Ever see an old barn or garage full of old, rusty tools and other metal items? Many of those items could be used if the rust and corrosion were removed. The same goes for your old rusty padlocks, and they're even easier to clean because they're small and compact. Rather than spending a lot of money on a chemical rust remover, why not employ a more natural approach?
Place the padlock in a plastic bucket or other container. Pour in enough white vinegar to cover the lock completely.
Allow the lock to soak in the vinegar until the rust and corrosion have loosened. Check the bucket frequently; the process could take from a couple of hours to overnight. Remove the lock as soon as the rust is gone.
Brush the lock with a toothbrush to remove any remaining loose pieces of rust.
Pour some vinegar into a spray bottle. Spray it directly into the keyhole several times until the vinegar runs out clear.
Allow the lock to dry out completely. Spray the keyhole with a petroleum-based lubricant. Wipe any excess off with a towel or rag.