The ultimate ironclad armor was case hardened nickel @-@ steel . In 1890 , the U.S. Navy tested steel armor hardened by the Harvey process and found it superior to compound armor . For several years ' Harvey steel ' was the state of the art , produced in the U.S. , France , Germany , Britain , Austria and Italy . In 1894 , the German firm Krupp developed gas cementing , which further hardened steel armor . The German Kaiser Friedrich III , laid down in 1895 , was the first ship to benefit from the new ' Krupp armor ' and the new armor was quickly adopted ; the Royal Navy using it from HMS Canopus , laid down in 1896 . By 1901 almost all new battleships used Krupp armor , though the U.S. continued to use Harvey armor alongside until the end of the decade .