What was wrong with iron bridges?

Answer 1

There is nothing inherently wrong with iron as a bridging material, just as there is nothing inherently wrong with virtually anything used to make a bridge, not excluding fiber ropes and wood used by the Incas. Cast iron was plentiful and easy to work and was used in many types of bridges from the 18th well into the 19th Centuries. Some beautifully crafted iron bridges are still in use today. Technically, steel is just iron with more of the impurities removed. Steel has greater tensile strength (resistance to breakage) than iron but until the Bessemer process was invented in the mid-19th Century it was harder to make, so iron was the material of choice. Iron and steel do rust, but paint helps to resist weathering, and any material exposed to weather will eventually deteriorate unless maintained.