Found it:
http://electrochem.cwru.edu/encycl/art-a04-archaeology.htm5ma per cm2 up to 50ma per cm2. Constant current power supplied at .005A per cm2 will not form hydrogen formation.
Something like an S2 oil filter would be 408 sq inches (5x8 in cylinder - rough dimensions). 408in2 = 2632cm2 * .005ma/cm2 = 13amps (low amp) or 2632cm2 * .05ma/cm2 = 131A (!) for moderate amp electrolysis.
That's really out of the range of most 'battery chargers'.
Lower amperage will work, but will take longer and since the effect is 'line of sight' you have to really play with the part to get it completely rust free.
For small parts, its not a bad way to do it, molasses is easier (slower) but media blasting is quickest.
