What is CP

Cathodic protection (CP) is a technique that can control the corrosion of a metal surface by making it the cathode of an electrochemical cell. The simplest application method connects a metal to be protected to a piece of another more easily corroded "sacrificial metal," which will serve as the anode of an electrochemical cell. Once connected via an electrically continuous path, the sacrificial metal then corrodes rather than the protected metal.
-- Wikipedia