With regard to the longevity of an alloy steel in place of an ordinary mild steel, an alloy might not neccessarily be more corrosion resistant, the alloyed steel could be harder or tougher, however, to be corrosion resistant, it's usually a mixture of chrome and nickel in various percentages that give it it's resistance: commonly known as stainless steel.
Cast iron is a very brittle high carbon iron that is no use as a method of making modern air pistols, steel is made fron cast iron that has had most of the carbon burnt off in a blast furnace, to bring it to a soft state (less than .02 carbon, if my memory serves me right and I did all this about 50 years ago, so bear with me), by adding small amounts of carbon and other materials, the steel can be made to undergo different properties: ie spring steel, (tough), knives and cutting tools (hard), gun barrels, (tough and strong) etc, the varieties are endless. I would think that the casings are made of a zinc based alloy with just the barrels and mechanisms inside made of steel. When I lectured in the past, we had often cast alloy items brought in for welding: our collective name for them was s**t metal as they were virtually impossible to weld properly. They would be easy cast and cheap to produce, but not that strong and it was easier to scrap and obtain a new one to replace it. Hope this helps a bit, this subject could go on for ages and to greater depth than I have just skimmed on here, and of course there are new properties and alloys that have been discovered since I did my college work on such a long time ago!