Ironclad can refer to two different things, depending on the context.
Adjective[]
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).
Noun[]
An ironclad is another name for any craft (usually a ram) covered in iron sheet armor. The strakes of ironclads are covered in braice to prevent corrosion from the vinegar seas.