i chipped a large portion of the paint on the tank...and it's exposing the metal (silver) part of the tank...are there anything i can put on it to prevent rusting and corrosion??

Dab some touch up paint as close to the original color as you can find in there.  Put several coats as to build it up a little higher than the original paint.  Sand with 1500 or 2000 grit wet sandpaper.  Polish out with any fine cut polising compund.  I use 3M brand Perfect-It.  Thats what I would do.