Yes, you are using solder.
Solder for electrical purposes usually is 63/37 (Sn/Pb) or 60/40 (Sn/Pb), so it would not be suitable because of the lead (especially if it was immersed in water). Outside of water, it would be OK, but I wouldn't use it (if you have children/pets around, etc).
Plumbing solder compositions can vary, depending if the solder was meant for pipes that carry potable water or not. Solder for potable water pipes would probably be 95/5 (Sn/Sb)...