Been a ferrous metallurgist for 20 years. Previous posters have explained it well. Chrome plating is essentially really, really, really tenacious paint. Rust is a generic term used to generally describe the oxides of iron, which form at the temperatures, pressures, and humidity levels found on earth. But really, rusting is the same as oxidizing. Just about all metals "rust", including chromium (gold is one of the few that really doesn't rust). The neat things about chromium oxide or chromium "rust" (which is actually a greenish color), is that (1) it is chemically very stable; not much reacts with it. Once it forms it does not change over time. (2) It is very thin. So thin that its greenish color cannot be detected with your eyes. The chromium plating may be a variety of thicknesses, but relative to fletcher's diagram, the thickness of the oxide is totally insignificant. (3) Chromium alloyed in steels can give the same basic effect as chromium plating, and thus we have stainless steels. The more chromium that is alloyed in the steel, the more stainless the steel becomes. Any chromium content in steel above 12%, makes that steel a stainless steel. Above 18% chromium, the corrosion resistance is very, very good, but the mechanical properties of the steel suffer. Stainless steel with 12-18% chromium (used for gun components) can still have good corrosion resistance and they can have very good mechanical properties. This is why you'll sometime hear stainless steels used for guns are "not 'real' stainless".... This is really a matter of degrees. The stainless steels used for gun components are a trade off between corrosion resistance and mechanical properties.
Stainless steel silverware is an example of a stainless steel containing 20-25% chromium. Silverware has excellent corrosion resistance, but at a cost. You can take your average spoon and bend it easily. So the mechanical properties of these types of steels would not work well for guns.
Also mentioned was chrome plating showing signs of iron rust. The only way the iron rust can show through the chrome plating is if there has been some wear on the chrome plate and it has been physically removed (allowing air and humidity to reach the steel surface) or there can be some manufacturing issues such as less than optimal adhesion of the plate to the steel or something like significant porosity in the chrome plate which allows oxygen and humidity to reach the steel surface. This rusting is accelerated by something called "small anode-large cathode", but the best analogy is a small scratch through the paint of your car, exposing the metal underneath. The exposed metal will rust very quickly because the area that can rust (steel exposed by the scratch) is very small relative the area of steel that cannot rust (the whole rest of the painted door).
Sorry for the long post, but metallurgy is my life..... ;^)
