulet of resist corrosion grants its wearer corrosion resistance, providing a 90% chance of negating any corrosion attempt upon your gear. This does not stack with other sources of corrosion resistance; only the more effective source is used.
Additionally, it provides its wearer with one level of acid resistance, greatly reducing damage from jelly-type monsters and other acidic foes. Stacking an amulet of resist corrosion with an item which provides preservation does not grant you any additional corrosion resistance, but does increase your acid resistance, further reducing damage taken. This reduction is fairly minor, however.
Acid damage and corrosion are generally restricted to certain areas (the Slime Pits, the Realm of Zot, oklob plant vaults). While this amulet is extremely useful in those circumstances, it is a poor choice for regular wear.