Even though good extension cords have rust inhibiting plating, they are still subject to oxidation and rust. When this happens, they reduce their current carrying capacity, possibly causing a fire hazard and reducing the voltage that the extension cord can deliver.
Try these tips to minimize this oxidation-
When stored outside or transported, plug the male connector into the female connector to cover the conductors.
Periodically plug and unplug both the male and female connectors to clean the conductors.
Spray a little WD40 or other rust inhibiting spray onto the connectors to keep them oxidation free.