If you dispense your beer with a party pump, which uses air instead of CO2 to drive the beer, you can only expect your beer to remain fresh for 8-12 hours. If you keep your kegs warm, and drive them with CO2 through a jockey box or other faucet, they will most likely last a couple of months.
Rust by definition is the result of the oxidation of metal. One engine manufacturer refers to the process of corrosion in aircraft engines as galvanic corrosion which is a type of corrosion caused by dissimilar metals being in close proximity of each other. In this process an electrolyte such as water also needs to be present. Almost all metals will oxidize or corrode.
Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration. All metals can corrode.