What is cathodic protection? How is it done by using impressed current and sacrificial anode? Explain with suitable examples.

Cathodic protection: the metal to be protected is forced to act as the cathode of an electrochemical cell, so it does not corrode. Corrosion is shifted to an auxiliary anode.

1. Sacrificial anode method: the structure is joined by an insulated wire to a more active metal (Mg, Zn or Al), which is higher in the galvanic series. The active metal becomes the anode and corrodes ("is sacrificed"), while the structure stays cathodic. Spent anodes are replaced from time to time.

      ground level
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   iron pipeline (cathode)        insulated Cu wire
   ===========================------------+
                                           |
                                      [ Mg block ]  sacrificial anode
                                      in backfill (gypsum + bentonite)
   Mg -> Mg2+ + 2e-  (anode corrodes)
   e- flow to pipe: O2 + 2H2O + 4e- -> 4OH- (on pipe)

Examples: Mg blocks on buried pipelines and underground tanks; Zn plates on ship hulls; Mg rods in domestic water heaters.

2. Impressed current method: a DC source (battery or rectifier) applies a current in the direction opposite to the corrosion current. The negative terminal goes to the structure, which becomes the cathode. The positive terminal goes to an insoluble anode such as graphite, high-silicon iron, scrap iron or platinum. The anode is buried in a backfill of coke and gypsum to improve contact.

            [ DC source / rectifier ]
             (-)               (+)
              |                 |
 ~~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~|~~~~~~~ ground
   ===========+=======          |
   buried pipeline (cathode)  [graphite anode in coke backfill]

Examples: long buried pipelines, oil and water tanks, transmission-line towers, ship hulls, marine piers. It suits large structures in soil of high resistivity and can be controlled from outside.