Once you get about half an inch to three quarters of an inch, the steel creates an oxide layer that protects the material underneath. You can look at ship wreck

Once you get about half an inch to three quarters of an inch, the steel creates an oxide layer that protects the material underneath. You can look at ship wrecks all over the world from a hundred years ago, the steel for the steam boiler is usually the last to go. The steel cables holding up the suspension bridge Golden Gate Bridge in San Francisco (87 years old), clouded in fog and arguably one of the harsher saltwater environments (near-constant 20mph winds 8 months out of the year) is still using it's original steel cables, and there are no plans to replace them.