My guess is that those bars are fairly inexpensive and you could get some rust removal/protectant treatments that would temporarily keep the rust away but eventually you will have spent the cost of a new set of bars for these chemicals. Only other way to keep the rust away is to paint, re-chrome or frequent polishing and waxing or spraying with something like Boeshield which will leave a dull finish and needs re-application from time to time. Leaving your bike outside all of the time is really speeds up the corrosion process and deterioration for everything on your bike as well. If you can find a way to store it inside your bike will last much longer.