What keeps steel which is fresh out a rolling mill from rusting is a combination of mill scale and a coating of oil. A surface layer of corroded iron, "rust", isn't protective because when an iron atom oxidizes, it gets smaller, allowing iron molecules in underlying layers to oxidize, and so on. Plating or coating adds protection either by providing an electrical polarity which counters the oxidizing process (zinc galvanizing) or by the plating substance's molecules becoming larger upon becoming oxidized (chromium). Alloying agents work along the same principles as galvanizing, except that they are distributed throughout the iron, not just on the surface.