Ironclad can refer to two different things, depending on the context.
Ironclad describes the state of being covered in riveted strakes, or sheets, of cast iron. They are thick enough to halt a cannon ball, though they might buckle under the shots of a sixty-eight pounder, and repeated hammering from multiple shots can weaken the rivets holding the strakes to the side of a vessel. Eventually they will come away, exposing the wooden planks beneath as weak points and targets. Ram captains preferring shoot-out strategies attempt to pound off their opponents' strakes, leaving them vulnerable and ready to "strike his colors" (lower the ship's flag and give in).