I have geothermal heating so I have 2 water tanks the first for the residual hot water from the geo and the second electric. The cold and geo come into the first tank then they connect to the cold side of the powered heater. On both there are dielectric unions on both hot and cold. On both the bottom part of the union is rusting. On the first tank the rust is also on where the metal top contacts the side and where the electric access is on the top. Any ideas on what could be causing this?

Rust on a dielectric coupling is caused when the union is not perfectly seated or it was screwed on and no sealant was used. Are the unions sweated on or screwed on?

They are screwed onto galv pipes stubs from the tank. I have attached 3 pictures that may help.

The hot union (orange) did not seal inernallly. I would completely remove that union and the nipple going into the tank. Install a new nipple using a paste sealant rather than tape, this will seal better than tape since the tank threads are going to be iffy. Do not try to recycle the top half of the union, repace both sides. Test and watch for leaks for several days. I sometimes wrap a new joint with toilet paper which will show even a small drop of a laek

I have been told this was caused by electrolysis. They are telling me to install a copper connector (like a ground) from the cold water pipe on the first tank to the hot water pipe on the second tank.

I forgot to mention this is only a 1 1/2 year old new house.