Penetrox, Corrosion X HD, etc on the threads themselves, and made up tight, REAL TIGHT. Thats enough for average underground soil conditions, for saturated soil conditions I offer this suggestion; mate the pipe up tight... Finish the installation. Then a good tight and smooth taping of the whole joint with pipe wrap, or before burial, a good liberal coating or Sicaflex, or Fire and Ice roof patch (The stuff used on membrane roofs), and wait until dry before burial, either method is just an encasemnet of the electrically complete joint, not part of the joint. Niether will do anything to keep the rest of the pipe from rotting if the ph level of the soil with corrode the pipe anyway. There is PVC coated RMC you could use for that, and they have special sleeves and compounds that do this. Then RNC if you can use that...
