Steel was also an obvious material for armor . It was tested in the 1860s , but the steel of the time was too brittle and disintegrated when struck by shells . Steel became practical to use when a way was found to fuse steel onto wrought iron plates , giving a form of compound armor . This compound armor was used by the British in ships built from the late 1870s , first for turret armor ( starting with HMS Inflexible ) and then for all armor ( starting with HMS Colossus of 1882 ) . The French and German navies adopted the innovation almost immediately , with licenses being given for the use of the ' Wilson System ' of producing fused armor .