My flash has developed a current leak of sorts
I have a four year old Nikon SB-600. It eats batteries, mostly while it is just sitting there. When I went to use it yesterday, it wouldn't power up so I opened it to put in fresh batteries and I noticed new corrosion around the contacts on one battery. I know the batteries have been in the flash less than a month so this was puzzling. I cleaned off the contacts, put in fresh batteries and the flash still wouldn't power up. Uh oh. I played around with the contacts a bit but no go. The flash is well out of warranty and Nikon's website doesn't even offer authorized repair facilities for flash units.
Facing the prospect of having to drop $325 on a new flash (more than my first SLR kit cost) I decided to see if the damage was isolated to the battery case by rigging up a set of batteries I could attach to just the last contacts in the series. While rigging this up I put a voltage meter across the contacts of the flash with no batteries installed. The reading was .4V. My flash was putting out .4 volts weeks after the last use. I assume the capacitor is bleeding off charge back into the battery pack explaining the death of the batteries and the corrosion.
Next I tried cleaning the contacts with baking soda and water. This worked so now the flash will power up and functions. I guess as long as a I store the flash with the batteries removed I should be okay. Anyway, if you have a flash you don't use very often, you might check the battery compartment.