From the Super Mario Wiki
The Metal Cap is an accessory that makes its debut appearance in Super Mario 64. It is usually found in Metal Boxes. The Metal Cap can turn Mario into Metal Mario, which makes him invulnerable. The side effect of this temporary invulnerability is that the user becomes extremely heavy (with a side effect of extremely powerful Ground Pounds), can no longer make very high or long jumps, cannot swim, and will sink faster in quicksand. Fortunately, while in metal form, the player is also unaffected by toxic fog, and cannot drown.
In Super Mario 64 DS, the Metal Cap does not appear. Instead, Wario becomes Metal Wario when he comes in contact with a Power Flower; but still retains the same properties as Metal Mario. As Wario couldn't use other Cap forms, it was not possible to get a Vanish Metal form in this game.
A similar item, the Metal Mushroom Orb works almost identically to the Metal Cap. Although, even with Metal Cap the player is still subjected to fall damage.
 Courses in which it Appears