The best way to accelerate rust on iron is to introduce acids or oxidants to the metal. Vinegar and muriatic acid are both excellent acids to use to accelerate rust.
Rust occurs when impure iron comes in contact with water, oxygen and other acids. Salt can make the iron rust quicker. Iron is not affected by rust when it comes in contact with just air or pure water. It takes an electrochemical reaction in order to degrade the metal. Rust can be prevented by applying coatings of chemicals that resist rust formation. Galvanization involves coating the iron with a layer of metallic zinc.