Why do some ships have only sacrificial anode cathodic protection, but no impressed current cathodic protection, and why do ships without engines have no double protection?

          Definition of cathodic protection: technology that mitigates the corrosion of a protected metal surface by taking measures to act as a cathode in an electrochemical cell. The forced current flows from the surrounding dielectric to the surface of the protected structure, so that all the metal surface is in the cathode state, and the release of electrons on the metal surface in the cathode region can be inhibited. The greater the applied current is, the stronger the cathode polarization will be, until the anode area on the surface of the structure is eliminated, and the corrosion will be inhibited. 

          There are two ways to achieve this goal: impressed current and sacrificial anode cathodic protection. In theory, if the coating is complete and defect-free, then the coating itself can complete the task of anticorrosion. However, it is actually very difficult and uneconomical to make the coating without defects. Therefore, the combination of coating and cathodic protection, 99% of the anticorrosive tasks with the anticorrosive coating, cathodic protection to protect the defects of the anticorrosive coating, is the most economical and effective anti-corrosion measures. The cathodic protection of steel pipeline is mostly made of potentiostat. The latest type is made of CNC high frequency switch potentiostat, which has more functions and higher cost。