Aluminum Armor
Aluminum is much lighter than steel and relatively strong. Since it doesn't rust, it seems like the ideal metal for ships. However, no one uses aluminum for fighting ships because it can catch fire when hit, which produces intense heat that cannot be extinguished with water orHMS Sheffield regular fire extinguishers. In fact, aluminum is so volatile that powdered aluminum is a prime component in rocket fuel. The US Air Force 15,000 BLU-82B bomb contains 12,600 pounds of low-cost GSX slurry (ammonium nitrate, aluminum powder, and polystyrene). The British built a few frigates with aluminum in the 1970s. During the Falklands war, one of these ships, the HMS Sheffield, was hit by a single Exocet anti-ship missile. The damage was not fatal since the missile failed to explode, but its rocket fuel torched the Sheffield's aluminum hull, which ignited and burned until it sank. In contrast, the steel hull frigate USS Stark was hit by two Iraqi Exocet missiles in 1988 and survived.
Another disadvantage is that aluminum is less resistant to damage. A heavy machine gun round, rocket, or land mine striking a steel hull makes a small hole. However, cast aluminum shatters like plate glass. Hits cause large holes as hundreds of aluminum fragments blast forward causing casualties.
The US Army learned this when it tested early versions of the aluminum Bradley infantry fighting vehicle. In the Bradley tests, small RPG explosions caused only minor damage, but ignited the aluminum body and caused most vehicles to literally burn to the ground. Even worse, burning aluminum produces deadly fumes which instantly kill anyone inside. Only after heavy criticism and actual live-fire tests did changes occur. (see "The Pentagon Wars", by J. Burton, pp. 136-193) A steel armor coat was added and "spall liners" were installed inside to catch fragments. The latest model of the Bradley has an all-steel body.