Single beam in New York City Parking Garage showing Rust and coating Animation's require the Flash 5 plug in. All metals have a potential. You can think of this as the amount of energy required for refinement being stored in the metal as a potential. The greater the amount of energy required for manufacturing, the higher the potential of the metal.
Gold has a very low potential, carbon steel much higher and zinc higher still.
The different metallic components each having different electrical potentials. It is even possible to have areas of different potential along a single sheet of steel or along a single beam. Add moisture and salts from rainwater, humidity or melting snow and a variety of other sources and the electrolyte is present. Oxygen from the air around us provides the final element necessary for corrosion to occur. When all of the elements for corrosion are present, the anode, which is at a higher potential than a cathode, attempts to achieve equilibrium.