By casting this spell, the wizard gains the power to corrode ferrous metals and alloys at a touch. Iron and iron-based alloys such as steel, meteoric iron, mithral, and adamantite are affected, but noble metals such as gold, silver, and copper are not subject to reduction through rusting. Any ferrous metal touched by the wizard must make an item saving throw vs. disintegration (usually a 17 or better on a d20) or be destroyed. Magical arms or armor may apply their bonus to this save, so a sword +3 would gain a +3 to its roll. Other magical metal items may receive a +1 to a +6 bonus based on the DM's estimate of their power.
The wizard may employ rusting grasp in combat by simply touching the equipment of metal-wearing characters or creatures. If he tries to touch the armor of a character, the wizard need only hit the opponents unarmored AC. If the armor fails its save, rusting grasp permanently destroys 2d4 points of AC through corrosion. For example, plate mail +3 (base AC 0) could be reduced to a base AC of 2 to 8 if it fails its item saving throw.
Weapons are more difficult to grasp; the wizard must make an attack roll against AC 4 (modified by the opponent's Dexterity) in order to touch the weapon. If the weapon fails its saving throw, it is destroyed. Important note: The wizard must touch the weapon and not the other way around! Unlike a rust monster, he doesn't corrode weapons simply by being hit.
Against metallic creatures, rusting grasp functions like the priest spell cause serious wounds in that it inflicts 2d8+1 point of damage per successful attack. The spell lasts for one round per level, and the wizard can make one touch attack per round.
The material component is an antenna from a rust monster.