scale is not rust. a section of pipe that is rusting will not collect scale and vice versa; the rust is forming by an anode reaction, and scale forms by a cathode reaction. cathodic protection systems actually increase scale deposit on the metal that is being protected from corrosion, because the electrons that enter the water attract and concentrate the ions that form scale. To protect a surface from scale electrochemically, the impressed cathode becomes covered in scale and must be periodically removed and cleaned; the same way a sacrificial anode in a corrosion protection system must be periodically replaced.