If steel can rust with saltwater... then why are ships made of steel? can't we just use other metals like aluminium etc?
Ships are made of steel and also aluminum. Some minesweepers are made from fiberglass. Steel is a comparatively cheap and easily used material. But don't be confused with the type of steel that is used. Plates do not have to be replaced every year. In fact ship s can go for years with no major steel work, and this can be extended with proper maintenance like sand blasting and painting. There are many different steel alloys with different properties. Some of them resist rusting and instead of flaking they just form a powder like barrier. Check out this link.
Steel became the medium of construction almost be accident. The Royal Navy began to coat its wooden ships with a coating of steel to give them added protection in battle. They were known as the Iron Clads. From this it was a small step by such people as Brunnel to start construction in iron, then various other steels. The Great Eastern is a classic example. Over the last one hundred years the main improvement is in welding rather than a riveted construction. Improvements in steel have lead to a reduction in the scantlings and through this a reduction I the weight of the ship which has allowed for an increase in the carrying capacity. There always have been specialist ships built with alternative materials. Aluminium, wood, concrete and GRP. GRP. Also it must be remembered that there has been great advances in steel, from the early days of cast iron.
also add to answers above (now below), zincs are mounted on the hull and shaft to draw the electrolysis to them instead of the hull and are easily replaced by a competent diver
aluminium is not good in saltwater Steel rusts in fresh air let alone saltwater but its alot cheaper than other mass produced metals you can rivet or weld it cheaply unlike stainless steel or Aluminium We could make them of wood but thats been tried Glass fibre and plastic is good for small boats but to do and Aircraft carrier size ship i doubt it
