Help why is my battery terminal corroded and sparking?

If you have an inverter it may be the cause of sparking when reconnecting batt wire. Mine did this for years even though inverter was shut off. I proved it to myself by disconnecting inverter 12v wire and had no more sparking. I concluded there must be a capacitor in the inverter that discharges when batteries are disconnected, then draws some juice again as soon as batt wires are connected again.