Kegs are stainless steel. As long as the PBW was mixed in the proper concentration, it doesn't do any damage. It's Aluminum (and probably other soft metals) that PBW will corrode.
I've done the same thing before. There will be some residue that builds up on the metal that will come off by putting about a half gallon of water into the keg and shaking it for a minute or two.
I use it to soak my FV's for a few days,never noticed a film. But it def needs a good rinse. I can smell it till I rinse it thoroughly. Then it smells clean.