1. Turn off water and power to water heater.
2. Use garden hose to drain water from heater's tank.
3. Cut copper pipe just above the corroded connection using a tubing cutter.
4. Use two pipe wrenches to loosen the old union.
5. Remove the old union from the steel pipe.
6. Wrap threads on end of the steel pipe with Teflon tape.
7. Install the lower half of the dielectric union to steel pipe; tighten first with pliers, then with pipe wrench.
8. Solder coupling and small length of pipe to end of existing copper water pipe.
9. Remove the rubber gasket from dielectric union.
10. Solder top half of union to copper pipe.
11. Allow union to cool, then replace the rubber gasket.
12. Use wrench to tighten together the two halves of the union.
13. Turn power and water back on, and check for leaks.