Shipwreck

Steel and iron, depending on their thickness, may retain the ship's structure for decades. As corrosion takes place, sometimes helped by tides and weather, the structure collapses. Thick ferrous objects like cannons, steam boilers or the pressure vessel of a submarine often survive well underwater in spite of corrosion.
Propellers, condensers, hinges and port holes were often made from non-ferrous metals such as brass and phosphor bronze, which do not corrode easily.