Corrosive Form

A target that touches the creature or hits it with a melee attack while within 5 feet of it takes X acid damage. Any nonmagical weapon made of metal or wood that hits the creature corrodes. After dealing damage, the weapon takes a permanent cumulative -1 penalty to damage rolls. If its penalty drops to 5, the weapon is destroyed. Non magical ammunition made of metal or wood that hits the creature is destroyed after dealing damage. The creature can eat through 2-inch-thick, non-magical wood or metal in 1 round. (Black Pudding, pp. 241, MM)