A/G pipe to U/G interface-cathodic protection

I might be mis-remembering this but it seems to me that when a pipe runs in the pipe rack and then drops and goes underground, you have to put in a set of isolation flanges to keep the pipe from getting eaten away at the point where it goes underground. Is this true or am I mistaken?

Yes, that is right. These are known as isolating flanges.
Piping which are buried in soil ;have to be given a positive current to nullify the corrosive electrochemical reaction between soil and buried piping.

based on the soil analysis and the surface area of the underground piping; the constant DC current amount to be supplied is calculated and accordingly rectifiers are installed at site. But, you do not want to waste the current to above ground piping (which are free from such corrosive effects).Otherwise,the intensity of the DC current supplied to underground piping would get reduced and the soil corrosive electrochemical reaction takes over.

Hence, insulating flanges are used.