You have been given many good suggestions from previous posts. I have had this white corrosion before on electrical contacts when a defective battery is encountered. As is with a car battery, this is a corrosive residue that must be removed and the contacts cleaned to prevent further problems. I have had no problems with my SB600 but I have a (and it still works) Vivitar 283 that years ago got corroded with a battery leak and it took some super fine steel wool and rubbing alcohol to cleanup. It also would not work after cleaning the contacts. After replacing the batteries, I held the flash unit in one hand and drove the side of the unit into my other open flat hand. At once I heard the capacitor/ thyristor start to whine as it built up the charge to flash the unit.
The one thing I have learned over my many years of electronics is to always remove the batteries from any valuable device when you will not be using it because it is too easy for good batteries to ruin an electronic device and that doesn't account for cheap batteries. The only things I do not remove them from are my wireless mice and keyboards because I use them everyday. One day I went to replace one of my wireless keyboard sets of batteries because the the computer said the batteries were out of power and sure enough one of the "copper tops" was starting to corrode. Good luck with the SB600.