Ford Bronco won't start unless jumped

I have an full size '87 Ford Bronco with a 351 with an odometer which reads 156396. The vehicle has sat since it left me stranded about four months ago, I had jumped it and drove it home with no problems. Now everytime I try to start it, the vehicle sparks at the battery and then it is completly dead until I mess around with the cables. I replaced the battery and the alternator. It worked that night(I shut it off and restarted it with no problems) the next morning it sparked upon starting and then was dead again. I didn't touch it for a week and tonight when I went back to it, the negetive terminal was fizzing and when I pulled the cable off it had been partially disolved. I have no clue what is going on and would appreciate any help that anyone could offer. Thank you for taking the time to read all of this.
akf