Rust is a pretty good abrasive, mixed with lube even better. One option if you have a syringe and needle is
to squirt a dilute phosphoric acid solution (ie diet coke) into the lock or immerse in same and let sit overnite
in warm area. Then wash out lock with clean water repeatedly, then use 90% alcohol to rinse out the water
several times or just put in very warm area after water rinse. Because of the nature of locks some sort of
injector is best with this approach. Then lube with a lock lubricant.
What this does is explained here:
https://en.wikipedia.org/wiki/Phosph...ersion_coating
Iron phosphate is a good rust inhibitor by itself.
Google on lock lubricants shows a bunch, graphite included, might be sl messy tho:
https://www.amazon.com/Powdered-Grap.../dp/B00ENQ3BAI