"Ospho" from a local body shop supply place will halt it, turns iron oxide (rust) into iron phosphate (an absolutely inert substance). You can patch if you so desire after the iron mites have died.

My brother when moving farther north, decided to purchase a sprinkler, you know the kind with lots of sprayers on one tube that just sprays forward and back. He parks the car in increments every so often to clean the salt from underneath. One word of warning though...while it is a quite bright idea he had, one must be very careful walking to one's car after performing this routine procedure...after all...water does have a freezing point...and there is a point on my brother's back side that knows this all too well.

Chris