Corrode Metal

Any nonmagical weapon made of metal that hits the creature corrodes. After dealing damage, the weapon takes a permanent and cumulative 1 penalty to damage rolls. If its penalty drops to -5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the creature is destroyed after dealing damage. The creature can eat through 2-inch-thick, nonmagical metal in 1 round. (Grey Ooze, pp. 243, MM)