Most metals rust rapidly due to the high voltages involved and rust inhibits electrical flow. All the metal parts of our fences are made from stainless steel, which will not rust.
The high voltage combines with the nylon in the tape to produce a mild form of chlorine gas that attacks and oxidises all metals. If any part starts to turn black that is a warning that the chemical change is starting. Remove the tape and clean the black off tape and fitting. It is a wise precaution to coat all metal parts, and particularly energiser lugs, regularly with WD40, silicone wax or petroleum jelly as this stops this oxidisation process.
If left two things happen. Power drops dramatically and, in time, the tape filaments become brittle carbon. Carbon is non conductive.