The metal rusts where it's in contact with the concrete because concrete nearly always has free moisture residing in it. That's also where the railing's coating tends to get chipped. Shoes, shovels, bikes, and whatever else bangs into it down low, and sidewalk salt makes it all worse.
You can be sure to have a high-quality coating on the metal parts, and you can protect them from impact damage. You can't stop the ravages of time, though. 10 to 20 years is a reasonable life expectancy for iron. Go with something else if you need more time.